Practically in most windscreen wiper delay circuits the wiper speed is self-governing of the speed of the vehicle.
However the quicker the vehicle travels, the more rain falls on the windscreen, as a result, preferably, the shorter the delay ought to be.
A variable delay circuit could possibly be handled by a sensor fitted in the speedometer cable.
How the Circuit Works
On the other hand this process would be pretty difficult. The less complicated option followed in this article, is to obtain the control signals from the contact breaker, in order that the wiper speed is assorted relative to the engine speed.
The input of the circuit is attached to the contact breaker; when the contacts open, the full battery voltage shows up around the input, with the consequence that T1 offers a short output pulse.
The resultant pulse stream is employed to trigger the monostable multivibrator created by N1 and N2.
The frequency of the multivibrator is then split by ten by the counter, IC2. The output of the counter is fed to a second monostable, N3/N4, which supplies an output pulse duration of around 0.5 s.
Based upon the speed of the engine, the time between successive pulses will probably be between around 10 and 40 seconds.
Therefore transistor T2 is frequently switched on for a short while, causing the wiper relay to pull in and the wipers to execute a single sweep.
By organizing a capacitor of approximately 2.2 u F to be switched in parallel with C4, the wipers can be designed to execute a double sweep every cycle.
The zener diode D1 is included to protect the variable windscreen wiper delay circuit from extremely large surge voltages showing up over the contact breakers, although diode D2 guards T2 against the back EMF caused by the relay.
Ideally, the holding current of the relay must not surpass 100 mA; if that is the situation, nevertheless, a transistor with a higher output current capabilities needs to be made use of.
Simple IC 555 Wiper Delay
We faced some challenges when attempting to fit a thyristor type wiper delay unit to the car. We finalized the issue to be the design of the car’s wiper circuit. Furthermore, noise spikes also aggressively triggered the thyristor. The circuit shown in Figure 1 can overcome these issues in both negative and positive earth vehicles. IC1 is attached in an unstable mode, powering RLA. C3, D1 and D2 deter spikes from the relay coil and the wiper motor from activating IC1. VR2 is controlled to provide the minimum interval that is needed. On the other hand, VR1 is the prime delay control and delivers a range from 1 second to 20 seconds. SW1 is an override switch to retail RLA permanently on (for typical wiper operation).
The relay should have a resistance of at least 150 Ω and occupy heavy-duty contacts. Figure 1 also shows a set of change-over contacts which are required if the circuit is applied on a car whose wipers are wired distinctly. The suppression circuit depicted in Figure 2 was noted to be essential to shield IC1.
