I have offered ammo counter kits for nerf dart guns for a while now. With a tweak to the ammo counter software’s min time between fire events and the addition of a photo sensor, you can install my ammo counter kit on a nerf gun and keep track of shots remaining in your clip even while full auto firing.
What allows this to work is a light break sensor. The part number I recommend is the OPB100Z. It comes with an IR LED and a Phototransistor. The LED creates a beam of light, and the phototransistor acts as a switch. When the light shines on the sensor, the transistor conducts and the ammo counter reads voltage. However, when a shadow is cast on the sensor the circuit is broken and the ammo counter fire input is grounded.
More info on how Phototransistors work: http://learn.parallax.com/node/258
Below is how to wire the light break sensor into an ammo counter. The wire leads on the light break sensor are color coded, follow the colors in the diagram below. Place the sensor and light on your barrel so each dart fired creates a shadow between the sensor and light.
- During your install and testing, you will be unable to see the light with your eyes. Your human eyes are not sensitive to IR light. Instead, point a camera or smartphone at the light and you should see the white/blue light on the screen.
- I do not currently sell light break sensors, but do tell me when purchasing a kit that you intend to use one. I will program with special software and include the extra two resistors seen in the diagram.
- Places to buy a light break sensor, part number OPB100Z: