I have made a sensor which uses six IR phototransistors, three per sensor block, opposite it is a matched IR diode(940nm). This is in turn connected to a MCU.
The function of the circuit is an airrifle chronograph, the circuit generates a signal when a pellet passes over one of the sets of three transistors, thus breaking the circuit giving a pulse. Look at the first image on the linked page here.
Firstly the device is detecting an object with a 4.7mm cross section passing between sensors, the original application used an audio port and the internal amplifier and some pull ups to create a pulse for each crossing of the sensor. When I added a external pull-up and measured the voltage, only a small change was visible. This yielded results such as \$ 0.4v < VOut < 0.9v\$, which obviously will not be picked up by an interrupt. I concluded that I would need to use a compartor to give a clean signal for the input to the MCU, this would be a standard circuit with hysteresis non-inverting.
Question: How do I calculate the needed pull-up resistor to give me a voltage range of 1.5 - 3V from a 5V supply with 6 phototransistors(linked above). This will be feed into the comparator circuit.
I know that transistors can be simplified to equivalent resistors but I do not know how to determine them for a phototransistor, I am assuming one can do this so as to find a pull-up.
The datasheet for the IR led.
Here is a snapshot of the wave form from the audio port and software used to read the signal:
Each spike is a break of a three set sensor of the six.