At this point I have all the electronics working. I just had to add the phototransistor by following a tutorial. A quick test showed it works really well. I connected a cardboard disc to the motor, taped a few coins on the bottom side in one place, and put the sensor under the disc. There is a very large difference in the values you get back from the sensor when it’s covered with something and when not. Here’s a first attempt at visualizing the schematic with Fritzing: Note that for now this includes a pot (to set the motor speed) and no external power (doesn’t seem necessary for now). And this is a simple program that will turn on the laser when something is in front of the photo sensor: const int transistorPin = 9; // connected to the base of the transistor const int laserPin = 13; // connected to laser TTL control (white wire on

void setup() { // set the transistor pin as output: pinMode(transistorPin, OUTPUT); // set the laser TTL pin as output: pinMode(laserPin, OUTPUT); Serial.begin(9600); } void loop() { // read the potentiometer: int potValue = analogRead(A0); // map the sensor value to a range from 0 - 255: int outputValue = map(potValue, 0, 1023, 0, 255); // use that to control the transistor: analogWrite(transistorPin, outputValue); int photoSensorVal = analogRead(A1); Serial.println(photoSensorVal); // turn on laser if something is close to the photosensor if(photoSensorVal < 100){ digitalWrite(laserPin, LOW); } else { digitalWrite(laserPin, HIGH); } } That’s actually it for the electronics. This part went easier than expected, thanks to some great tutorials on SparkFun, Bildr, the physical computing wiki from ITP – and of course Alex’s great courses. What’s next: I have to find a nice design to keep all the parts together. Now everything is literally kept together with tape, which is not very safe. I already sent some mirrors flying around the room. I think I’ll start with moving the optical sensor related things to a second breadboard and connect it to the other one with some longer wires.