I have a internally regulated vehicle alternator which uses a conventional battery light wired in-between the exciter terminal and positive.
Obviously when the alternator isn't turning the exciter terminal is ground so the bulb is on, when the alternator is turning and charging the terminal becomes live thus no voltage difference and the bulb goes out.
I want to get rid of the bulb and have everything on my engine controlled and monitored with my own MCU.
As the alternator needs to 'see' a current on the exciter terminal I have no option but to replace the bulb with a low value resistor. Nothing above 25Ω works so I'm using a 20Ω resistor. This works and allows the alternator to charge.
I now want to hook up a 3v3 MCU to monitor the state of this circuit and to replace the visual element of the bulb. But it doesn't seem to be as simple as it first seems.
This allows the alternator to charge and outputs 2.8-3v to the MCU when the alternator is charging, but when is 0.7V when not charging. Obviously I can get the voltage a bit higher by using a 3.3v zener ( I only had a 3v to hand), but how can I get the 0.7 lower. Ive tried putting a pull down resistor in parallel with the zener creating a voltage divider but this affects the 3v when the alternator is charging.
I've also tried using a onto coupler but no matter what value of resistor I use I cannot get the opto to trigger.
It may seem that 0.7 is low enough to register low but I want to make sure that this circuit will work with all alternators. So I need the value to be as low as possible.
What is the best way to monitor the exciter terminal of a alternator with a MCU?