how to wire a volt monitor?

im trying to wire a voltage monitor/meter in my car and want to go directly to the battery for a more accurate reading. my problem is i dont want it to stay on when i turn the car off. i know a switch is an option but i dont really want to go with a switch... does anyone have any information about using a switch transistor to make it come on only when the car is on and have it go off when the car is off?Boosh

Use a relay controlled by an ignition source to switch the meter on and off by interrupting the input going to the battery. What kind of car? Do you have a lighter socket that only works when the ignition is on? Otherwise pull up (or request) the wiring info for your car to determine which wires are suitable for switched ignition. You can follow the relay diagrams on the site if you are not familiar with relays.

its a 2000 camry. and yes i do have a cig thats only on when the ignition is on.Boosh

You can wire up the relay as follows: 87 - Battery (fused at battery) 30 - +12v to the voltage monitor 85 - Gnd 86 - IGN controlled +12v (Ignition, Cig lighter) When the relay is energized via ignition circuit it will provide battery power to the meter.

GREAT!!! thanks!!!!   and can i use any single pole double throw relay or is there a certain kind i need to get? just anything that will handle 12VDC???Boosh

Nevermind, i understand. i can use a SPST cuz i dont need the 87a. THANKS AGAIN!!!!!Boosh

FWIW, I fitted a blue-LED voltmeter behind a 5x5cm (2"x2") blanking plate in a friends car. I used a on-off-momentary mini-switch to control a DIP relay. The DIP relay looks like an IC/chip with 8 pins in a 14-pin DIP (Dual Inline) package. The switch was only to allow monitoring whenever, though normally I'd just use a push button. But in this case I though the ability to turn off was essential (in case it annoyed). Normally the switch is on so the voltmeter is on with IGN.      I intend to add a delayed off of about 1 minute or 2. Hence after IGN is turned off or the PB (momentary) switch, the meter stays on for a while. I reckon that's handy to monitor surface charge decay (why??) or confirm the battery voltage. Normally the relay would be closer to the source (battery), but the (timer,) meter, relay & switch are all behind the plate with one wire to IGN +12V and two wires to battery-negative & battery +12V with a 500mA "205" fuse near the battery +12V.