The eyes on the robot are phototransistors . The level of light on each sensor is measured from time of decay of the voltage measured across a capacitor connected across the [ Select ] light emitting diode phototransistor . The [ Select ] current voltage is measured after the relevant pin on the Arduino is programmed to act as an [ Select ] input output . The [ Select ] voltage current has to fall below a threshold level determined within the programme. The higher the light level the [ Select ] shorter longer the decay time because of the [ Select ] higher lower of the resistance of the [ Select ] phototransistor light emitting diode . The [ Select ] difference in sum of the light levels normalised to the [ Select ] difference in sum of the light levels from the two sensors is used to find the direction (left or right) of the brightest light, while rejecting ambient light. The illuminated beacon on of each robot is a [ Select ] light emitting diode phototransistor with the current limited by a [ Select ] resistor capacitor connected in [ Select ] parallel series . The beacon is chosen to be in the infra-red wavelength range to reduce the sensitivity of the system to ambient visible light. The algorithm sets the differential speed of the robot's drive wheels to head towards the direction of brightest light.