School Science Lessons
(UNPh38.1)
2024-04-09
Electronics, Switching circuits
Contents
38.5.0 Switching circuits
38.5.0 Switching circuits
38.5.1 Switching circuits - Introduction
38.5.2 Automatic lighting control, front steps light
38.5.3 Bridge fire alarm
38.5.4 Flashing circuit
38.5.5 Headlights ON warning
38.5.6 Heat-operated switching circuit, fire alarm, emf protection
38.5.7 High speed flash photography
38.5.8 "Holding" coil
38.5.9 "Kill switch" with safety RCD
38.5.10 Latching relay circuit, bistable \ flip-flop
38.5.11 LDR --> LED in the dark
38.5.12 Light-operated switching circuit
38.5.13 Moisture detector, water indicator
38.5.14 Reed switch, reed relay, "make-and-break"
38.5.15 Relays, magnetically operated switches, "make-and-break"
38.5.16 Relay with matching contacts
38.5.17 Room air conditioning
38.5.18 Seat belt warning
38.5.19 Semiconductors, Transistors
38.5.20 Sound-operated switching circuit with latching, crystal microphone, burglar alarm
38.5.21 Switches in motor vehicle ignition system
38.5.22 Switches in parallel
38.5.23 Switches in series
38.5.24 Time delay
38.5.25 Time-operated switching circuit
38.5.26 Traffic lights
38.5.27 Transistor amplifier with magnetic earphone
38.5.28 Transistor amplifier with crystal earphone
38.5.29 Two tone police siren
38.5.1 Switching circuits - Introduction
Manually operated switches
See diagram 39.2.1: SPST and SPDT reed switches.
Switches stop and start flow of current through a circuit.
The switch is "off" when the switch terminals are not connected and "on" when a conducting part of the switch connects the terminals.
When a perfect switch is "on", current flows, when "off" no current flows.
The Morse code key is a switch controlling the current flowing to a circuit.
Name a mechanical switch by its "poles" and "throws", e.g. Single Pole Single Throw, SPST, and Single Pole Double Throw, SPDT.
The "pole" is the moving arm or lever.
The "throw" is the terminal to which the pole can connect or be "thrown".
A Morse Code key is usually a SPST switch.
Multipole switches have many moving arms and terminals selected by the position of the moving switch arm.
So, a 6-PDT switch has six poles each of which can switch between two, i.e. double, terminals, such a switch will have 18 terminals in total, 6 for the poles and 12 for the throws.
A push button is a SPST switch that has a return spring so that when a finger is lifted the switch returns to the open position.
This may be identified as an "SPST mom", meaning momentary.
Some Double Throw, called DT, switches have a central rest position where the moving arm makes no contact.
Call this a Centre-Off, or CO, switch.
38.5.15 Relays, magnetically operated switches, "make-and-break"
See diagram 38.5.02: Latched relay.
The relay is a switch operated by an electromagnet instead of by hand.
It allows small currents to switch large currents with electrical isolation between a low voltage and a high voltage circuit and to improve safety by operating the switch from far away.
A relay has a coil energized by the low voltage circuit and switch contacts connected to the high voltage circuit.
When the relay is off, the arm is at its rest position and the normally closed (N.C.) switch rests in contact with the common switch contact.
When a current passes through the coil, the resulting magnetic field attracts the arm so the normally open (N.O.) switch contact is in contact with the common switch contact.
Double pole double throw, DPDT, relays are common for controlling several circuits with one relay.
Use one relay to control the direction of a motor.
When a relay operates, hear a click as the relay switches on and off and see the contacts moving.
Relay parts can wear out as the switch contacts become dirty and high voltages and currents cause sparks between the contacts.
Relays have a slow response and the switch contacts can rapidly wear out due to the sparking.
Place a diode across a relay coil to avoid the back emf created when the relay coil switches off.
The back emf can damage components.
If using a relay to switch a 60 W lamp, the switch contacts must be rated for 250 mA at 240 V AC.
Use gold switch contacts for low voltages and tungsten for switching high voltages.
The relay must have a coil that can be energized by the low voltage control circuit.
The circuit powering the coil must supply enough current for the relay to operate.
Coil resistance means the range of voltage for the relay to operate.
Contact rating means current and voltage that the contacts can switch.
The number and type of contacts means whether relays have a single switching action or more, with single throw or double throw and number of poles, with single or double throw action.
A single pole double throw contact is a changeover contact.
For more contacts use one relay to switch another.
A plug-in circuit board with a make-before-break switch for showing slow RL time constants on the oscilloscope.
38.5.10 Latching relay circuit, bistable \ flip-flop
If a relay becomes "latched" on when pressing the Turn on button energizes the coil, the only way to turn the relay off will then be to cut the power supply by pressing the push-to-break "reset button".
This type of "bistable circuit" has two stable states for its output, on and off.
Connect the normally open switch contact of the relay to a motor that will then run indefinitely until the "reset button" is pressed turning off the coil for the trigger button to be pressed again.
The "reset button" can be pressed automatically to cut the power to the relay coil after the model has been running for a certain time.
Make a relay latch with a multi-contact relay so that one set of contacts bypasses the switching transistor keeping current flowing through the coil although the transistor stops conducting.
When it pulls in, it stays in no matter what happens in the rest of the circuit.
The only way to make the relay drop out again is to disconnect power.
38.5.14 Reed switch, reed relay, "make-and-break"
See diagram 38.5.03: Reed switch.
A reed switch has flexible metal reeds with contacts at the tips in a sealed glass tube.
The 2-reed switch has normally open (NO) contacts that close when the switch operates.
