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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number.

PICAXE-based traffic light sequencer

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This circuit uses a PICAXE-18M2 microcontroller to sequence model traffic lights at a 4-way intersection. The east-west direction also includes a pedestrian crossing signal which is triggered with a pushbutton and then occurs at the appropriate point in the sequence.

The usual red-green-amber-red sequence is implemented with appropriate delays. With the circuit as shown, the sequence does not start until pushbutton S1 is pressed but it can be omitted and replaced with a link so the sequence automatically starts at power up. Pushbutton S2 is used to trigger the pedestrian crossing signal, which turns green simultaneously with the north-south traffic lights.
Pushbutton S3 causes the amber lights in all four directions to flash, which normally occurs when the traffic light control system fails. All three pushbuttons have associated pull-down resistors so the PICAXE can detect when they are pressed.

The lights are appropriately coloured LEDs. Since only one LED within a set is illuminated at a time, they are wired with a common cathode and a single 390Ω current-limiting resistor. This sets the LED current to about (5V - 2V) ÷ 390Ω = 7.7mA. The LEDs and resistors for the south and west directions are wired with their anodes in parallel with those for the north and east directions respectively but with separate current-limiting resistors.

An in-circuit programming header is provided and the software can be downloaded from the SILICON CHIP website (TrafficLights18M2.bas). The software can be modified to change the timing if desired.

Phillip Webb,
Hope Valley, SA. ($60)

Electric fence tester uses Xenon flash tube

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This circuit is an adaptation of the Maxi Electric Fence Tester published in SILICON CHIP, May 1999. Like that circuit, this one uses a Xenon flash tube to give a visible
indication of a working electric fence. However, this particular circuit can handle higher-power Xenon tubes.

Power for the circuit is derived directly from the fence. And while each flash involves a hefty burst of energy, this energy is built up progressively through successive pulses from the fence so that the average power consumed by the circuit is relatively small. As a result, the circuit can be permanently connected to the fence without significantly diminishing the effectiveness of the latter.

A feature of this circuit is that it operates independently of the polarity of the fence pulses, by virtue of its bridge rectifier. The three capacitors (470nF, 22nF & 10nF) are progressively charged via the diode bridge and a resistor ladder consisting of 20 680Ω 0.5W resistors (to provide an adequate voltage rating).

When the voltage across the 10nF capacitor is close to or exceeds the breakdown voltage of zener diode ZD1, programmable unijunction transistor PUT1 conducts and triggers SCR1, to dump the charge from the 22nF capacitor into the primary of trigger transformer T1. TI steps up the pulse voltage to fire the Xenon tube which then discharges the 470nF capacitor to produce a bright flash.

While the circuit should work with most types of Xenon tubes, such as found in typical camera flash units, it is preferable to use a tube specifically designed for strobe applications, to ensure a reasonably long operating life.

Herman Nacinovich,
Gulgong, NSW. ($45)

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