The circuit described below monitors your car's brake lights, and indicates by a light emiting diode whether they both function correctly. In that sense, it can save you money by preventing your being fined for driving with defective brake lights, and it also leads to increasing road safety.
Circuit diagram :
The monitor depends inevitably on the voltage drop across the supply lines to the two lamps. For the circuit to work correctly, that drop needs to be greater than 0.6 V. If this is not so, the drop must be increased by adding a 5 V diode in series with each lamp. Transistor Ti and T2 in figure 1 form a Schmitt trigger, which reacts to the voltage drop across the supply lines to the two brake lights. This reaction manifests itself in Di lighting via T3. If one of the brake lights is faulty, the switch-on cur- rent drawn by the other lamp will cause Di to light briefly when the brake pedal is pressed. If both brake lights are defective, Di will not light at all. All three possible states of the brake lights are thus indicated.
The hysteresis of the trigger, and, therefore, the sensitivity of the circuit, can be adjusted within narrow limits with Pi. The preset is best adjusted with one lamp out of action in a manner which makes Di light briefly as described above.
If you find it disturbing that Di lights every time you brake, the operation can be reversed by replacing the BC557B in the T3 position by a BC547B (n-p-n). The collector of T3 is then connected to the positive supply line, and the emitter to R6. On the printed circuit board this means that the flat edge of T3 must be turned the other way. A second base connection has also been provided on the PCB. Note, however, that this configuration no longer makes it possible to ascertain whether one or both brake lights are faulty, i.e., when the LED lights, one or both lamps need replacing.
The printed circuit board is not available ready made. In figure 1, Si is the brake pedal switch, and Lai and La2 are the brake lights.