Push button latching circuit

Thread Starter

rtm038

Joined Oct 18, 2009
2
Hello all,
I'm working on a 12+VDC project and need some help figuring out the circuit. Basically, I want to control the ON/OFF states of a standard Bosch automotive type (SPDT) relay with a momentary push button switch that is supplied with a switched 12+VDC feed. Here is how I want the circuit to work:

1. Push the button and the relay turns ON

2. Push the button again and the relay turns OFF.
 

gerty

Joined Aug 30, 2007
1,302
The easiest thing to do is go to an electrical supply store and buy a light switch that does just that. Push on, push off.
 

elec_mech

Joined Nov 12, 2008
1,500
Hi Rtm038,

Gerty's solution is certainly the simplest. If you want to use a momentary switch, you can do it with a 555 timer and a handful of components like so:

http://todbot.com/blog/2010/01/02/momentary-button-as-onoff-toggle-using-555/

You can also use a CD4013. I did this with 12VDC:

http://forum.allaboutcircuits.com/attachment.php?attachmentid=31563&d=1308768439

Just use a pulldown resistor at the input (pin 3) to prevent accidental switching. Connect one side of the momentary switch to pin 3 and the other to 12VDC. Use D1, C7, and R14 if you want the output to stay off when you first apply power to your circuit.

If you want to control a relay, you'll want to add an appropriately transistor between the output and the relay coils whether you use a 555 or a 4013 - they will not be able to provide enough current to energize the coils. Don't forget to add a diode across the coils to prevent back EMF from damaging the momentary circuit.

Hope this helps!
 
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