Detect vehicle ignition on

[futureroads]

I am looking for a simple circuit to determine the status of a vehicle's ignition based on voltage on the vehicle's OBDII power pin (16). With the ign off the voltage is around 12.5V and with it on its around 13.5v. I want to be able to "switch" 12v to a device lead when I detect the ign is on. Seems there should be a simple way to detect this voltage increase. I've looked a zener diode, MOSFET, etc circuits but not sure which approach would be best. Any suggestions?

 

[xaccto]

a comparator circuit is what you need. It has 2 input terminals, one (-) goes to the voltage your detecting via a potential divider,
the other (+) goes to a reference voltage (can be produced by a zener, or a more accurate voltage reference).
The potential divider you want to be adjustable, its purpose is to divide the input voltage down to close to the reference voltage
on the other terminal.
You can switch (+) and (-) terminals around depending on what output behavior you desire - whether it goes high or low when the voltage
threshold is crossed.

you can do this comparator with an general opamp, eg LM358,LM741 etc or a comparator device LM339,LM301 etc
google these, find heaps of results.

oh, one more thing, I have done exactly this sort of thing in the past, do not underestimate temperature variations in the
voltage reference effecting the threshold you set in a automobile application.
If using a zener reference, use 5.1V as that has the minimum temperature coefficient, or really recommend a reference like LM335.

 

[kak111]

Voltage Comparator Information And Circuits

This page provides basic information about voltage comparator integrated circuits and is to act as reference material for other circuits.
The circuits shown are based on the LM339 Quad Voltage Comparator chip or the LM393 Dual Voltage Comparator chip.
These devices are functionally identical. The LM311 Voltage Comparator can be used for these applications as well and
also has a number of unique features.

**broken link removed**

comparator output controls NPN driver and relay

 

[DeLorean]

Not as simple as a comparator but a more reliable way would be to use a micro reading the voltage. Then when it senses the increase in voltage you know the engine is running. This way you don't need accurate references since your looking for a change.

Of course you will want a circuit that is run on a switched power source so you don't drain the battery when not in use.

 

[futureroads]

I had been playing with a couple of zener's (12.1v and 5.1v) using a voltmeter to see what voltage came thru them when connected to the 12v source (13.5). It made me wonder if I couldn't use a 13v zener to detect when the voltage changed from 12.5 to 13.5 (ign on). Seems in a perfect world I should only see voltage thru a 13v zener when the ign was on. If that were true I was thinking maybe I could use that connected to a transistor gate as a "switch" or something. I'm trying to keep this down to a couple of small components if possible. Needs to fit in a cable/cable connector if possible.

 

[logicon]

You have a tight 1V margin to play around the base of your transistor gate. It may work but you must match current and power capability of the zener (at the emitter) to the switching device (e.g relay, solenoid, e.t.c). Bear in mind that the zener voltage will respond to switching current if that is within your tolerance limit, then you are fine. A 10~22 Ohms resistor at the base will help limit the 13.5V signal current to appropriate level required for your relay or load. Hope this is in line with your approach. Good luck!