The 3-reed type has a wiper, a NO contact and a normally closed (NC) contact, thus achieving an NO and an NC pair.
When the switch is operated, both these pairs change to the opposite state.
Reed switches are actuated by the magnetic field from a nearby magnet or an electromagnet.
The magnetic field magnetizes the metal reeds so that their ends attract, or repel (as in the 3 reed case), each other and the contacts open or close.
When the magnetic field is removed, the reeds spring back to restore the contacts to their rest state.
As reed switches have only a momentary action a latching relay circuit is needed to keep the circuit on.
Reed switches are small, easy to install, cheap and easily obtainable.
However, the contacts and reeds are small and delicate and so cannot handle large voltages or inductive circuits, which may cause the reeds to arc when switched.
If asked to carry heavy currents the reeds may heat up and lose their springiness.
Because the mass of the reed is very small, reed relay switches can be quite a bit quicker than normal relays.
Reed switches can be built into the fixed parts of door and window furniture, the jambs and casements, and magnets attached to the moving parts.
These can be the basis of a security system, e.g., counting the number of entries to a secure building or setting off an alarm.
38.5.19 Semiconductors, Transistors
Semiconductors
The main semiconductors used in this section are diodes and bipolar junction transistors, BJT Diodes.
Diodes are used for protection, steering or illumination.
Protection and steering diodes can be 1N914, 1N4148, OK up to about 75 mA, 1N4001, 1N4002, OK up to about 1 A, 1N5400 series, OK up to about 3 A.
These are silicon and are fairly robust and cheap.
Germanium diodes are fairly fragile and often more expensive.
Diodes for illumination are LEDs, almost any type numbers are suitable.
All are silicon and have about the same forward volt drop.
Transistors
Most circuits in this section use NPN BJTs, because early transistors were made from germanium and were mostly PNP.
Germanium transistors are quite heat sensitive and can easily be damaged by excessive heat, including soldering leads.
After some use these transistors become leaky.
The amplification of base current may be lost in the high ICE leakage current.
Most modern transistors are made from silicon, because it is easier to make NPN transistors in silicon.
They are not so heat sensitive and do not develop the same leakage and ageing effects as germanium transistors.
Be sure to select suitable BJTs for these demonstrations.
Check with local electronics manufacturers for end-of-run components.
Even unmarked ones or ones with specific in-house markings can easily be identified.
38.5.6 Heat-operated switching circuit, fire alarm, emf protection
See diagram 38.5.1a: Alternate back emf protection.
To protect the bipolar junction transistor, BJT, a protection diode in reverse bias is wired in parallel with the coil.
When a high back-emf is induced in the coil, it can be clamped at the forward bias voltage, ~0.6V if silicon, of this diode.
The same protection can be achieved by connecting the emitter of an NPN control BJT to the control coil.
Here, the base emitter diode withstands the coil back emf, in this case, the coil still develops the full back emf.
Check the reverse Vce specification of the BJT, or experiment by measuring the collector current through a reverse biased BJT as the reverse bias is increased.
The BJT may be destroyed, so use this as a demonstration rather than as a class experiment.
Such use of a transistor as both control and protection reduces assembly time and cost of components.
1. Connect the bell (or a lamp) to the normally open, NO, contacts on the relay.
2. Adjust the variable resistance to just stop the bell ringing, or just extinguish the lamp.
3. Heat the negative temperature coefficient (NTC) thermistor with a very small flame or focus sunlight on it with a magnifying glass.
4. When a thermistor is heated it has a lower resistance, so more current flows through the BJT base, increasing the collector current and the bell rings.
38.5.3 Bridge fire alarm
See diagram 38.5.1b: Bridge fire alarm.
This is the basis for fire alarm systems.
The thermistor is placed at the highest point of the room, or in the main air exhaust duct in the ceiling, as hot air rises, the ceiling will be the hottest part of the room, normally.
Because of the variability of NTC thermistors, they are usually set in a mounting receptacle, along with a small heating element, to keep the thermistor at
about constant temperature, above normal room temperature, to improve the response time.
The thermistor is connected into a Wheatstone bridge circuit.
Where the other three arms are kept at a constant temperature, one arm is variable to set the trigger point.
Any change in the resistance of the thermistor can be detected quite quickly and reliably and the bridge circuit is also quite sensitive to small changes.
If a fire starts, the heated air rises even more rapidly to the ceiling or passes through the exhaust duct, changing the resistance of the thermistor, setting off the alarm.
Go back to the transistor, relay, variable resistance, thermistor demonstration.
1. Swap the variable resistor and the thermistor.
2. Leave the bell, or lamp, connected to the NO contacts.
3. Adjust the variable resistor so the bell just starts ringing, or the lamp starts glowing.
4. Heat the thermistor as before.
5. The bell, lamp, should stop working.
This kind of circuit can be used for turning off a night time heating device, or a night time insect killer.
If the house is air conditioned and there is no temperature difference between day and night, the thermistor will need to be outside, where it can sense the usual diurnal temperature variations.
Some thermistors are positive temperature coefficient, PTC, devices.
38.5.12 Light-operated switching circuit
See diagram 38.5.2: Light-operated switching circuit, front steps light.
In the dark, the resistance of the light dependent resistor, LDR, is high, so less current flows through it, more current flows to the base, base current is high, collector current is high, and the lamp lights.
In the light, the resistance of LDR is low, so more current flows through it, VBE falls, less current flows to the base, collector current is low, and the lamp does not light.
If a 10 k ohm variable resistance replaces the 10 k ohm fixed resistance, adjust the level of light / darkness at which the lamp will turn on.
This circuit is the basis of child care night lighting, automatic street lighting and automatic factory lighting.
