Light falling on the transistor changes both the mark and the space of the generated signal thanks to the diode bridge. The diodes are general-purpose types, such as the 1N4148 and 1N914. Any phototransistor can be used in this circuit. The central frequency depends on C and the resistance presented by the transistor in a reference light level. The circuit shown here can use any inverter. For signals in the audio band, C can assume values between 4,700 pF and 0.47 µF. The lowest value recommended for C is 47 pF.