I am trying to build a circuit that can trigger the starting and stopping of a typical digital timer using a light source as the trigger. I came across an old web page that does almost what I need, but as I know almost nothing about circuits I would like some advise if it will do what I need. This project is for an espresso machine to time how long the espresso water pump is on when making the espresso. The timing range is from 0-60 seconds. I figure I can either use a current sensor to trigger when the pump is activated, or use a light sensor (photodiode/photoresitor) to "see" when a light is activated (by using a LED wired to the pump circuit to illuminate when the pump is on).
The circuit I came across is here
I'm unsure about the value of C1 as it states "value of C1 depends on the exposure time to be measured and on the photo diode used". So would this circuit work for my use on a typical timer like this? Also it says that a photoresistor could be used instead of the photodiode, how would I modify the circuit to work with a photoresistor (my local store sells photresistors, but not photodiodes)?
The circuit I came across is here
I'm unsure about the value of C1 as it states "value of C1 depends on the exposure time to be measured and on the photo diode used". So would this circuit work for my use on a typical timer like this? Also it says that a photoresistor could be used instead of the photodiode, how would I modify the circuit to work with a photoresistor (my local store sells photresistors, but not photodiodes)?