I recently revisited some of my early laser harp work and thought it worth making a posting of the original beam detection circuit. This is an active low comparator circuit and the output line can be hooked directly into an Arduino.
Shine the laser/light onto the Light Dependent Resistor and turn the 100K potentiometer until the LED just comes on. Then shading the LDR should turn the LED off. All the parts are easily available from Radio Shack (Note: the power and gnd connections for the LM324 are not shown).
Part 1 is the power hookup – you need to connect the 5 volts output from the Arduino (or another source) and gnd to the two lines that run along the longest edge of the breadboard.
Part 2 is the first of 2 potential divider circuits. These divide the input voltage up from part one into different levels. This first one is adjustable with the potentiometer (100K). If you connect the resistor and the potentiometer as shown, then measure the output of the divider ( the middle tab of the potentiometer) you should see a voltage change as you turn the control knob of the potentiometer.
Part 3 is the second potential divider circuit. This uses a light dependent resistor in place of the potentiometer alter the voltage. Same as before though, if you wire this up and measure the output (the line that is connected to pin 3 later in the circuit) you should see the voltage change as more or less light shines on the circuit.
Part 4 uses the op-amp integrated circuit to compare the 2 voltages from parts 2 and 3. When the voltage output of part 3 is above that from part 2 the pin 1 from the IC will go “high” and cause the LED in part 4 to light up. In addition to wiring in pins 1,2 and 3 you will need to connect the power and gnd pins to the power and gnd lines we created
in part 1.
Part 5 is the output stage – this uses a simple LED and current limiting resistor to light up when the pin goes high. You want to have this LED light up when the laser pointer is pointing at the photocell, and go off when it is not. Note the polarity of the LED, it should only go in one way round.
I used 8 of these circuits in the very first static laser harp I built: