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Circuit Notebook

(1) Traffic Lights for Model Cars Or Railways; (2) LED Torch Uses Blocking Oscillator; (3) AFX Slot Car Lap Counter; (4) Simple BFO Metal Locator; (5) Capacitor Leakage Adapter For DMMs; (6) Simple AM Radio Receiver.

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.

Traffic lights for model cars or model railways

Kids these days seem to have most things you see in the toy shops, so if you have a son or grandson who has a collection of cars, here is something he will really appreciate. And it will be really special as you will be giving something made by you - a set of traffic lights for his cars.

This traffic light circuit uses a 555 timer IC as the master timer. The 220kΩ timing resistor and 10μF capacitor control the timing pulses, giving a period of about three seconds.

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The 3-second output pulses are used to clock a 4017 decade counter whose outputs directly drive the green, orange and red LEDs. To obtain a longer time for the red and green lights compared with the orange light, two outputs are ORed using 1N4148 diodes for the red and green LEDs, while the orange is driven by one output only. This gives about 6 seconds for the red and green LEDs and 3 seconds for the orange.

When power is first applied, the RC network connected to pins 1 and 15 of IC2 resets the 4017 and the green LED cycle begins. The orange and red cycles follow and at the end of the red cycle, pin 1 will go high to reset the 4017 to start the green cycle all over again.

You can experiment with the cycle times by adjusting the 220kΩ resistor or by combining more or less 4017 outputs to achieve different ON times for the three LEDs.

The circuit is designed to be powered by a 9V battery and this is the maximum voltage that is recommended. This is because the LEDs are directly driven by the 4017 with no current limiting resistor being used. The 4017 naturally limits the current that it can supply to 15mA.

An extension of this project would be to make a second set of lights for the cross traffic. Here you would use the same 555 as a master timer for both sets of lights (otherwise chaos would ensue) and a separate 4017 to drive the three extra LEDs. Of course, you would have to take care and ensure that green and orange outputs on each set of lights correspond with red on the other!

Jack Holliday,
Nathan, Qld. ($35)

LED torch uses blocking oscillator
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This simple LED torch is driven by a 2-transistor blocking oscillator which steps up the voltage from a 1.5V cell. It relies on the inherent current limiting of the 150μH choke to protect the white LED from over-drive. With a 9V zener diode in place of the white LED, it could also provide a 9V supply provided the current drain is modest.

Peter Goodwin,
Southland, NZ. ($30)

AFX slot car lap counter

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AFX slot car sets are very enjoyable but you can increase the fun with a lap counter. This circuit will count from 00 to 99, with independent counters for each track.

The sensing device used is a Hall effect sensor (UGN3503; available from Dick Smith Electronics). One of these sensors is glued under a section of each track (printed side up); between the slot and one of the track rails is the best spot. In this position, it will allow the ground effects magnets on the cars to pass over them.

The sensor will provide a voltage of about 3V when a car passes over it and about 2V without a magnetic field.

Both counter circuits are identical, with dual op amp IC5 handling the signals from both sensors. IC5a and IC5b are wired as comparators, with a 2.5V reference derived from zener diode ZD1 via the 10kΩ and 12kΩ resistors.

Each time the output of IC5a goes high it clocks IC1a, a 4518 BCD counter. NAND gates IC2a & IC2b provide a carry out to the other half of IC1 for a 2-digit display. More counters may be cascaded this way to provide extra digits. The BCD outputs of IC1 drive 7-segment decoders IC3 & IC4 which drive common cathode LED displays.

Pushbutton S1 resets the counters to 00 for both tracks for the start of a new race.

Placid Talia,
Oakleigh, Vic.

Simple BFO metal locator

This circuit uses a single coil and nine components to make a particularly sensitive low-cost metal locator. It works on the principle of a beat frequency oscillator (BFO).

The circuit incorporates two oscillators, both operating at about 40kHz. The first, IC1a, is a standard CMOS oscillator with its frequency adjustable via VR1.

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The frequency of the second, IC1b, is highly dependent on the inductance of coil L1, so that its frequency shifts in the presence of metal. L1 is 70 turns of 0.315mm enamelled copper wire wound on a 120mm diameter former. The Faraday shield is made of aluminium foil, which is wound around all but about 10mm of the coil and connected to pin 4 of IC1b.

