Indicator Circuits

Indicator and Hazard circuits rank pretty high in the 'problem' stakes when building a car. The problem is - there is no single, universal way to do it. There are hundreds of switches, lamps and relays available and hundreds of ways to wire them. If you're in any doubt over your wiring it always make sense to mock-up a circuit on the bench. Here, we've made a simple indicator cuircuit on a small piece of plywood using four #RL15 indicator lamps, a pair of #SR5 side repeaters, a flasher relay #FRLY7 and an ON/OFF/ON toggle switch #TSN4. A few short pieces of 5 Amp wire, some spade connectors and a #QCON3 connect it all together. A 12 volt battery is all that's needed to make it work and prove your design. When you have the indicator circuit working you can add the Hazard function. All the information you need is in this catalogue. Just take it steady and work through it one wire at a time - you may surprise yourself !!


The traditional, 2-terminal flasher relay is the mainstay of most indicator and hazard circuits. When you're problem-solving your circuit it's useful to understand how these little beauties work. Well, inside the aluminium can is basically just a switch whose contacts open and close at a specified time interval to turn the indicator lights on and off. But, getting the correct time interval is the clever bit. One of the switch contacts is mounted to the main metal frame inside the aluminium can which continues through the base to one of the spade terminals -'L'. The other switch contact is riveted to a bi-metallic strip which is isolated from the main frame and is connected to the other spade terminal - the +12 volt supply terminal - 'X'. (terminal desinations can vary. There is a list of different ones in the catalogue). Also connected to this terminal is a tiny heater coil which is fixed to and is in direct contact with the bi-metallic strip. When the indicator is turned on current flows through the heater coil, through the made contacts and lights the indicator lamps on the vehicle - at the same time, warming-up the bimetallic strip which bends when heated and opens the switch contacts, turning off the lights and the heater coil. The bi-metallic strip cools down, bends back and the switch contacts are made again - and so on and so on. The heater and bi-metallic strip are made to exacting specifications so that the correct bulb wattage - 2 x 21 watt and one x 5 watt - total 47 watts (two indicators and a side repeater) is the exact power needed to open and close the switch contacts at exactly the correct flashing interval. A hazard flasher relay does the same but at a total of 94 watts. An added benefit of this type of flasher relay is that, if you have a dodgy bulb you'll know because the flash rate will change or not flash at all.