Help needed, 3v to 12v circut

[harolto]

Hey,
im having trouble with a circut im trying to build.
basicaly i have an unstable voltage of 1.5v-3.1v that i need to switch a 12v relay
and the relay needs to stay switched when the volts are removed, untill another switch is pressed (momentary switch).
i built a circut using a transistor, a low current (3a 12v) relay, a higher current (40a 12v) relay, some resistors, and a diode, and i cant get it to work.
i have a VERY basic knowledge of circuts, could some one help me out? a circut diagram would be great and parts that i need,
thanks
-Jake

 

[Kevin Weddle]

I don't understand. Post a schematic so we can see if your circuit has potential.

 

[jtjohnston]

Created a simple car alarm once.
http://jtjohnston.net/relay.PNG
Put a jumper from pin 1 to the normally open pin (pin 5).
Pin 3 has continuous juice. Juice pin 1 once and the relay will lock on until the ground or juice on pin 3 is broken.
Sounds like it might help?

 

[harolto]

that is sorta the circut, there is bits missing im sure,
basicaly i need it to latch when 3v is applied, then unlatch by a button
also i need it to stay latched if 12v is disconnected, or stay unlatched (memory sorta thing)
any help is much appreciated
-Jake

 

[Kevin Weddle]

It looks like you want the small relay to power the big relay. Disconnecting the 12V will turn the small relay off, if it's on, and the big relay won't stay on. The pushbutton switch needs to break series, it looks parallel to the load.

The hardest part will be saturating the transistor.The source must be able to supply enough base current, which can be higher than many signal sources output.