switch

[ajcletus500]

Im looking to make a switch which will turns on when power supply is given and can be turned off only is the power supply is off. Which means that again when the power is made on the switch should be pressed once to get the circuit running. Power supply will be of 12volts. Can anyone help me to do it??

[kam]

one relay DPDT (or DPST). Connect the relay coil in series with your power supply and in series with your ON pushbutton. So when you have power and you press the pushbutton, the relay arms. Now, use one of the Normal Open contacts of the relay and connect them in parallel with your push button. So when you press the pushbutton, the relay arms and the relay contact which is parallel to the switch will hold the current as long as there is power. This is a typical latching relay connection in automation.

http://www.pcbheaven.com/scripts/imagepresent.php?filename=%2Fblogpages%2Fimages%2Fmyjob_felder_110522_1306915881.jpg
http://www.pcbheaven.com/scripts/imagepresent.php?filename=%2Fblogpages%2Fimages%2Fmyjob_felder_110522_1306915881.jpg

You will see that the start pushbutton arms a relay, and one of the contacts of the relay is parallel to this pushbutton