This circuit is supplied as a hobby project. In this case, check with the Highway Patrol or Department Of Safety to make sure that this is legal in your state. The 555 timer IC is connected for astable multivibrator operation. This is a flasher circuit modified to turn on and off a bulb instead of a LED. It will create a flashing condition on the brake lights while the brake is pressed. (Normally when depressed, the brake lights stay on but do NOT flash.) The flashing rate, for the attention of drivers behind you, can be varied from very fast to a maximum of once in 1.5 seconds by varying the preset variable resistor VR1. (You might also wish to see our Astable 555 Square Wave Calculator.) The ON time of the circuit is given by the formula T(ime) ON = 0.69 x C1 x (R1 + VR1) seconds and the OFF time is T(ime) OFF = 0.69 x C1 x VR1 seconds.
You can increase the value of C1 in varied values to 100uF to get a slower flashing rate of up to once in 10 seconds.
