Relay

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NickLee

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I have built a 555 timer that uses 2 9v batteries and i need it to power a relay... so i need to know where i can get a relay that will close when 2 9v batteries hit it, but i also need it to allow the current from a car bettery to go through it without messing it up.. if any onw can help me out with a radio shack part number or somewhere else i can get it or how to build it please let me know...
Thanx,
Nick
 
Use a low drop out 12V-regulator. It stabilizes the 18V to 12V and also the car battery voltage if it exceeds 12V (normally starting 500mV above 12V).

At the output connect a 12V relay.

The described method is cheap and the circuit will work properly at both input voltages.

If the 2X9V batteries are connected in parallel use a 6V regulator and a 6V-relay. In that case you don't need a low drop out regulator.

Boncuk
 
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Get a relay with a 12v coil and as high impedance as you can. It can stand 18v occasionally.
It doesn't know what battery is feeding it, only what current goes through it.
Relays have a current gain from 25 to 200, so if your relay coil draws ~10 mA you will probably only be able to switch ~500 mA.

Don't work on house wiring, yet.
 
Did you re-invent the ubiquitous 555 timer? You meant to say " ... 555 timer circuit". It sounds like you need a multi-throw relay, one that switches between 2x9V and 12V feeds. You might consider my circuit as an inspiration, if it answers your inquiry:
 

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