For automatic street lighting and factory lighting there may be a half hour delay so that the circuit does not respond whenever a cloud passes overhead.
The delay may be combined with a 24 hour clock to prevent lights being turned on in the middle of an overcast day, or to turn off lighting in an office when nobody is be present, e.g. public holidays.
The delay may have an override circuit, e.g. operated by a security circuit, or by a telephone line, so that lights can be turned on automatically when there is smog, or when an intruder is detected.
To prevent positive feedback between the building or street lighting and the LDR, which would cause the control circuit to latch up, the LDR is aimed carefully away from the controlled lighting.
For a factory in the Southern Hemisphere, the LDR may be just under the southern eaves of the roof, but opposite for the Northern Hemisphere.
For street lighting, the LDR may be positioned on top of the luminaire's reflector pointing to the sky.
Two LDRs may be required, one to sense sunrise and the other to sense sunset.
These can be wired in parallel.
LDRs have some heat sensitivity, so the LDR may be mounted in a holder that has an infrared band stop, or RG band pass filter.
38.5.11 Latched relay
See diagram 38.5.02a: Latched relay.
The relay will become latched on when pressing the "Turn on" button energizes the coil.
This is an "SPST NO mom" switch.
The only way to turn the relay off is to cut the power supply by pressing the push-to-break "reset button" or "stop button".
This is an "SPSTNC mom" switch.
Connect the normally open switch contact of the relay to a motor that will run indefinitely until the "reset button" is pressed turning off the coil, waiting for the "turn on" button to be pressed again.
Operate the "reset button" automatically to cut the power to the relay coil after the motor has been running for a certain time.
This circuit is called a latching relay circuit bistable, because the circuit has two stable states for its output, on and off.
In binary logic terms, a latching relay is a "flip-flop".
38.5.16 Relay with matching contacts
See diagram 38.5 02b: Relay with matching contacts.
Make a relay "latch" with a multipole relay so that one set of contacts bypasses the switching transistor keeping current flowing through the coil although the transistor stops conducting.
When the armature pulls in, it stays in no matter what happens in the rest of the circuit.
The only ways to make the relay drop out again are to disconnect power to, or to short out, the coil.
38.5.2.3 "Holding" coil
See diagram 38.5.02c: "Holding" coil.
When contacts "make and "break", they wipe over each other.
This usually clears any oxidation caused by arcing from the previous turn-off.
If the return spring becomes weak or the control current is low, the speed of "make" may be slow, and the wiping less effective.
Some motor starter relays have two windings, a low resistance one for a rapid "make", which then switches over to a high resistance "holding" coil to latch the NO contacts on.
For motors running for a long time, this high resistance coil runs at a much lower temperature than the low resistance one would have, thus reducing risk of damage to the coil and switching gear.
38.5.9 "Kill switch" with safety RCD
38.5.9 "Kill switch" with safety RCD
See diagram 38.5.02c: "Kill switch" with safety RCD.
Latching relay "kill" switch
Safety consideration
A good application for a latching relay is as a "kill" switch where inexperienced people may have access to dangerous voltages, e.g. an electrical experimentation class.
At the beginning of the year, the instructor shows the students where the "kill" switch is and show its operation.
At the start of a class, the instructor pushes a "Start button" to energize all the mains circuits in the room.
Under normal circumstances, at the end of the class, the instructor pushes the "reset button" to de-energize all the mains circuits.
In case of an emergency, e.g., a student gets an electric shock, anyone can run to the kill switch and press the "reset button".
Turn off the mains BEFORE trying to touch or move a victim.
A further refinement is to use a residual current device, RCD, or an earth leakage circuit breaker, ELCB, as the trigger for operating the "reset button".
38.5.13 Moisture detector, water indicator
See diagram 38.5.3: Moisture detector.
Put spaghetti insulation around the connecting wires used as probes, leaving about 20 to 30 mm uninsulated at the sensing ends of the probes.
The operation of this circuit is similar to that for the light detector in the light-operated switching circuit, with the probes replacing the LDR.
This circuit does not work in distilled water, deionized water, methylated spirit and oils.
In soil moistened with water, inorganic salts in the soil dissolve and ionize to provide electrons for carrying current.
Most inorganic salts ionize to about the same degree, but organic salts are much more variable in their ionization.
The probes have a voltage between them provided by the sensing circuit.
Electrons flow from the probe connected to the BJT's base, through the ionized salts in the moistened soil, to the other probe.
If there is very little moisture, the available ions are further apart, and hence the "resistance" of the soil is high.
If there is a great deal of moisture, ions are readily available, and the resistance between the probes falls.
Beyond a certain increase in moisture level, no further drop in resistance occurs.
When the resistance between the probes falls, Vbe rises, base current increases, collector current increases and the LED lights.
The automatic washing line is out of favour, because the copper strips in salt will leave marks in the clothing that are difficult to remove!
38.5.23 Switches in series
See diagram 38.5.01: Multiway light switching, local switch and remote switch.
When switches are in series, current only flows if both switches are on.
Use these circuits for isolation and safety purposes.
For example, the main switch on the mains power distribution board is in series with all the power and lighting circuits in the house.
Use several switches in series to guide current to a circuit.
Burglar alarm circuits may use several switches in series, and all turned on when there is no danger.
If a burglar opens a door or window that has one of these on switches that switch turns off and signals an alarm.
This method of wiring several switches in series uses less wire than if all switches are in parallel.
The binary logic diagram for series switches is a form of AND gate, for two switches in series, both switches A and B need to be on for the circuit to be on.
It is a form of OR gate, because no current can flow if either switch A or B is open.
