By Shartwig79
I've read a lot about some of your frustrations with the Emergency Lights/Beacons. I have developed a simple solution for an alternating lamp flashing device.

It's a very basic design utilizing an Automotive 12v Flasher, an Automotive 12v Relay, 2 lamps (1156 or 1157 styles) some wire, and a power source.

I did not include a fuse in the schematic, but it should be used. How you decide to switch it is entirely up to you.

What happens here:

The flasher is a bimetal strip. As current passes through it, the 2 different metals heat up and expand at different rates, causing the strip to curl up and break the electrical connection. The lead leaving the flasher is then interrupted and reconnected every second giving it the "Flashing" effect. That lead then runs into the electromagnetic solenoid section of the Relay. The way the field collapses on and off will cause the contacts inside the relay to connect and disconnect accordingly. On an Automotive Relay there are 3 poles to the contactor section. 1 is the input side, the other 2 are output. When one side of the output is hot, the other is not and vice versa. This gives us the opportunity to use the alternating capacity of the relay to make our lights flash opposite of one another. You may note that one of the lamps is attached directly to the flasher as opposed to the opposite pole of the contactor. This is because there must be a load on the flasher in order for it to function (Remember how that strip has to get hot?)

Now, depending on the style of lamp you use 1156 or 1157 are the best choices. If you must use LEDs, you must get a solid state electronic flasher. Reason being the LEDs do not have enough of a current draw to overheat that "Strip" inside the flasher.

The 1156 is a single filament bulb. The 1157 bulb is a double filament. The wiring difference is minimal between the two schematics. If you choose to use 1157's you can wire it so that while one light is flashing bright, the other one is illuminated with the dimmer filament and alternates between the two.

This is the simplest way I can come up with to get your lights to flash alternatively.

I hope these links work. When it comes to analog technology, I'm tops.... but this modern stuff doesn't always cooperate. ... quired.jpg ... ematic.jpg
By D1gger
ALL the relays I've ever purchased from auto parts stores I have gotten from the auxiliary lighting section (fog or off-road lights) have all been like that, with the main circuit being turned on and off by the relay being both outputs on or off, not alternating. I have seen the relays you mention, that alternate the output between the output posts, but those are usually found on the main relay/fuse block under the hood. They look similar from the outside and fit the same sockets, but you have to look closely at the schematic printed on the side of the relay to be sure which type you have.
By D1gger
Right now in my hand is a brand new, in package, heavy duty 40A relay from Napa, part number 192D that I had in the car as a spare for my roof lighting. The diagram printed on the relay clearly shows 85 and 86 as the control side, and 30/87a/87 as the main "controlled" side and 85/86 switch BOTH 87a/87 on or off from 30, not alternating between 87a and 87.
By Shartwig79
Well shut my mouth, I stand corrected.

They must've started making those for lighting specific applications to make it easier for novices to hook up lamps in a parallel arrangement.

The reason I arranged the circuit as I did, was to make the lights flash on and off opposite of each other. If I wanted them to flash together, I would've just used a heavy duty flasher.
By D1gger
I get what you're saying, but fog light relays, like the ones that came with my Bosch fog lights back in the 80's, were like this. Only certain under-hood relays do the alternating thing...

I just watched GB1 again and noticed that the PAR46 deck lights on the roof in front of the front lightbar don't alternate, they flash on and off together.

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