Power-on Delay Circuit

[kpatz]

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.

[jimbonzzz]

I like it. Might try it out, every once in a while after I start the car the V1 flickers between the remote display and detector until I cycle power, might be just what the doctor ordered...

[amoney]

8)

[HighVoltage]

FYI, in case someone isnt sure you can pretty much substitute any integer number for the X in diode labled 1N400X. Its just a flyback diode, so a 1N4007 should work just fine. You could also eliminate it if you substitute the relay with one that has an integrated flyback or snubber.

[ronsc1985]

You could have used an N-Chanel FET instead of the npn's, the zener and a few other parts along with a higher resistor, a much smaller capacitor or both. If your criteria is getting parts at R.S. it's just fine. I find places like Digi-Key a much better source.

[RacerX]

this works similar to the POWERSENSEnode from cheetah. http://www.speedcheetah.com/ (yes, i'm allowed to put this link up )

i've been testing it out and it works well.



turns on when car is started and turns off when car is turned off

[proudNMAmember]

Looks cool racerx. I am sure others on here would like one of those. Mk

[vocaltrance]

i see the schematic's been psoted however i was wondering if someone who ends up building this circuit wouldn't mind posted a pic of it... schematic's aren't really my uh forte

[kpatz]

i see the schematic's been psoted however i was wondering if someone who ends up building this circuit wouldn't mind posted a pic of it... schematic's aren't really my uh forte

Here's a pic, but the schematic tells you what gets connected to what:



Note that there are LEDs in the circuit that aren't shown on the schematic; the red LED lights during the delay period before the relay turns on, and the green LED lights when the relay turns on.

[bauzer71487]

RacerX. . so how does that thing work . . and where did you have to cut wires and stuff? I don't get how it quite works. . does it connect directly to battery?