This is a plug-in voltmeter for monitoring battery voltage. It plugs into a standard cigarette lighter socket.
This is what it looks like in operation. (My battery is getting weak.)
The LED voltmeter module came from Deal Extreme (dx.com).
Item: RD04 Mini 0.36" Yellow LED Digital Voltage Measuring Meter Module - Black + Blue-223908
The case is made from Lexan polycarbonate .093" thick sheet plastic. It is glued together with 5-minute epoxy.
I found that using a utility knife to scribe straight lines, clamping the sheet along the scribed line between two boards, and bending the sheet until it snapped off, worked well for cutting the pieces. I sanded the edges to make them square and flat, using a sheet of 100 grit sandpaper on a granite tile.
The meter box is mounted on a Philmore 48-700 "Auto Power Plug", from Altex Electronics. Cost: about $3.20.
I cut the plug off 1 inch from the cord end, to leave a 0.2 inch portion of the flatted cylinder portion.
Here is the drawing of the plastic parts. The oval opening in the bottom had to be trimmed out slightly larger than shown; I used a 'Dremel' tool.
The LED module has three wires. The black goes to the ground connection in the plug, and the red and blue wires both go to the +12 (center pin) connection.