Recently I've purchased some IV drop counter sensors from Ebay. I want to use these to measure sporadic drops of coating that might fall. However I'm having a bit of difficulty understanding what is happening from what the datasheet is telling me. (https://www.medonegroup.com/pdf/manuals/techManuals/Alaris-7100-7200-Service-Manual.pdf)




At the moment I've hookup the sensor to a bench PSU and current limited the diode and tested the sensor without all the electronics above. This does seem to generate a signal of about 10-50mV, but sometimes it does not even flinch when a drop passes. I was hoping that rebuilding this driver would make it more reliable and give me a more usable signal? (The board itself does not seem available anymore)
The thing is that I don't understand what is happening in the area outlined in red. I don't see how the ambient light signal is subtracted from the drop signal, or how the signal from the sense capacitor is transferred to the sample capacitor. It seems that this should be something fairly obvious that they didn't put in the block diagram, but I can't seem to figure it out.

Help is very much appreciated.

 

It uses a 32khz signal to pulse the led, this is to eliminate daylight sensing, feeds the signal to the receiver for comparing.

 

It uses a 32khz signal to pulse the led, this is to eliminate daylight sensing, feeds the signal to the receiver for comparing.

Yes, this would be my guess as well. However I don't understand how the parts in red translate to an actual schematic. As the sense capacitor will get charged with the low frequency ambient light signal and thus never get to the sample capacitor, but I'm probably missing something.