Hello all, I would like to pick your brains about designing a voltage actuated switch .hear are the details: . automotive application so 12v dc and also 5v dc are available to power the circuit .i need a electronic switch that can sense when a 5v variable supply voltage drops to 3-3.5 v. Then will output a 12v. It doesn't have to latch as the output just triggers a latching relay. Ive been out of the electronics for 20 years an what i thought of and tried works on paper but not in actuality using a ka393 comparator, couple resistors and a pair if zener diodes. Any info to point me in the right direction or schematics would help . thanks

 

You should be able to do it using the KA293/KA293A, KA393/KA393A, KA2903 comparator you mentioned. Keep in mind the comparator has an open collector output but should work in a standard comparator circuit. If you can post a diagram, even a rough diagram of what you tried someone can likely fix it up for you.

Ron

 

Here is an 8 pin DIP that does that. 7665S CMOS Micropower Over/Under Voltage Detector Just use it for the brains and build around it and use it's alarm out to trigger a relay or transistor switch.

 

Ok .pic of schmatic i used cant upload from my phone .i know im missing something simple. The aging memory has struck again. Ill look into the7665s chip when i can use my desktop. Thanks guys.

 

Is the 5 V supply regulated ?
If so put 3V on _ input on comp. using 2 Rs or a pot; Variable 5V to + input. Output via R to base of PNP transistor rated for relay.