Table 38.5.3.1
| Switch A |
Switch B |
Outcome |
| OFF |
OFF |
OFF |
| OFF |
ON |
OFF |
| ON |
OFF |
OFF |
| ON |
ON |
ON |
The shorthand form of truth table found in binary logic texts uses "1" for ON and "0" for OFF:
Table 38.5.3.1a
| A |
B |
O / P |
| 0 |
0 |
0 |
| 0 |
1 |
0 |
| 1 |
0 |
0 |
| 1 |
1 |
1 |
This is the truth table for an AND gate.
Draw the shorthand truth table as in 38.5.3.1.
In transistor-based switching circuits, the output of a transistor switch can be changed by either an ON or an OFF switching event.
38.5.22 Switches in parallel
See diagram 38.5.01g: Motor reversing.
When switches are in parallel, current flows if any switch is on.
Such circuits can be used for signalling danger when there are several risky events that need the power to be switched on.
For instance, water sensing switches could be placed in the bathroom and under the house to alert the owner to possible flooding danger, or to turn on a pump.
Burglar alarms can use several switches in parallel, all turned off when there is no danger.
If a door or window with one of these off switches is opened by an uninvited guest, the switch turns on and signals an alarm.
This method of wiring several switches in parallel uses more wire than if all switches were wired in series.
By using several alarm circuits, some in series and some in parallel, a burglar cannot know in advance whether to short out or open a circuit to avoid setting off an alarm.
The binary logic diagram for parallel switches is a form of OR gate, for two switches in parallel, either switch A or B needs to be on for the circuit to be on.
It can also be seen as a form of AND gate, no current can flow if both switches A and B are opened.
Table 38.5.3.2
| Switch A |
Switch B |
Outcome |
| OFF |
OFF |
OFF |
| OFF |
ON |
ON |
| ON |
OFF |
ON |
| ON |
ON |
ON |
38.5.20 Sound-operated switching circuit with latching, crystal microphone, burglar alarm
See diagram 38.5.4a: Check sensitivity of a circuit.
See diagram: 38.5.1: Sound-activated burglar alarm.
Tap on the crystal microphone to make it produce enough base current to switch on the transistor.
The crystal microphone can easily produce up to 1 V AC if tapped sufficiently vigorously.
The base emitter junction of the BJT acts as a rectifier so that voltage excursions from the crystal microphone that exceed the base bias will increase base current and thence collector current.
The collector current passing through the transistor to the emitter provides gate current to trigger the thyristor and current flows through the lamp.
The lamp will stay on until the battery is disconnected, because the thyristor acts as a latching switch.
To make a sound activated burglar alarm, substitute a relay for the thyristor and a bell for the lamp.
A crystal microphone is quite sensitive, i.e. its output is quite high for any defined change in sound pressure, any NPN signal BJT
running in this open loop fashion, has very high gain.
The gate turn-on current of the thyristor is not specified very accurately, and depends on temperature.
Consequently, this is really a demonstration of the principles.
It could not be a serious useful device as it stands.
Discover how variable is this device:
Potentiometers, (Commercial).
1. Insert a small resistor in the lead between the BJT's emitter and the SCR's gate, or in the BJT's collector lead, say, 1 k ohm.
2. Connect a CRO across the 1 k ohm resistor.
3. Experiment with making increasingly louder sounds, perhaps an audio oscillator fed to an audio amplifier and a loudspeaker, until the lamp is triggered on.
4. Experiment with temperature variation by holding the BJT and the thyristor in the fingers.
5. If a sound pressure meter is available, plot the range of levels that provide reliable turn-on.
A sound detector can be further developed by connecting the crystal microphone across a 1 M ohm logarithmic potentiometer, with the wiper connected to the BJT's base.
38.5.7 High speed flash photography
See diagram 38.5.4b: Flash gun delay.
This demonstration is the basis for firing flashguns for high speed photography.
Usually, the SCR is in the flashgun.
This system can be used for capturing the effect of crash-testing motor vehicles and for observing the effects of small arms projectiles on targets.
Such experiments need to be set up in the dark, because the camera shutter is left open.
The delay between hearing a shot, pressing a shutter control and the operation of the shutter, even in very fast electronic cameras, is too long and variable.
In electronic and digital cameras, there is always a finite delay between operating the local or remote electronic shutter release, and operation of the shutter.
Sports photographers learn to anticipate this delay in each camera used.
In the more sophisticated flashgun firing circuits, a variable delay circuit is used to allow for different transit times.
When photographing handgun projectiles, allow for the sound to travel from the hand gun to the crystal microphone and for the speed of the projectile.
Sound travels at about 340 m / s, and handgun projectiles travel at between 250 m / s and 500 m / s.
For the stone and glass experiment, the crack in glass can travel at up to 400 m / s.
Try photographing someone sneezing violently, the material exiting from the mouth and nose can reach the speed of sound!
There is no need to allow for the time between firing the SCR and initiating the flash, the delay is usually less than 1 ms, Metz, Rollei.
38.5.27 Transistor amplifier with magnetic earphone
See diagram 38.5.5, Transistor amplifier with magnetic earphone.
The size of the base current depends on value of the fixed resistor.
The capacitor stops DC passing through the microphone, but allows the AC signal from the microphone to flow to the base.
The diode can be seen as a very nearly perfect switch.
When the voltage applied is in the forward bias direction, current flows.
When the diode is biased in the opposite direction, there is almost no current flow:
* connect a diode, ammeter and current limiting resistor in series.
* connect this circuit to a variable voltage power supply,
* plot the current flow vs the applied voltage,
* reverse the polarity of the power supply,
* Plot current vs applied voltage.
