Today, I performed validation testing of the breakbeam module selected to handle bullet-detection for high speed photography. The sensor module is a Sharp GP1A57HRJ00F from SparkFun Electronics (Datasheet). This is an infra-red emitter / detector combo: one half of the module holds an IR LED, the other half an opto-transistor detecting the output.

The output on Pin 2 is at TTL High until the beam is broken, then it goes to TTL low.

I wanted to make sure the sensor does as advertised (remember, I’m a mechanical engineer by trade, that happens to dabble in electronics and coding). So I hook it up with a breakout board and populated the current limiting resistor with a SMT 231 ohm resistor that we’ve got lying around the lab. Here’s the schematic:

Connecting Vcc to a 5V DC power supply in the lab, and connecting a probe from an oscilloscope between SIG and GND, I was able to get the following response:

As you can see, the output on the signal switches rather quickly. The oscilloscope was set to trigger off the falling edge of the signal at approximately 3.8V and the drop in voltage occurred within 20 *nano* seconds.

If we assume a .22 round that’s spherical in shape (say, a BB), travelling at 1000 fps:

Each milli-second the round covers 1 ft
To cover .22, or 0.0183ft, the round will take 0.0183 millisecond, or 18.3 microseconds.

I’m fairly confident that the round firing through the beam can trigger an electronic response. The next challenge is then designing electronic circuit to lengthen this response to a switched pulse to fire a pocket wizard radio transmitter.