Fig. 1 My basic photodiode test circuit.

Photodiode Op-Amp Circuits

Here we will use operational amplifiers or op-amps to convert the photodiode current to a measurable voltage - this is called a trans impedance or current to voltage amplifier. This enable an analog-to-digital port on an Arduino or PIC microcontroller to measure light intensity.

Here in all cases the photodiode is reversed biased. We won't use the photodiode in it's voltaic mode. For more on these subjects see the following:

Fig. 1 shows a basic test circuit I constructed. When the LED is turned on reverse current flow through the photodiode from cathode to anode to the base of Q1. The current is amplified and use to light up a LED. This is an ON/OFF circuit useless for measuring light intensity.

Fig. 2 basic LM741 photodiode trans-impedance amplifier.

In Fig. 2 we use a LM741 to convert the small leakage current to a voltage by the formula Rf * Ip. Depending on the value of Rf the voltage output is positive 0 to 10 volts. the brightness of the LED is proportional to light intensity on the photodiode.

Note this is a bipolar power supply circuit.

Fig. 3

Fig 2 differs in that the anode of the photodiode is connected to the -12 volt supply. This reduces capacitance and improves switching response. Note this is a demo circuit - for real high speed performance use high speed op-amps such as a Analog Device ADA4817-1 or Burr-Brown OPA640. Also use a PIN photodiode.

For more on photodiode capacitance see: Photodiode Circuits Operation and Uses

Fig. 4

In Fig. 4 we connect the cathode of the photodiode is connected to the +12 volt supply. This produces a negative voltage output. This is another bipolar power supply circuit.

Fig. 5

Here is an experimental circuit to read light intensity with a photodiode with and Arduino. The maximum voltage out is 5-volts with a 7-volt supply. Output 0-5 volts.

This is a unipolar circuit.

Also see Optimizing Precision Photodiode Sensor Circuit Design by Analog Devices. (PDF)

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