The circuit and program could easily be adapted for photographic triggers for water droplet images or any timing beam project.
Step 1: Components
- the Arduino Duemilanova
- the ioBridge io-204 monitoring system and serial module
- two IR emitters
- two IR phototransistors
- a 47 ohm resistor
- two One megohm potentiometers
- a few bits of wires, glue and some plastic tube
Step 2: Details
The emitters are wired in series from the Arduino 5 volt supply via a 47ohm resistor to limit the current flowing through them.
The phototransistors are individually wired in series with the 1 megohm potentiometers to create a pair of voltage dividers. When the IR beams are not broken the monitored voltages will be at 5 volts and when they are interrupted the voltages drop towards 0.6 volts.
The variable voltages are fed into two of the arduino analogue pins and are monitored using a small program on the Arduino. When each beam is broken the voltage drops below the user defined threshold and the programme measures the time taken between the beams being broken forming a simple speed trap.
The calculated velocity is then sent via the Tx/Rx pins on the Arduino to the serial module on the ioBridge. The ioBridge dashboard is configured to listen for the data being sent from the Arduino and a serial widget is used to redisplay the velocity on the Burp the Frog webpage.
To waterproof the emitters and transistors I bonded them into some small pieces of plastic tube which were then glued to an ornament in the aquarium. The tubes also eliminate a lot of the ambient light which may affect phototransistors which are not specifically designed for IR light.