Note: do not exceed the VRRM specification.
The diode can be seen as an automatic switch, when the polarity is correct, current flows, no human intervention is required.
This can be used as a safety device to protect polarity sensitive devices.
Zener diodes act as normal diodes when forward biased, their current carrying capacity is limited, the Zener effect can fail if the forward current limit,
IFM, is exceeded.
38.5.28 Transistor amplifier with crystal earphone
See diagram 38.5.5b, Transistor amplifier with crystal earphone.
It can be used as a simple two-wire signalling device by connecting a current limiting resistor in series with a pair of LEDs, e.g., one green and one red,
each in series with a reverse bias protection diode, but the two branches of LED and diode are in reverse polarity to one another.
At the far end of the two wires, voltage of one polarity will light the green LED, reverse the polarity and the red LED lights.
A bell, a buzzer or an incandescent lamp can be substituted for each LED.
If the ground is sufficiently conductive, one of the two wires can be replaced by a ground spike at each end of the circuit, a higher voltage will be required
to sound the bell or buzzer or light the lamp.
This is the basis of some early telegraphic and telephonic signalling.
The beauty of the diode as a switch is that it is silent and has no moving parts.
The main drawbacks of the diode are the slight reverse current, negligible with silicon diodes, and the unpredictable failures.
Whether the diode will fail short circuit or open circuit cannot be predicted.
A gradual build-up of current is likely to weld the elements.
38.5.25 Time-operated switching circuit
See diagram 38.5.6a: Car courtesy light switch.
When switch 1 closes, the lamp lights and the capacitor starts to charge until VC > VBE, then current flows in the BJT's base to switch on the BJT,
relay contacts open, lamp switches off.
Open switch 1 and close switch 2 to let capacitor discharge.
Open SW2 and close SW1 to recommence.
What determines how long it takes for lamp turn-off?
Time constant.
Try different value capacitors to vary the turn-off time.
38.5.6.1 Courtesy lighting
This was the basis for automatic courtesy lighting circuits in motor vehicles, opening the door operated SW1, turning on the ignition switch operated SW2.
So the lights stay on long enough for the driver to insert the ignition key in the ignition lock.
Opening the door with the engine running may not turn on the courtesy lights unless another circuit is used.
Perhaps SW2 is operated by the starter motor solenoid, but then the capacitor will stay charged.
After the door closes, a bleed resistor, say 100 k ohm to 1 M ohm, across the capacitor could fix that.
Nowadays, an integrated circuit like the 555 would be used, with a diode feeding a strobe circuit to short the capacitor.
38.5.4 Flashing circuit
See diagram 38.5.7: Flashing circuit.
The LEDs flash alternatively, perhaps at too fast a rate to see.
When the switch is closed both capacitors start charging.
Either capacitor could start charging first.
One capacitor will charge faster than the other.
If capacitor 1 charges faster some current flows from the capacitor 1 to the base of transistor 2, which stops the capacitor 2 connected to its base from charging.
Some current flows from the capacitor 2 to the base of transistor 1, which stops the capacitor 1 connected to its base from charging.
When capacitor 2 is fully charged current stops flowing to the base of transistor 1, which no longer stops capacitor 1 connected to its base from charging.
The capacitor with the least capacitance will charge faster.
At start-up, both capacitors are like short circuits, so, both bases start at VBAT less VDIODE + V390R + VLED.
This should turn both BJTs on hard, but one BJT will turn on a bit ahead of the other, because of component differences.
If BJT1 turns on faster, VCE falls thus reducing VC of BJT1, this stops the capacitor, C1 connected to its collector, from charging.
Meanwhile, C2 keeps charging and when it is fully charged, no more current flows in C2 and BJT1 turns off.
VC of BJT1 now rises allowing C1 to charge, leading to BJT2 turning on.
As long as the battery is connected and has life in it, the LEDs will keep flashing alternately.
In binary logic terms this is called a bistable, symmetrical flip-flop.
Alter the values of C1 and C2 to see the effect on the flash rate.
By setting C1 significantly different from C2, which LED will turn on first can be predicted, but now the bistable is no longer symmetrical.
Tolerance of capacitor values.
If measuring capacitance, show that two capacitors with the same markings have different values of capacitance and so you can predict which LED will come on first.
38.5.7.1 Measuring capacitance
If a capacitance tester is not available, use a high input impedance multimeter, e.g. a DMM with 10 M ohm input on the voltage ranges a 100 k ohm fixed resistor, a DC power supply, a SPDT switch and a stopwatch.
* Connect the components as in diagram 38.5.7a.
* Set the output of the PSU to about 90% of the rated voltage of the device under test, DUT.
* Set the output of the PSU to constant voltage and a very low current.
* Turn on the DMM and allow it to auto zero.
* Turn the switch on to "Charge" for several seconds, then "Discharge" the capacitor with the 100 k ohm resistor.
Repeat this charge discharge cycle several times until when the DMM shows full voltage.
As in step 2, there is no charging current shown on the PSU meter.
This action should remove any polarization effect.
* Calculate 63% of the voltage in step 2.
* At the same time as turning the switch to "Charge", start the stopwatch.
* When the DMM indicates the 63% value, stop the stopwatch.
* Calculate the value of C.
* To be certain of the value, discharge the capacitor through the 100 k ohm resistor and repeat steps 7 to 9 several times.
The time constant for a 100 uF capacitor and a 100 k ohm resistor is 10 s.
38.5.2 Automatic lighting control, front steps light
See diagram: 38.5.8: Using IR.
