How to trigger on releasing a push button?

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thecritic

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I am trying to design an automatic Door Bell curcuit.
I will be hiding a large push button under a door-mat. Whenever someone steps over the door mat and then moves out, I want the door bell to ring for some time and then stop. Please keep in mind that the door bell should start ringing only after the person leaves (after the push button is released).

I know that there are ICs that meet my demands. But I would like to do it as simple and as cheap as possible. I can afford a couples of relays (I already have) and large or small capacitors and some resistors. I am in no way able to buy a couple of ICs.

So, this is in some way a problem of economy too. Otherwise it would be very simple to do what I want by using, say, a microprocessor.
 


Can you afford a 6 pin micro controller (base line family) from microchip? this will cost you only 25-INR.(less than a 1$), so that you can give the intelligence what ever you wish...and make the circuit simple by avoiding bulky analogue... n passive RC time components....
 
I am sorry but I don't know how do I use them. Furthermore I don't know if those chips are available in my nearby local store or not.
 
I have just got an idea, but need some improvement. I need to use a double push button. Can you see some way round for using a single one.
 

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Here is a simple circuit using a slow release relay and a DC bell.
 

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Pressing SW1 activates the slow release relay K which connects power to the bell through its contact K1 and its coil. The bell does not ring because it is shorted by SW1. When the button is released, the bell rings until the relay drops. Note that the current through SW1 must be equal or greater than the relay pull in current, and the current through the K/Bell circuit must be less than the relay release current.
 
thecritic said:
I have just got an idea, but need some improvement. I need to use a double push button. Can you see some way round for using a single one.
Click to expand...

Use a single pushbutton SPST, the bottom contact in your drawing defeats the whole purpose of your charging network. Also, buffer the capacitor current with a transistor to energize the relay coil.

Also, the coil needs to be connected across the cap to work. On second thought, I don't like the idea at all. Instead, find a switch with NO/NC contacts and do this:
 

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