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.
How Conventional Wiper Systems Work
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.
Enhancing an Instant 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.
Circuit Diagram
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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NvD says
sir one doubt..i think this circuit works only when water is in contact between a&b..when water leaves it stops
Swagatam says
NVD, yes that's right, the water sensing option makes the circuit automatic, while the switch provides the manual option, so the circuit is facilitated with both the options here.
Jite Bently says
Sir I am jitendra. I'm making a large scale helicopter so can u please help me in making its rc circuit board.
Swagatam says
Hi Jitendra, I will try to publish it soon….
Anonymous says
Hi Swagatam.
There is a room thermostat that I turn on, which gives power to
the circuit.
1) Timer for the pellets- 15 seconds then off
2) Timer for the glow plug – 2-3 minutes then off
3) Timer for the fan – pulsing 2-3 seconds on then 30 seconds off
(this needs to cycle until the photo cell is activated,).
4) Photo cell – when it sees fire, it is to stop the entire cycle.
Then the next cycle begins:
5) Timer for the pellets – 8 seconds on, 30 seconds off until the water
temp reaches 75 degrees C (the boiler will cut the power at this temp).
So every time power in on it should do the steps above, I hope it's clear!
Let me know if you need more info..
Thanks for your Help again!
Vasilis K
Swagatam says
Hi Vasilis,
Thanks for the explanation!
I'll inform you as soon as it gets published.
Swagatam says
Hi Vasilis,
you may see it here:
https://homemade-circuits.com/2013/04/programmable-timer-controller-circuit.html
Anonymous says
Hi there Swagatam!
I have used your circuit on my homemade pellet burner that uses a 12V Ac motor for the feeder screw running 8 sec on, 30 sec off , and it works great, my idea for my project was similar to your circuit,I also have a monostable circuit istalled using the famous 555 ic for the igniter which is a 12V glow plug that stays on for 2-3 min which is enough to ignite the pellets, here is the fun part now, I run in to some problems during ignition, my idea for this project was for me to use a plc controller for all the action, money is short and i find building the circuit more interesting.My question is if i can produce a sequence of actions for the ignition process, when power on the circuit 8-10sec on time 30 0ff time for the feeder, at this point glow plug to stay on around 2-3 min, and the fan blower which is a 220Vac to pulsate for the shortest 2-3 sec on 10- 15 off to it's lowest speed if possible, when fire is detected using a LDR to activate the second circuit ( 8 sec On 30 sec off)and the fan blower to its regular speed via a relay..
I hope i explained everything correct!
Greetings from greece
Vasilis K
Swagatam says
Hi Vasilis,
Thanks!
Can you provide your requirement in a step-wise manner because I am finding it difficult to understand the sequence in the above explanation.
If you can provide the circuit operating sequence serially with the relevant timing functions, then I can try working on it.
adam0carter says
This can be very useful in rainy weather, how much this circuit system costs? i would love to get this for my car, gonna check my local area auto shop, i hope it won't give any problems.
Swagatam says
Thanks! In India this would cost around a dollar…
keval thacker says
thank you so much sir.
Swagatam says
You are welcome….