brake lights monitor

elektor July august 1985 it+044.1 11 brake lights monitor R Kambach The circuit described below monitors your car"s brake lights, and indicates by a light-emitting 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 increased road safety. 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 A diode in series with each lamp. Transistor T1and 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 D1lighting via T3. If one of the brake lights is faulty, the switch-on current drawn by the other lamp will cause D1to light briefly when the brake pedal is pressed. If both brake lights are defecti...