Photodetector - Input Unit

What does it do?

Photodetectors are semiconductor devices which respond to light. They can replace light dependent resistors and have the advantages of lower pollution and smaller size. Signal input sensor

How does it operate?

There are several kinds of photodetector.

Photodiodes are similar to normal diodes but, if they are reverse biased, the current through the diode increases with the light level.

Phototransistors (and photodarlingtons) are like ordinary transistors (and Darlington drivers) but the ‘base current’ is produced by the light falling on the device – there in no actual electrical connection to the base.

Phototransistor circuit

Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.

The circuit diagram on the left is for a phototransistor.

As the light level increases the current through the phototransistor and R1 increases, so the output signal voltage increases.

To allow the sensitivity to be adjusted the fixed resistor R1 could be replaced with a variable resistor.

To produce a ‘dark sensor’ the positions of the phototransistor and the resistor are interchanged, so that the output signal voltage increases as the light level falls.

Note – the output current available from a phototransistor is small – enough for the input signal to a PIC, a CMOS integrated circuit or a MOSFET, but not large enough to drive a transistor.

Possible applications

  • Sensing if it is night or day
  • Sensing if an object has blocked a beam of light


The pin connections and PCB shown are applicable to both the SFH309F phototransistor (Rapid Electronics Order code 58-0425) and the SFH300-4 (Rapid Electronics Order code 58-0480).

How part of the PCB might look

Note – the shorter leg of the phototransistor, and the side with the flat, is the collector. The collector should be connected to the +Vs supply voltage. This is the opposite way round from a LED (hotlink to data sheet), where the shorter leg is the cathode or negative lead.


Make sure that the signal going out (on the green PCB track) changes from high to low when the photodetector is covered.

Fault finding

If there is a fault, check:

  • the voltage on the collector (the leg identified with the flat) is +Vs;
  • the value of the resistor
  • that the detector has been connected the right way round.

If there is a fault, check the tracks and solder joints.


  • Light dependent resistors (LDR) – cheaper, but cause pollution because it contain cadmium. LDRs are also larger.
  • A cheaper L-610MP4BT (Rapid Electronics Order code 72-8968) phototransistor is available. However, this produces very, very small currents and can only be used reliably with process subsystems that only need a very low input signal current.

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