Most, if not all, recent
cars have an impressive amount of electronics, whether it be ABS
brake systems, engine control with injection calculators, airbag
activation, or other various functions, called comfort functions. Among
them is one which we tend to forget because it has become so common
today. It turns on the windshield wipers automatically for a few seconds
after the windshield cleaner. This practice is almost indispensable
because it avoids any dripping of excess rinse product right in the
middle of a just-cleaned windshield.
Unfortunately, many ‘low end’ cars or some of the older cars are not
equipped with this automatic function which is a very nice convenience
to have. So, since all that is required is a handful of components that
any electronics hobbyist worthy of the name already has in his/her
drawer, we will discuss the circuit proposed here. This project is super
simple and simply keeps the windshield wiper activated for a few
seconds after the windshield washer control contact has been released.
While the windshield washer pump is operating, the 12 volts
delivered by the battery are present at the terminals and are therefore
charging capacitor C1. Once the windshield washer has stopped, this
capacitor can only discharge through R2, P1, R3, and the T1 emitter-base
junction, due to the presence of diode D1. It thus keeps T1 in the
conductive state during a certain time, the exact period of which
depends on the setting of P1. T1 in turn saturates T2, which then does
the same for T3.
The Re1 relay is therefore connected which maintains the windshield
wiper in operation because its work contact is wired in parallel to the
control switch. Once C1 is sufficiently discharged, T1 is blocked, which
then blocks T2 and T3 and deactivates relay Re1. The type of components
is not really critical, even if we indicate specific reference numbers
for T3, any low-power npn transistor with a gain over 25 will work.
However, considering the amount of power consumed by the windshield
wiper motor, relay Re1 will imperatively be an ‘automobile’ relay.
You can find very low-priced ones at many car accessory shops (and
even at some component retailers). These relays maintain contact under
12 volts and often do not have more than one work contact but they are,
in general, capable of cutting off about 20 amps. Finally, the only
delicate point of this project is to properly identify the control wire
for the windshield pump on one hand, and the windshield wiper motor on
the other. Observing what is happening at the various connections with a
simple voltmeter, should get it right without too much difficulty.
Author: Christian Tavernier
Copyright: Elektor Electronics