Could use 38.5.2, above, if all that is required is that the lights come on when ambient daylight falls below a predetermined level.
1. For acting on the presence of humans or warm blooded animals, a Passive Infrared, PIR, detector, or an active IR, AIR, detector could be used.
With a PIR, or an AIR, detector, the device senses both temperature and movement.
The Perkin Elmer LHi954 and 958 detectors, PIR devices, are sensitive in the 0.1 Hz region and have a total included sensing angle of 110o.
The detector has an FET wired as a source follower in the same case.
If the PIR detector is wired into the circuit of 38.5.2 in place of the LDR, the resistance between the drain and the source, RDS, increases with movement
and incoming IR radiation.
It acts in the opposite manner from the LDR and needs to be wired as in the second circuit, 38.5.2b, if the controlled circuit is to be turned on.
Warning!
As the PIR device is sensitive to both heat and movement, it cannot be hand-held.
2. For a more sophisticated detector, use an IR light aimed at an opto-transistor.
Be sure to select IR emitters.
They may require at least 20 mA current flow, connected in series with an ordinary diodes, a 220 ohm resistor and a 6V battery.
A suitable detector photodiodes, or NPN phototransistors.
Each device has only two leads, either cathode and anode, or collector and emitter, connect the photodetector in place of the LDR in the circuit of 38.5.2b,
anode / collector to the + supply and cathode / emitter to the base of the BJT.
So that when it detects emission from the IR emitter, the circuit is turned on.
These photodevices have some sensitivity to ordinary light, they can be set off by sunlight, a car's headlight or a torch.
Put the device into a light tight tube and fit an IR bandpass, or an RG bandstop, filter in the end of the tube.
Similarly, put the IR emitter into a tube with a double convex lens to
focus the beam on the receiver.
If an ordinary incandescent lamp is used instead of the IR emitter, put
an IR bandpass, or RG bandstop, filter before the lens.
This will make it very difficult for the casual observer to see the beam.
3. To make the demonstration more visually dramatic, use a red LED pointer torch as your emitter, the kind used by slide show hosts, and leave the IR
filter off the detector, blow some smoke across the path of the beam to reveal the beam and show the effect of interrupting the beam.
Then replace the red LED pointer with your carefully constructed IR emitter
in its tube and replace the IR filter over the IR detector.
Blowing talcum powder over the beam will not change the frequency of the light, so, the IR beam will not be revealed to the human eye.
If fibre optic cables are being articulated in your country, ask your local telecommunications installation firm if it can spare any photodevices.
Some devices may have the IR filter already fitted.
Some may be side viewing, and some axial.
38.5.24 Time delay
Seat belts, traffic lights, police siren, bar code, morse code, headlights on, air conditioning.
The Morse code key is a switch controlling the current flowing to the circuit.
See diagram: 38.5.9: Monostable time delay.
In 38.5.01d, the idea of slowing down operation of a mechanical relay was introduced.
In 38.5.02, the idea of the flip-flop was introduced, this is a bistable electronic delay.
By removing BJT2 and leaving R2 and C1 in place, you have a monostable electronic delay.
Varying the value of the R or C will alter the time before turn on.
However, while VBE, the voltage at which the BJT will turn on, or off, is fairly constant, as the battery ages, the time taken to turn off will increase.
If instead of a battery, a constant voltage power supply is used, this limitation is removed.
A constant voltage can be achieved by using a Zener diode in series with a current limiting resistor as the supply for the RC circuit feeding the base of the BJT.
If the whole circuit were powered from the Zener diode, the voltage would drop a little when the circuit turned on and you might get some instability or hysteresis.
With tantalum capacitors, which have very low leakage, and metal oxide resistors, which are very stable, quite long time delays can be achieved.
Applications for this include motor vehicle seat belts and headlights-on warnings, traffic lights, police sirens and room air conditioners.
38.5.18 Seat belt warning
See diagram: 38.5.9a: Seat belt warning.
A switch under each person's seat in series with a switch in the seat belt buckle receptacle controls the current fed to the resistor, R, in the circuit above.
If the delay is set to, say, 12 seconds, this allows enough time for each person to get seated and "belt-up".
If the driver's circuit is kept separate and operates a relay to control the ignition, the vehicle will stop if the driver fails to belt-up in time.
All the other circuits can be used to operate a display in front of the driver to show which passenger has not belted-up in the required time.
The time delay also allows a person to disconnect a seat belt for a very short time, e.g. to belt-up a child.
A resistor across the C can make the turn-on and turn-off times quite unsymmetrical.
Because of the range of operating voltages of a car battery, 10 V to 15 V, the RC chain would be fed by a Zener set at, e.g. 6.8 V.
The battery voltage may drop as low as 6V during starting, but shortly after when charging commences, the voltage would rise quite quickly, and 6.8 V
would be available within a couple of seconds to warn of seat belts not done up.
38.5.5 Headlights ON warning
See diagram: 38.5.9b: Headlights ON warning.
If you drive to work in the morning when it is dark or foggy and when you arrive at your destination the sun has risen or the fog has dispersed, you may forget to turn off your headlights.
Motor vehicles are normally fitted with about a 60 Ah battery.
Normal headlamps are 60 W each.
A pair of headlamps, plus the side, tail and number plate lamps draw about 12 A.
So, in about 5 hours the battery will be flat.
If you leave the car for a normal 9 hour day, not only will the battery
be flat, but also you may have damaged it permanently.
Some people actually get into a habit of turning off the headlamps BEFORE turning off the ignition, just as they turn on the headlamps
AFTER the engine has started.
These habits are actually kind to the battery.
However, some people never learn these habits.
