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Circuit Notebook

(1) DC Automotive Tester With Current Probe; (2) Winter Charge Booster For 12V Car Batteries; (3) Triple-LED Version Of Torch; (4) One-Of-Nine Sequencer; (5) Simple AM Transmitter; (6) Battery Status Indicator; (7) Smoke Alarm Battery Life Extender.

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

DC automotive tester with current probe

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The inspiration for this circuit came from the Auto Ammeter published in the June 2002 issue of SILICON CHIP. It avoids the hazards of trying to break into automotive circuits to make measurements.

A 3-terminal regulator, REG1, supplies the 5V needed for the Hall Effect sensor whose output at pin 3 is nominally at +2.5V and is fed into opposite inputs of comparators IC1a and IC1b. The other comparator inputs are connected to resistive voltage dividers and their thresholds set by trimpots VR1 & VR2. The trimpots are set so that LED1 and LED2 are just turned off. This setting is sensitive enough to detect the current drawn by a 5W globe in a 12V circuit.

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The two LEDs will show which way current is flowing in a circuit which is particularly useful when testing wires (which do not have the correct wiring code) within a wiring harness.

A simple current probe can be made using a 30A battery clip (DSE cat P-6420) and a small toroid (DSE cat R-5410). The toroid should be cut in half and the halves kept oriented in the same way. To do this, you score all round the surface of the toroid with a hacksaw and then carefully snap it in half. Be prepared to make a mess the first time you try this, so purchase several toroids.

The Hall Effect sensor is sandwiched between the toroid halves and silicone is applied to one side. Silicone is also used to hold the two toroid halves inside the clip. The battery clip also holds the lot together as the silicone cures.

Howard Anthony,
Huntingdale, WA.

Winter charge booster for 12V car batteries

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This is really a project for winter when the colder temperatures reduce battery capacity and make engine starting harder. This circuit is designed to shift the regulator's earth reference voltage up by about 0.6V to increase the maximum charging voltage.

The circuit can be fitted in a small box and mounted as close as possible to the regulator. The switch can be set to summer or winter, as the case may be. The extra voltage to the battery will do little harm given that it is less willing to accept a charge in cold weather and greater demands are made of it with more difficult starting, lights, wipers, etc.

Stephen Butcher,
Masterton, NZ. ($25)

Triple-LED version of torch

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This circuit includes the additions by Duncan Graham in January 2002 and Rick Matthews in the May 2002 issue of SILICON CHIP to make quite an improvement on the circuit originally published in the December 2000 issue.

It drives three white LEDs, increases the operating voltage range and regulation and allows the use of two cells rather than one.

Q4 is now a high-gain Darlington transistor and its associated base resistor R5 is increased from 220Ω to 560Ω.

L1 was replaced with a pre-wound coil from an old computer power supply. The coil is a toroid of 13mm diameter with approximately 70 turns of 0.5mm enamelled copper wire.

C3 was increased from 47μF to 100μF for extra filtering at higher current. R11 (10kΩ) was added in series with the current regulator as without it Q5 would stop the oscillator altogether instead of just altering the pulse width.

R7 was changed from 24Ω to 8.2Ω to allow the circuit to deliver 60mA to drive three LEDs instead of one. Resistors R8, R9 & R10 (4.7Ω) were added to ensure that the LEDs shared the current equally. The three white LEDs are 15CD (yes, 15 candelas!) available from Oatley Electronics for $6.00 each.

Philip Chugg,
Rocherlea, Tas. ($40)

One of nine sequencer

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This novel circuit uses a flashing LED as the clock input for a 4017 decade counter.

Typical flashing LEDs (eg, DSE cat Z-4044) flash at about 2Hz so the outputs Q0-Q9 will cycle through at that rate.

For example, Q0 will turn on for half a second, then Q1, then Q2 etc up to Q8 then it will start at Q0 again. Up to nine outputs can be used. If you want fewer outputs, connect an earlier output to MR, pin 15. If MR is not used, connect it to 0V.

Uses for the circuit include sequencing different strings of Christmas lights etc.

The resistor from CP0 to ground can be anywhere from about 330Ω to about 10kΩ. Lower values will cause the LED to flash more brightly if that is required.

With a 4.7kΩ resistor as shown, the clock input CP0 (pin 14) will alternate between about 2V and 7V.

To drive loads of up to 40W at up to 60V, connect each output to the gate of a 2N3055E or equivalent Mosfet (MTP3055E etc), as shown for Q0.

Peter Olsen,
Lugarno, NSW ($35)

Headlight controller for motor bikes

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This circuit automatically turns a motor cycle's headlight on and off, independently of both the light and ignition switches, provided the battery is fully charged.