The two oscillator signals are mixed through IC1c, to create a beat note. IC1d and IC1c drive the piezo sounder in push-pull fashion, thereby boosting the output.

Unlike many other metal locators of its kind, this locator is particularly easy to tune. Around the midpoint setting of VR1, there will be a loud beat frequency with a null point in the middle. The locator needs to be tuned to a low frequency beat note to one or the other side of this null point.

Depending on which side is chosen, it will be sensitive to either ferrous or non-ferrous metals. Besides detecting objects under the ground, the circuit could serve well as a pipe locator.

Thomas Scarborough,
Cape Town, South Africa. ($35)

Capacitor leakage adaptor for DMMs

Used with a DMM on the 20V range, this circuit gives a rapid and direct measure of the leakage current of capacitors. There are two ranges, with maximum readings of about 20μA and 2mA, and the test voltage can be varied. This lets you test leakage at or near the capacitor's rated voltage.

In addition, the circuit can help determine the amount of internal electro-chemical activity, which reduces the capacitor's lifespan. For example, one 0.33F 5.5V super capacitor I tested has an open-circuit voltage that rises exponentially to about 0.8V over a period of 10 days.

Note: super capacitors are technically called electro-chemical capacitors but they store energy electrostatically like other capacitors.

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To quantify the internal electro-chemical activity of a capacitor using this circuit, simply measure the capacitor's "leakage" with the test voltage set to zero. If the reading is negative, the capacitor is self-charging with its plus terminal becoming positive with respect to its minus terminal. If the reading is greater than zero, the capacitor is self-charging with its minus terminal becoming positive with respect to its plus terminal.

In the circuit, the 10kΩ potentiometer (VR1) adjusts the test voltage. Zener diode ZD1 limits the maximum test voltage to ensure that the output of IC1a can swing to at least 2V above the test voltage.

IC1b and associated components derive the ground rail from the single-ended supply. The negative supply voltage is fixed at -3.3V by ZD2 to give more range to the test voltage, which is derived from the positive supply.

The circuit will operate from any voltage in the range 9-36V but keep in mind that the maximum test voltage is 8.4V less than the supply voltage.

With S1 in position 1, IC1a is configured as a unity gain buffer and the DMM reads its output voltage. Without a test capacitor (CUT) connected, the DMM will display the test voltage. When a CUT is connected, it will be rapidly charged to the test voltage via S1a.

The 100kΩ resistor in series with the inverting input to IC1a protects the op amp in case a capacitor charged to a high voltage is connected to the test terminals, particularly when power to the circuit is off. However, it offers no protection against a charged capacitor being connected to the test terminals in reverse.

Position 2 of S1a configures the circuit to display the leakage of the capacitor. The feedback resistor around IC1a is set to 100kΩ or 1kΩ by switch S2, while S1b connects the DMM to show the difference between the test voltage and the output of IC1a.

In this position, IC1a maintains the test voltage across the CUT. Since no current flows into the op amp input, any leakage current flowing through the CUT must also flow through the selected feedback resistor (R). IC1a will therefore raise its output voltage above the test voltage by I x R volts, and this difference will be shown on the DMM.

To use the circuit, first set S2 to the desired range, then place S1 into position 1 and adjust the 10kΩ pot until the desired test voltage is shown on the DMM. That done, connect the capacitor to be tested and wait for the DMM reading to stabilise at the test voltage. Now switch S1 to position 2, whereupon the DMM will show the leakage of the capacitor.

Andrew Partridge,
Kuranda, Qld. ($45)

Simple AM radio receiver

This circuit is essentially an amplified crystal set. The inductor could be a standard AM radio ferrite rod antenna while the tuning capacitor is a variable plastic dielectric gang, intended for small AM radios.

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The aerial tuned circuit feeds diode D1 which functions as the detector. A germanium type is far preferable to a silicon signal diode because its lower forward voltage enables it to work with smaller signals.

The detected signal from the diode is filtered to remove RF and the recovered audio is fed to a 2-transistor stage which drives a set of 32Ω phones from a Walkman-style player.

Peter Goodwin,
Southland, NZ. ($30)

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