> Main > Tech Index > DIY Modifications > Tail Light blinker
Tail Light Blinker Circuits
Install a momentary blinker for trailer running lights
Visit Geno's Garage
for Truck accessories.
Geno's Garage Truck Accessories

From a TDR thread

From Evan A. Beck - All that is needed is a momentary contact switch, and a relay. Basically all you need to do is interrupt power to the running lights. Simply use a normally-closed momentary contact switch, and regular high-amp relay. The headlight switch would no longer carry the load for the running lights. The relay would be carrying the load instead. The reason for using a relay, is because most momentary-contact switches are rated for only an amp or two. This way, you can use a small momentary-contact switch to work a relay, which controls a high amp load.

This circuit will momentarily blink the lights off if the parking lights or headlights are on.

From C. D. Day - If you go with two double throw relays and a normally open momentary contact switch you can turn the markers on when the lights are off and when the tail/markers are on at night the same switch will momentarily turn them off. You won't have to mess with the main light switch

The circuit below will flash the tail lights regardless of the position of the headlight switch.

How it works   

A 12V tap from under the dashboard (pick a circuit) feeds the push button switch. The Black/Yellow lead from the headlight switch is cut and routed through the relay pair. Note: This will blink ALL clearance and tail lights on the trailer.

My revised drawing based on the circuit from C. D. Day's original drawing below.

Larger version of C. D. Day's drawing (1500x1200, 125Kb)

Northwest Power Products has the relays, 30 amp p/n 29-939 and 40 amp p/n 29-940 on web at

The Juliana Company can supply Bosch relays. This guy only sells Bosch relays and sockets. So he carries a COMPLETE line and always has stock. He has good prices too (a few cents plus/minus as Northwest Power) and they get better if you buy more than 9 at a time. 800/334-0476

You can get a complete Bosch relay catalog online (PDF format) after submitting an electronic form at

Junkyards have oodles of Bosch relays under the hoods of many vehicles. Usually you can get a handful of them for less than $20.


Last Update: August 23, 2000