I'm trying to monitor current going to a device, and in case current is drawn, light a led and provide a low signal (<5V) to a microcontroller.
The current goes through a half-ohm resistor and the voltage difference is amplified by AD8210. Output of the amplifier is compared to a reference value set by a potentiometer using a LP311. Reference value is set by hand so that the led illuminates when the load is active.
This far everything goes fine, but trying to read the status to a microcontroller won't work, as LP311's collector stays at ~11V even when active.
My only idea so far has been that the output transistor is not saturated. However, increasing the load current and therefore increasing the comparator input difference should then bring collector voltage lower, but this was not the case.
So, why is emitter-collector voltage so high? I was expecting it to be 1V or less.