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Momentary Relay question....

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Hi there everyone!

I am wishing to perform a simple hack on my new car which would allow the doors to lock (via the central locking) when the driver's seatbelt is engaged.

I have read the tech manuals on the car and have isolated the wiring which needs to be used.

I am at a loss however when it comes to the actual switching part.

Here is my problem.

Seatbelt engaged gives a 12v output on wire A.
Seatbelt disengaged gives 0v on wire A.

When seatbelt is engaged, this 12v must switch over wire B from 12v to 0v for approx 1 second, but allow wire B to return to 12v (even though wire A is still at 12v).

Can anyone help? I appreciate this may actually be far simpler than I may think, and I am very grateful in advance for all answers.

Thanks for the speedy reply Mike.

Sadly you will have to guide me somewhat further!

So I assume the 555 provides the timer component but how would I connect this to enable the scenario I postulated above?

Thanks again.


Super Moderator
1) Take a look at the web sites
2) Read about the 555 chip
3) Get one and try experimenting with different configurations
4) Put together a circuit
5) Then if you still have questions, come back and ask away.
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Well-Known Member
A simple circuit...but making some assumptions.
How much current does the +12v>0V>+12V lock-circuit have to switch?
That will determine the current requirements for the relay contacts...and by specification determine an appropriate relay and it's coil resistance and the time constant for the circuit. My assumption in this circuit is a low current, there for a 200Ω relay coil and a pull-in voltage of 8VDC. That gives an on-time of 1 second.

Or this relay could switch a bigger relay.



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