Remote Turn On wire live only when engine/alt on

**mike938** · 07-21-2007, 07:28 PM

Does anyone know of a way to have a relay going only when the car is actually running? In my car (97 camry) I am unable to find any fuses that are live only when the car is actually on but instead they all come on when the key is switched to on.

This was my attempt at creating such a thing however when tested it did not work properly. To test: I used batteries as the fuses with the starter fuse as a battery switched with a lightbulb after the switch before the diode so I could easily see that the starter is switched on. The remote turn on wire was also a lightbulb so I could easily see what was happening. Anything needing a ground (batteries and bulb on the Remote wire) were attached to a common ground.

The problem with my test circuit is that whenever the ignition (on whenever car key is switched to On in car) battery was attached the starter bulb wouldn't come on when the switch was on even though it would still trip the relay and turn the remote on bulb on. When the starter was turned off the Remote bulb would stay on (like it should) while the ignition fuse battery was connected and would shut off when the battery was removed. Also like it should, if the ignition battery was reconnected, until the starter switch was turned on the Remote bulb wouldn't turn on. When the starter switch was on and the ignition battery was not connected the light between the starter switch and the diode on that line would light up.

Does anyone know what I should use or what is wrong in my circuit?

Thanks~

**2k1Toaster** · 07-21-2007, 07:57 PM

The only thing I can think of that most cars would have and would be easy access to and is only in an on state when the engine is running would be the tachometer wire. When off you get nothing. When on, it sends a bunch of pulses. So with a frequency to voltage converter you will now get some sort of variable voltage with the RPMs. Then you can ramp that up and trip your relay.

**mike938** · 07-21-2007, 08:10 PM

Very interesting idea but would this really be too safe? It seems like if I had it so idle and higher rpm would be enough voltage to trip the relay, if I get to higher RPMs it would potentially produce too great a voltage and destroy the relay or something.

I do know however, other than potentially disabling my starter (lol?), the relay circuit I had diagramed did work to do what I wanted. Any ideas on how to get that to work properly

**2k1Toaster** · 07-21-2007, 08:20 PM

The chip you use will take a v+ and a v- as well as the pulse input. The higher the RPM, the faster the pulses.

The chip will put out a signal between v- and v+. So 0rpm would be v- and the maximum output you could ever get would be slightly under v+.

So yes it is safe. Then you could just use a comparator with one comparator input being the vout of the freq chip, and the other comparator input being v-. That way it would always trip if greater than v-, meaning anything other than v- in this situation.

**mike938** · 07-21-2007, 08:28 PM

Ok, that actually does sound great, I will definitely look further into this. Thanks.
I guess I'll be looking for the tachometer wire tonight.

**brodgar** · 07-22-2007, 02:44 PM

You can also tap into the wire which runs from the alternator to the charging warning light on the dash. Again it is only live when the engine is running.

This is the recommended method when switching the relays for towed caravans (trailers) for battery charging and refrigerator.