I built this circuit to cause the power-up of my hardwired V1 to be delayed for a couple seconds after the engine is started. This prevents possible damage and performance issues with the detector.
The circuit is based on the one provided by Bill Bowden on his website here, with some modifications. All the parts needed are available at Radio Shack. The relay I used is a 5-amp DPDT 12V relay.
Here's the schematic:
As shown the circuit provides a roughly 2-3 second delay from when power is applied before the relay closes. To increase the delay, increase the value of the 100K resistor, or the 100uf capacitor. In addition, a trimpot could be used to allow the delay to be adjusted.
For those who are wondering how the circuit works:
When power is applied, the capacitor is charged via the 100k resistor. Once the capacitor's voltage reaches roughly 5.1 volts, the Zener diode starts to conduct, which turns on the transistors, which in turn energizes the relay coil.
Once the relay energizes, power is supplied to the radar detector (or other device as desired) through one set of relay contacts. The 2nd set of contacts perform two functions within the circuit. First, it keeps the relay energized once it engages. Secondly, it discharges the capacitor through the 10K resistor and 1N914 diode. This "resets" the delay quickly so that if power is cycled (say, when you turn the ignition key to engage the starter), you'll get the full power-on delay again.
BTW, I didn't show this in the schematic, but a fuse on the +12V line is a Really Good Idea(tm). I used the inline fuse from my V1 hardwire kit.