Modulated signals up to a few megahertz can be detected with the circuit shown in the figure, which uses a photodiode as a sensor and a low-power field-effect transistor as an amplifier element. The sensitivity depends on the 100 kΩ resistor in series with the photodiode, which can be changed according to the application. The capacitor C has its value chosen in the range of 1 nF to 100 nF, according to the modulation frequency of the received signal. The power supply can be between 5 V and 15 V, typically. The sensor can be any photodiode or even a PN junction of a phototransistor.
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