Build a Brake Light Flasher Circuit

Somebody asked me about a brake light flasher circuit so I designed one that is easy to build.

I decided against using 555 timers because they are only rated for 15V and bike electrical systems often have transients above that. I have had 555 circuits on my bike die for no reason, so I am not so confident in them.

The brake light flasher circuit uses an LM358 op amp (rated for about 36V). The first stage designated U1-A is a free running oscillator that will drive the FET switch Q1 and make the brake light(s) flash. The second stage U1-B is a one shot that "waits" a certain period of time then goes high and locks the brake lights on solid. Between the two stages, the lights flash for a while then go steady ON. They are "OR" connected into R10 which activates Q2 and Q1, so whichever stage tells the lights to go ON over rides the other.

R7 and ZD1 are voltage protection for the circuitry.

S1 allows the whole thing to be bypassed and restore stock brake light operation. Not mandatory, good idea.

The power P-FET switch shown (Q1) is a 70 m Ohm device which would be good for about 3A total load current and not have any heat problems. Other P-FETs can be used if they have low enough on resistance so they won't overheat.

As shown on the schematic, the flash rate can be adjusted by varying R4. The length of time the unit flashes can be adjusted by varying R6. The values listed on the schematic produce the pattern shown on the schematic.

The legality depends on your state: I looked up California and it allows a MAXIMUM of four flashes (in a four second interval) when the brake light is applied before the light must go on steady. It can NOT flash continuously.

Check local laws before you put a flasher onto your brake lights.