Wednesday, March 13, 2013

Rain Triggered Instant Start Windshield Wiper Timer Circuit for Automobiles

The following circuit was requested by one of the readers of this blog Mr. Keval. The actual request was for a rain triggered windshield wiper circuit, but the idea here has been further enhanced with a quick start feature for better efficiency.


Normally, electronic wiper control circuit involve a bistable for switching the wiper mechanism into an oscillatory action.

The bistable switches ON the wiper motor and keeps it moving until the set time lapses, this process continues for so long the power is switched ON to the circuit.

A 555 IC is fundamentally utilized for implementing this function, which is generally configured in it standard bistable mode. However one disadvantage with a standard 555 bistable circuit is that it introduces a delay of of around 1.6 times the set RC time interval value.

Therefore if suppose the bistable delay is set to 10 seconds would mean a 555 bistable would require a 10*1.6 = 16 seconds to initiate the action, that can be very annoying.

Adding a Quick Start Action

The present design eliminates the above issue by wiring the 555 bistable in a smart way.

Referring to the circuit diagram, when the wiper switch S1 is depressed, pin#6 of the IC is immediately elevated to the supply voltage level of 12V via C1.
This resets the bistable, making its output go low, which energizes the connected relay and the wiper motor gets instantly activated.

The above process of instantly starting the motor using a 555circuit makes the present circuit different from the conventional circuits which employs the same IC but without the above modification.

Now once C1 is charged, which happens via R2 after the output gets activated, pin#2 of the IC falls below the 1/3 Vcc mark. This situation pulls the output to high, switching OFF the relay and the system.

After this C1 starts discharging through R1 and P1, once C1 completely discharged the cycle is yet again repeated as long as S1 is kept depressed.

The value of R1 and the setting of  P1 decides the OFF time of the circuit.

If the value of P1 and R1 is selected too low, then C1 may not get charged solely through them but also via R2, which will make the output and the relay system to stay switched ON forever, until S1 is switched OFF.

Adding a Rain Trigger

An useful rain trigger feature can be added to the circuit for enabling an automatic start of the wiper motor when rain fall is detected.

The two transistors T1 and T2 are configured as a high gain amplifier set up. The points A and B get bridged  with rain water droplets introducing a low resistance across the points.

This switches ON the transistors and the relay, which in turn switches ON the connected wiper motor.

The wiper motor remains switched ON as long as rain persists and points A/B stays bridged with water droplets.



Parts List

R1 = 47K,
R2 = 22K,
R3 = 1K,
P1 = 1M
C1 = 33uF/25V
C2 = 0.01uF
C3 = 0.1uF
D1,D2 = 1N4148
T1,T2 = BC547
IC1 = 555
RELAY = 12, SPDT

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