Understand LM393 circuit

Thread Starter

shein

Joined Sep 11, 2012
6
Hi All,

I'm trying to figure out how Roomba dirt detect circuit works but having trouble to understand how the final stage works. I've drown a schematic from the pcb and did some measurements with an oscilloscope while tapping on the piezo sensor.
2018-04-26-110731_826x625_scrot.png
roomba_dirt_signal_1.png
roomba_dirt_signal_2.png

As far as I understand IN+ is a reference voltage. When IN- becomes greater than IN+ OUT drops low and pulls IN+ low through diode D4. This is some sort of hysteresis to implement output latching.
What I don't understand is how OUT stays 1.7V in idle state when IN+ is 2V and IN- is 0V. I believe the output should go to VCC which is 14V but my measurement shows it is 1.7V in idle.
I double checked the pcb and it seems like D4 is the only feedback from the OUT.
I would appreciate any help.
 

Thread Starter

shein

Joined Sep 11, 2012
6
I should have looked more closely to the block diagram of LM939.
The output of the LM393 is an open collector. So it must be pulled up to the desired voltage when inactive.
Screen Shot 2018-04-26 at 12.52.19 PM.png
 
Top