Synchronization of Rear and Windscreen Wipers
In certain vehicles, synchronizing the operation of the rear window wiper with the windscreen wiper proves convenient. Nevertheless, it’s crucial to consider that the rear window is generally less prone to getting wet compared to the windscreen, especially while the vehicle is in motion. As a result, it’s desirable for the rear wiper to activate only intermittently, typically after a series of windscreen wipes. It’s worth noting that some vehicles are designed in such a way that their rear windows remain dry while in motion, rendering rear wiper operation unnecessary.
Synchronized Rear Wiper Operation
In the circuit illustrated in Fig. 2, the coupling mechanism guarantees that the rear wiper activates once for every four or 16 windscreen wipes, contingent on the position of switch S1 (currently set for once every four wipes).
Clock Signal and Noise Isolation
The circuit’s clock signal is derived from the return terminal (53e, green/black wire in most cars) of the windscreen wiper motor, as depicted in Fig. 2. This square-wave signal, transmitted to IC1 through K1, is kept below 5 V by the voltage divider R1-R2 to safeguard IC1 from any potential damage. C1 effectively bypasses any electrical noise from the car’s system. The g2 output (pin 1) of IC1 goes high every fourth clock input, and the g4 output (pin 3) activates once every eighth clock input.
Trigger Pulse Generation and Monostable Operation
The trailing edge of the signal at S1 is converted into a trigger pulse through R5-C3. This pulse’s duration, set by RT-P1-C6, is applied to monostable IC2. Adjusting the pulse length to approximately one second gives the rear wiper sufficient time to start moving.
Relay Control and Voltage Regulation
The monostable IC2 drives transistor T1, which regulates relay Re1. This relay, designed for motorcars, can manage the substantial switch-on current of a wiper motor. Often rated at 25 A or 35 A. IC3 supplies power to the circuit, converting the car’s 12 V voltage to 5 V. Additionally, this IC safeguards the circuit from large voltage spikes originating from the vehicle’s battery.