What resolution over what size do you need? So, how many X How many Y?
Phototransistors are cheap. Like 10 cents.
WhatIf??
- you illuminate a row of phototransistors with 2 or 3 MR16 type halogen lamps, thru a slit to control the illumination falling on them.
- Each phototransistor is wired from +5V with a pulldown resistor to Gnd. When all are illuminated, all outputs are HIGH.
- Each phototransistor goes to an input on a 74HC165 (See: http://arduino-info.wikispaces.com/Popular-ICs - Scroll down)
- Each phototransistor also drives an 8-input NAND gate (74LS30) When any phototransistor goes LOW, the gate pulses Parallel Load on the 74HC165.
- You clock out the data serially to Arduino. The bit for the trigger phototransistor is the only one LOW.
You could expand this to two axes, and expand the 8 X or Y to 16.
2 or 3 dollars in parts, Some circuit board or drilled plastic strip, plus the illumination and the bullet shields for the hardware. Cat5 cable to the firing position and the Arduino: +5, Gnd, Clock, Data, Bullet detected. (Illumination power separate)..
Maybe the phototransistors aren't fast enough? 60 uS seems OK.
Maybe??