Here is a brake failure indicator circuit that constantly monitors the brake status and gives an audio-visual indication. When the brake is applied, the green LED flashes and the piezobiZzer sounds for approximately one second if the brake system is intact. If the brake fails, the red LED lights up and the buzzer stops beeping.
The circuit will only operate on vehicles with negative ground. It also gives an indication of the failure of the brake switch. In vehicle hydraulic brake systems, a brake switch is mounted on the brake cylinder to drive the rear brake lights. The brake switch operates with fluid and does not operate if the fluid pressure drops due to leaks. Fluid leakage can not be detected easily unless there is a severe pressure drop on the brake pedal. This circuit detects the possibility of a brake failure by controlling the brake switch and reminds you of the brake status each time the brake is applied.
The circuit uses an IC-IC IC3140 (IC2) as voltage comparator and timer NE555 (IC3) in monostable alarm configuration. Voltage comparator IC2 detects the voltage level through the brake switch. Its non-inverting input (pin 3) receives half of the supply voltage through the 10-ohm-ohm potential dividing resistors R3 and R4. The inverter input (pin 2) of IC2 is connected to the brake switch via diode D1, IC 7812 (IC1) and resistor R2. It receives a higher tension when the brake is applied.
Normally, when the brake is not applied, the output of IC2 remains high and the red LED (LED1) shines. The output of IC2 is fed to the activation pin 2 of the monostable coupling capacitor C2. The resistance R1 is used for the IC2 input stability. IC1 and C1 provide a regulated uncurled supply to the inverting input of IC2.
IC3 is wired as a monostable to give pulse output of one second. The timing elements R7 and C4 cause the output to be high for one second to activate the buzzer and LED2. Usually, the IC3 pin is high due to R6 and the buzzer and LED2 remain off.
When the brake pedal is depressed, pin 2 of IC2 gets a higher voltage from the brake switch and its output goes down to turn off the red LED. The low output of IC2 gives a short negative pulse to the monostable via C2 to activate it. This activates the buzzer and LED2 to indicate that the brake system is working. When there is a pressure drop in the brake system due to a leak, the LED1 stays on and the buzzer does not sound when the brake is applied.
The circuit can be mounted on any general purpose PCB or perforated board. Connect point A to the terminal of the brake switch that goes to the brake lamps. The circuit can be powered from the vehicle battery.
The circuit requires a well-regulated power supply to prevent unwanted activation while the battery is being charged from the dynamo. IC4, C6 and C7 provide 12V regulated to the circuit. The power supply must be taken from the ignition switch and the circuit earth must be attached to the vehicle body. A two-color LED can be used instead of LED1 and LED2 if desired.
![[Image: 1A7_june1-_-vefy.png]](http://electronicsforu.com/wp-contents/uploads/2016/05/1A7_june1-_-vefy.png)