The first stage uses the 22Ω resistor and ZD1 to hold transistor Q1 off while the motor is not running; it draws about 2mA.

Once the battery voltage exceeds 7.0V during charging, Q1 begins to turn on. The last stage uses the 22Ω resistor and ZD2 to turn on transistor Q2, which pulls the base of Q1 down, switching it hard on. In conjunction with the Vbe drop of Q2, ZD2 will turn off Q2 at a battery voltage of about 6.7V.

In practice, this means that the headlight will be on most of the time while the motor is running and charging the battery.

Heatsinks are required for both transistors. The circuit can be mounted adjacent to the battery with a single lead going to the headlight power feed.

Stephen Butcher,
Masterton, NZ. ($35)

Touch-enabled fuel cut-off for diesels

Ignition immobilisers work well for petrol cars but are no use for diesel-powered vehicles which require a fuel cut-off solenoid.

This circuit fills that need. It is auto arming and gives an armed/disarmed indication via the fuel gauge. Under the bonnet, it can be disguised as a horn relay, making it easy to hide as well as costing less than $20 to build.

To switch on the circuit, turn the ignition on and then use your hands to connect the touch point to the chassis (say via the ignition key). The tiny current which flows through your body turns on the Darlington-connected transistors Q1 & Q2 which activate relay RLY1.

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One set of contacts of RLY1 is used to latch it on via diode D2 and also switches on relay RLY2 for the fuel pump or fuel cut-off solenoid.

The second set of contacts of RLY1 connects the fuel gauge to the fuel sender, making the fuel gauge operate normally.

If the circuit isn't switched on, the fuel gauge should indicate that the tank is empty and the fuel pump or solenoid won't operate.

Note: this concept won't work in those vehicles in which the fuel gauge indicates even when the ignition is turned off.

The 47μF capacitor and diodes D1 and D2 provide a 0.5-second time-constant, so the circuit is less sensitive to momentary power loss and therefore safer.

The 100nF capacitor and 220kΩ pull-up resistor at the base of Q1 are there to prevent false switch-on at ignition turn-on and they reduce the sensitivity of the touch point.

A. Doyle,
Rydalmere, NSW. $40

Simple AM transmitter uses crystal ear piece as micophone

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There are not many AM transmitters that are easier to build than this one because the inductor is not tapped and has a single winding. There is no need to wind the inductor as it is a readily available RF choke (eg, Jaycar Cat LF-1536).

To make the circuit as small as possible, the conventional tuning capacitor has been dispensed with and fixed 220pF capacitors used instead.

To tune it to a particular frequency, reduce one or both of the 220pF capacitors to raise the frequency or add capacitance in parallel to lower the frequency.

Q1 is biased with a 1MΩ resistor to give a high input impedance and this allows the use of a crystal ear piece as a low cost microphone.

Peter Goodwin,
Southland, NZ. ($30)

Battery Status Indicator

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This circuit gives a progressive indication of the condition of a 6V (4-cell) battery. It is based on a 3-lead tri-colour LED (Jaycar Cat ZD-1735 or equivalent).

With a fresh 6V battery, transistor Q1 is cut off and Q2 functions as a current source, feeding about 2mA to the green LED. As the voltage from the battery drops, the bias on Q2 is reduced and so it is turned on less and Q1's emitter is no longer held below its base. Thus Q1 turns on gradually (as battery voltage falls) and so the red LED is progressively lit, producing a colour change from green to orange.

Finally, as the battery falls below 4V, Q2 is cut off and Q1 turns on fully, to give a red indication.

While this circuit has been set for a 6V battery, the transitions can be changed to suit other voltages.

Tony Ellis,
Porira, NZ. ($35)

Smoke alarm battery life extender

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While smoke alarms are quite cheap devices, the cost of 9V batteries quickly exceeds their purchase price. Added to that is the irritation of random beeps from the alarm as the battery reaches the end of its useful life.

This circuit allows typical smoke alarms to be powered from the 12V supply in a burglar alarm while still keeping the standard 9V batteries in place. It extends the 9V battery life to that of its "shelf life" as the battery is only required to drive the smoke alarm in the event the 12V supply is removed or shorted out.

In normal operation, the LM317 supplies 9.7V and this is fed via diode D2, resulting in just over 9V at the smoke alarm supply terminals. Q1 is not biased on, so the 9V battery is disconnected from the circuit. If the 12V supply is removed, the output of the LM317 will be 0V and Q1 will be biased on via the 4.7kΩ resistor and thus the smoke alarm will continue to be powered.

The circuit could be assembled on a piece of Veroboard and fitted inside the smoke alarm. Alternatively, you could house the circuit and 9V battery within a standard electrical flush-mount box which the smoke alarm covers when mounted.

Paul Blackler,
Spreydon, NZ. ($35)

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