I have an automotive-type Hall sensor that is slowly failing and want to squeeze a bit more life out of it, since it's costly and not readily available. Not a critical part.
The sensor I assume is a Hall transistor-type arrangement - it takes 12V supply, a 6V pulled-up signal line and ground. On detection, it pulls the 6V to ground. I've measured the short-circuit current on the signal line to be 20mA.
The "computer" on the other side reads the low signal and can work with a bit of headroom, but I assume it needs to be down to at least 2-3V in order for the signal to register reliably.
I want to design a very simple circuit that raises the input impedance on the sensor side and provides a low impedance (>20mA) on the "computer" side. That is, the circuit should provide a 6V (Vcc/2) signal line with about 10k of impedance, and when that voltage falls below 5V (~Vcc/2.5) it should drive the output side low as well. Inverted should also be fine, as far as I can tell.
I already had an LM324 window comparator built before I knew how the circuit actually behaves. However I don't like the fact that it feeds 12V on the signal line, computer side. I could add a diode to allow only pull-down, or set the Vcc to 6V for the LM324, but the component count is already high. Also three gates are unused.
I could try a 2N222 common-emitter circuit but from my calculation it seems that the gain is too low. Maybe a Darlington would work?
The last idea, which I would like to avoid, is to just use an ATTiny85 board (have a few of those), reading the ADC through a large resistor, pulling the output line low if it goes below a threshold. This is a minimal part solution since it already has a regulator in place and would only need a diode on the output and a resistor (>25k) on the input. Edit: won't work, the ATTiny will see 6-7V on its output pin.
Edit: I've measured that the sensor can source 4mA to ground, assume it also sinks around that value. Likely the computer can work with 5-10mA (since it works sporadically), but targeting 20mA just to be safe. It needs a pullup. Can't seem to shake the feeling that this sounds like DTL / TTL.