EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: bborisov567 on August 11, 2021, 09:12:51 am
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I am trying to make a device that switches a relay when engine is started (voltage going from 12v to 13.5v). I am using the circuit below. Zener is 8v. I have a problem with the standby current - it's about 10-12mA which is a lot for a device that is connected to the battery all the time. I tried making R10, R30 and the trimmer pot around 3 times bigger but current is still the same. I believe what happens is since the threshold is quite small both transistors are already saturated at 12v. Could you give some suggestions or another relatively simple circuit for the task.
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Using a voltage reference and a comparator could work with less current.
TL431 needs a mA or two.
Going whole hog, a INA226 (at 0.5 mA) and an AVR8 will give you 15 bits of measuring voltage.
I simply poll the OBD from the ECM to see if the engine is running.
That way I can see running time and schedule my coffee breaks. >:D
Checking voltage is not the most reliable way to tell if the engine is running.
Do you really need to differentiate "ignition on" from "engine running"?
If your alternator has a separate "Stator" (S) output, that would be more reliable.
If your coil/ignitor pack has a tach output you could monitor that.