In many orders cars, the windscreen wash pump is not coupled to the windscreen wipe function. This circuit switches on the wiper motor for a predetermined time each time the pump is actuated. The wipers start to work when the pump switch is pressed, and keep on working a while after the switch is released.
The anode of diode D1 is normally taken to ground via the wash pump motor. When the pump is powered, C1 charges rapidly D and R1. Consequently, T1, T2, and T3 are switched on, and relay Re1 is energized. C charged as long as the pump motor and the wipers operate. When the pump switch is released. the washing stops. but the wiping continues for a time determined by R2- C2. Diode D1 prevents C2 being discharged via the pump motor. Diodes D2 and D3 protect the circuit against back e.m.f from the relay coil.
Installing the circuit in a car should be easy as there are only three connecting points (apart from the supply voltage). Note, though, that the circuit is designed to work with a pump motor that has a fixed contact to ground, while its positive contact is taken to the wash switch.
The relay on the board is capable of switching 10-20 A. Its contacts are connected across the wiper switch via terminals ‘A’ and ’13’ by means of heavy-duty wiring. The connections to the board are made via spade terminals and mating sockets as used in car electrical systems. The spade terminals are screwed direct to the board and soldered to minimize the contact resistance. Finally, the relay may be an Omron Type G2L-113P-4S-SV or a Bosch Type 0 332 016 101. The Omron type fits on the PCB, the Bosch type does not.
Parts list
Resistors:
Ri = 220 Ω
R2 = 120 kΩ
R3 = 100 kΩ
R4 = 3.3 kΩ
R5 = 10 kΩ
Capacitors:
C1 = 100 nF
C2 = 19 μF, 63 V
C3 = 10 nF
C4 = 100 μF, 25 V
Semiconductors:
D1, D2, D3 = 1N4148
T1 = BC550C
T2 = BC560C
T3 = BD140
Miscellaneous:
Re1= 12 V relay for PCB mounting.
Z>90 Ω,contact rating >10 A