So you want a circuit that will alert the driver within a short time of turning off the ignition that the headlamps are still on.
The headlamp circuit is normally in series with a relay, another set of contacts on the headlamp relay, or from the headlamp switch
provides a feed to the collector of the time delay circuit.
The ignition circuit provides a feed to the Zener diode powering the RC / base circuit, only when the ignition is off and the headlamps are on.
When headlamps and ignition are both on, the warning LED is OFF.
A small hooter or other warning device is connected to the delay BJT's emitter and the NO pair of contacts on the headlamp relay.
If the headlamps are left on after the ignition is turned off, the voltage on the base of the BJT will gradually rise and the BJT will turn on, and the hooter will sound.
If the headlamps are turned off before the ignition is turned off, there will be no power to the delay circuit or to sound the hooter.
Table 39.5.9.2
| Ignition |
Headlamps |
Warning |
| OFF |
OFF |
OFF |
| ON |
OFF |
OFF |
| OFF |
ON |
ON |
| ON |
ON |
OFF |
38.5.26 Traffic lights
See diagram 38.5.9c: For outline schematic, each time delay represents a delay circuit.
1. British traffic lights are red over amber over green.
British railways signal lights are green over amber over red, because the cowl around the light can collect snow above it and obscure the light above.
So the red is placed below the other lights where it cannot be obscured by snow on the cowls of the lights below it.
2. Some traffic lights are set on a fixed cycle during peak commuting times, e.g. 35 seconds green in the E-W direction, then 6 seconds of amber followed by 49 seconds of red.
The lights in the N-S direction are set to commence their green phase 4 seconds after the end of the E-W amber phase. to allow for late clearing of the intersection and "chancers", stay on for 35 seconds, change to amber for 6 seconds and then to red for 49 seconds.
This circuit can be achieved with six BJTs controlling six relays, by wiring some of the relay contacts in series, you can arrange that red and green cannot both be on at the same time.
Another pair of BJTs and relays can be inserted to achieve the British system of giving an amber + red phase as a prelude to the green phase.
The RC circuits are all fed from one Zener diode supply.
In place of the mechanical relays, "solid state" relays can be used.
These use no moving parts and are essentially tracks.
In some places, pulsed LEDs are used in place of the usual incandescent lamps.
Pulsed LEDs can be more efficient, brighter and last longer than incandescent lamps.
The pulsing of the LEDs can be achieved by a bistable set to cycle at about 60 Hz, faster than the human eye can sense.
The only drawback is that the initial purchase price of the LED cluster and control circuitry is greater than that for an incandescent lamp and its socket.
38.5.29 Two-tone police siren
See diagram 38.5.9d: (Click suppression not shown).
For the two-tone police siren in France and the ambulance sound heard in many parts of the world, the bistable circuit of 38.5.7 can be used.
Instead of one of the LEDs, fit a relay selecting between the output of two oscillators to be fed to an amplifier and speaker.
The armature is connected to the amplifier input, the NO and the NC contacts of the two relays are connected separately to the output of each oscillator.
Choose any two tones, e.g., 400 Hz and 500 Hz, sort of Ab and B#, but not quite C, for the oscillators.
If the oscillators can be set to square wave, the tones will be much more annoying / alerting.
As the tones are not quite in tune with standard pitch, they will annoy anyone with any musical sensitivity.
The oscillators can each be the bistable of 38.5.7, with much smaller values of RC product.
The RC product in the RC chains of one oscillator will need to be different from those in the other oscillator.
Fit a 1 k ohm resistor in place of each LED, from the collector of one BJT in each oscillator, connect a 100 nF capacitor to the NO, or NC, contact of the appropriate relay.
Connect a 22 k ohm resistor from the relay side of the 100 nF capacitor to ground to avoid loud clicks as the relays change-over.
38.5.17 Room air conditioning
Whenever a door to an air conditioned room is opened, the temperature and humidity are likely to change.
When people move around inside an air-conditioned room, closer to and then further away from the room sensor, thermostat / hygrostat, block, it will sense a change.
Such changes can be a signal to the air conditioner to come on or go off.
This rapid turning on and off can be wearing on the moving parts of the air conditioner and on the room occupants.
Feed the signal from the room sensor to a monostable, i.e. 38.5.7 circuit, as above, but with the R and C swapped.
Replace the LED with a mechanical or "solid state" relay to control the main air conditioner.
The timer can be set to, say, 10 minutes.
Such a delay can cover room deliveries, occasional visits to the balcony to have a smoke, savour the smog.
38.5.21 Switches in motor vehicle ignition system
38.5.10.0 Switching off
38.5.10.1 Switches in a motor vehicle ignition system
38.5.10.2 Coil ignition system in motor vehicles, "points", capacitor across points
38.5.10.3 Effect of switching an AC circuit
38.5.10.4 Coil ignition system in motor vehicles, high voltage at the opening of the contacts
38.5.10.5 Switching a car headlight bulb
38.5.10.0 Switching off
1. Switching off a DC circuit connecting only resistance is easy.
Connect a car headlamp bulb in series with an open contact switch, e.g. a knife switch, and a small car battery.
Use a proper socket for the bulb to avoid shorting the battery.
Observe the very small spark as the switch contacts open.
2. Switching off a DC circuit connecting an inductance is not easy.
The current tends to continue flowing as a high voltage builds up at the switch contacts.
See this as an arc at the contacts as the switch is opened.
38.5.10.1 Switches in a motor vehicle ignition system
Coil ignition system in motor vehicles, "points"
See diagram 38.5.00a: Spark coil without capacitor across points.
The coil ignition system in motor vehicles relies on this high voltage at the opening of the contacts, sometimes called "points", in the distributor.
