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Circuit Notebook

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.

Trailer lights
test circuit

Click for larger image

Trailer or caravan lights can pre­sent a major challenge if one or more of the lamps are not working. This simple tester is built with sufficiently long input and output cables so that it can be positioned on the vehicle’s boot and can be seen by the driver (or second person) while the controls are operated. It has two modes: “Thru” and “Vehicle Test”.

The Vehicle Test mode checks the vehicle wiring to the external socket. The steps are as follow:

(1) Ensure that rotary switch S1 is in the Off position and the external earth clip is protected from touching any metal parts.

(2) Turn the switch to position 1 (LH turn), operate the vehicle’s LH blinker switch. The 12V lamp in the unit should blink in unison with the vehicle’s lamp.

(3) Proceed to test the other functions. Typically a box trailer will have LH & RH turning, stop and rear lamps to test.

(4) If the Tester’s lamp does not illuminate, this could indicate that the earth return is broken. If so, the Earth socket on the tester will allow a connection to the vehicle bodywork via a clip lead. If this causes the lamp to light, it confirms an earth fault.

(5) Typically most faults are due to globe failures, poor connections due to corrosion or broken wires.

(6) If the LEDs illuminate in the incorrect sequence, the trailer or vehicle wiring needs to be checked against the industry standards.

In the Thru Test mode, the Tester is connected between the Vehicle and Trailer, both plugs to their respective sockets. This is a continuity test from the vehicle to the trailer. Ensure that the rotary switch is in the Off position and that the external earth clip is removed, preventing it from touching any metal parts.

The correct LED will illuminate as each function is activated. If a LED fails to light during the test, check the relevant light by swapping with a tested lamp. Check also for corrosion on the lamp socket and connector pins or for broken wires.

If multiple LEDs light, this could indicate that the earth return is open circuit. In that case, attach the external earth wire clip to a metal part of the vehicle and run the test again.

Bob Winchester,
Hornsby, NSW. ($40)

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