I would like to trigger a [7]555 timer when a phototransistor stops receiving light from an IR led, more precisely when the (expected to be smallish) signal from the light-sensitive voltage divider, consisting of a phototransistor and a resistor, changes quickly. How should I wire the timer? I expect if I decouple the light sensor from the timer with a capacitor I might be able to get a sufficient voltage swing to trigger the timer from a small but quick change in the signal?
I would like to implement a version of the LED staircase. The project idea is to build a number of as-cheap-as-possible photodetector-triggered lamps on one side of a 3-foot gap opposite IR LED throwies on the other. When someone passes by they break the beam and trigger an entertaining trail of lights that slowly turn off, one by one, a couple of seconds later.
I was overthinking the problem. After some experimentation it appears all I need is a photodarlington (a phototransistor by itself is not sensitive enough) in series with a resistor. This forms a voltage divider with a usable signal. If a potentially long low pulse is a problem then merely decouple the photodarlington from the logic with a capacitor differentiator.
Best Answer
I would start it using a LM567 to detect the presence of someone on the stair. I recommend this circuit: http://www.mondotronics.com/PDFs/3-337_Mod_IR_v22.pdf
Using the output of this circuit, you should trigger your LM555 to keep the light on as much time as you want, but not using a large capacitor as you mentioned, but just a LM555 in monostable operation.
The output from LM555 can drive a transistor to control as many LEDs as you want!
Some level or signal inversion maybe required.