On some cars, it is convenient to couple the rear window wiper to the windscreen wiper. However, since the rear window does not get nearly as wet as the windscreen, particularly when the car is moving, the rear wiper should operate only once for every umpteen wipe of the windscreen. Note that your car may be one of the fortunate ones of which the rear window, when the car is moving in the wet, does not get wet at all because of the car’s design.
The coupling shown in Fig. 2 ensures that the rear wiper operates once for every four or 16 wipes of the windscreen, depending on the setting of switch S1 (as shown, once every four).
The clock for the circuit is taken from the return (terminal 53e – green/black wire on most cars) of the windscreen wiper motor see Fig. 2. This signal, which is a square-wave, is applied to ICI via K1. Its level is lowered to not more than 5 V by potential divider R1-R2 to prevent any damage to ICI. -Any noise from the car’s electrical system is bypassed by C1.
The g2 output (pin 1) of IC1 goes high every fourth clock input, and the g4 output (pin 3) once every eighth clock input.
The trailing edge of the signal at S1 is transformed into a trigger pulse by R5-C3.
This pulse, whose length is determined by RT-P1-C6, is applied to monostable IC2. The length of the pulse should be set to about one second to give the rear wiper time to get going.
The monostable drives transistor T1, which in turn controls relay Rei This relay is a motorcar type that can handle the large switch-on current of a wiper motor: it may be rated at 25 A or 35 A.
The supply for the circuit is provided by IC3, which brings down the car voltage of 12 V to 5 V. This IC also prevents large peaks on the battery voltage from reaching the circuit.
