I'm looking at adding an additional light to my trailer that I want to activate when the hazard light circuit is active. Currently there is a standard 7-pin plug from the car to the trailer (RV/Caravan) so when the car's hazard lights are on the trailer's left and right indicators flash. How would I go about adding what is effectively a 3rd hazard light to the trailer? It should only be active when the hazards are activated from the car.

I'm assuming some sort of "AND" logic is required. If both indicators are flashing then supply power to the 3rd hazard light. If only one indicator is on (i.e. normal turn signal operation) then the 3rd hazard light should remain off.

The 3rd hazard light in question is LED (part of a multi-function rear light bar. separate wires for tail, stop, reverse and indicator) it is the indicator I want to trigger as the 3rd hazard light (and not use it as a turn signal since there is only one of these light bars fitted). The indicator circuit on the light bar draws 3.9W (approx 0.325 Amps).

thanks very much.

 

A relay or transistor (BJT or MOSFET) will do the job of ANDing the lamps:

 

boostbuck's circuit requires the LED not share a common ground with the other indicators.

If that's not the case, the circuit below should work:

 

oops, good call, I forgot automotive circuits use chassis as return.

 

Thanks you both very much. I'd say it will be a common ground with the other indicators. All earthed via the trailer plug to the car. I'm a beginner so apologies if I ask a stupid question. Any recommendation on what diode to buy for D1? ... also ... do I need any additional components to drop the 12v on the RTurn and Lturn wires to a lower voltage or will a TIP32 and 2N3904 be ok as is?
Also, do I need to confirm what amps are drawn by the Rturn and Lturn lights? ... is that relevant?

 

D1 is your LED lamp

 

No voltage dropping required, all will be good at 12V, and amps drawn by existing indicators is irrelevant.

 

Awesome. Thanks very much for your detailed and quick responses guys. Much Appreciated

 

Sorry guys. Another clarification please. What wattage should those resistors be? Cheers

 

R1 = 2 watt
R2 = 1/4 watt

 

Thanks very much @sghioto

 

R1 = 2 watt
R2 = 1/4 watt

Since this is for a hazard light where the duty-cycle is about 50%, R1=1W should be sufficient.

 

Thank you @crutschow much appreciated