Maker Pro
Maker Pro

Power OFF 555 timer trigger?

Oziam

May 26, 2012
10
Joined
May 26, 2012
Messages
10
Hi,

I have a 12v DC circuit that I want to trigger a 555 or 556 when the power source is turned off!
Is it possible without using a relay?

Thanks!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
Sure, it's easy. the 555 is triggered by a negative going pulse, so the main thing you need is a common ground and a small capacitor between the 12V rail and the 555's trigger pin.

The power being turned off will generate the negative going pulse.

Note that this assumes there are no capacitors in the 12V circuit that will maintain the voltage across the supply (or limit its rate of fall) after the switch is opened.

This page may help.
 

Oziam

May 26, 2012
10
Joined
May 26, 2012
Messages
10
Thanks Steve, yes of course, I was trying to re-invent the wheel as usual.

cheers :)
 
Top