Connect a spark coil in series with a car battery and an open contact switch.
Make sure that the secondary of the spark coil is shorted, or connected to a spark plug with its body connected to the battery or to one of the low
voltage terminals on the coil.
When the switch contacts are opened see a small spark at the switch contacts and at the spark plug.
38.5.10.2 Coil ignition system in motor vehicles, "points", capacitor across points
See diagram 38.5.00b: Spark coil with capacitor across points.
Connect a 1 uF 250 V capacitor across the switch terminals in the previous experiment.
When the switch is opened, less sparking at the switch, but a much bigger spark at the spark plug, because the capacitor allows much bigger build-up of voltage at the switch contacts and much less loss of energy at the switch contacts.
Arcing of switch contacts will cause interference you can hear as a crackling sound on a radio.
It will also erode the switch contacts that will fail eventually.
The capacitor across the distributor points in a car prolongs the life of the points.
Be careful!
There will be quite a high voltage at the switch contacts as they open, so the fingers must not touch the switch parts or the wires attached to them.
Use a CRO to show the difference in voltage across the coil primary and across the switch contacts with and without the capacitor.
With no capacitor across the switch contacts, the limiting seen on the CRO is the breakdown ionization of the air.
To see it on a CRO, use Horiz Magnify to see the short duration, high voltage pulse.
Be careful!
Do not try to use the CRO on the secondary of the spark coil!
Cars have capacitors across most DC switches to reduce wear of the switch contacts and to reduce interference to other electronic devices in the car.
38.5.10.3 Effect of switching an AC circuit
Use a low voltage step down transformer, at least 60 W, and repeat the experiment with a car headlamp bulb, with the switch in the secondary side.
In an AC circuit containing inductance, the rapidly changing polarity of the AC makes it difficult to open the switch at exactly the right moment to get either no spark or a big spark at the switch contacts.
Move the switch arm very gently to a position where sustained sparking at the switch contacts occurs.
A capacitor across the contacts may reduce the sparking, but the switch cannot actually turn the circuit off, because the capacitor will appear to continue to carry AC current.
Repeat the experiment with a choke in series with an incandescent lamp.
See the light dimming and hear the arc buzzing.
Disconnect from the AC and then look at the blackened contacts.
The choke can be likened to the wiring in a household.
So use snap action switches in AC circuits to avoid contact erosion and maintenance.
A suitable combination of capacitor across the switch and circuit inductance could result in resonance.
Think about what happens to the current flowing in a series resonant circuit.
At resonance, current flow in the capacitor and inductance is at a maximum and much larger than the current flowing in the rest of the circuit.
Better quality equipment use DPST AC mains isolating switches.
Think about what happens with a SPST switch in circuit if the active (or live, or phase) and neutral wires are swapped around.
This happens quite frequently when untrained people put power plugs on AC equipment or even on extension leads.
Switch contact materials are often made of copper, silver, gold or platinum, because whether machinable, proneness to erosion, ease of oxidation, conductivity of oxide, cost, and the required life of the equipment.
38.5.10.4 Coil ignition system in motor vehicles, high voltage at the opening of the contacts
See diagram 32.5.2.2: Ignition system.
The coil ignition system in motor vehicles relies on this high voltage at the opening of the contacts, points, in the distributor.
Connect a spark coil in series with a car battery and an open contact switch.
Make sure that the secondary of the spark coil is shorted, connected, to a spark plug with its body connected to the battery or to one of the low voltage terminals on the coil.
When the switch contacts are opened see a small spark and a spark at the spark plug.
Connect a 1 uF 250 V capacitor across the switch terminals.
When the switch is opened, see less sparking at the switch, but a much bigger spark at the spark plug, because the capacitor allows much bigger build-up
of voltage at the switch contacts and much less loss of energy at the switch contacts.
Arcing of switch contacts will cause interference like a crackling sound on a radio.
It will also erode the switch contacts, causing them to fail eventually.
The capacitor across the distributor points in a car prolongs the life of the points.
Modern cars have capacitors across most DC switches, because switching off an AC circuit connecting resistance only, is easy.
Use a CRO to show the difference in voltage across the coil primary and across the switch contacts with and without the capacitor.
With no capacitor across the switch contacts, the limiting seen on the CRO is the breakdown ionization of the air.
Be careful!
Do not try to use the CRO on the secondary of the spark coil.
38.5.10.5 Switching a car headlight bulb
Use a low voltage step-down transformer, at least 60 W, and repeat the experiment with a car headlamp bulb, switch in the secondary side.
In an AC circuit containing inductance, the rapidly changing polarity of the AC makes it difficult to open the switch at exactly the right moment to get
either no spark or a big spark at the switch contacts.
Move the switch arm very gently to a position where sustained sparking at the switch contacts occurs.
A capacitor across the contacts may reduce the sparking, but the switch on actually turns the circuit off, the capacitor will appear to continue to carry AC current.
Repeat the experiment with a choke in series with an incandescent lamp.
See the light dimming and hear the arc buzzing.
Disconnect from the AC and then look at the blackened contacts.
The choke can be likened to the wiring in a household.
Use snap action switches in AC circuits to avoid erosion and maintenance.
A suitable combination of capacitor across the switch and circuit inductance could result in resonance.
Think about what happens to the current flowing in a series resonant circuit.
Better quality equipment use DPST AC mains isolating switches.
What happens with a SPST switch in circuit if the active, live, phase, and neutral are swapped around?
Switch contact materials are often made of copper, silver, gold or platinum, because of such factors as whether machinable, proneness to erosion, ease of oxidation, conductivity of oxide.