You can test an IR (infrared) link with a circuit that converts an IR-generated photocurrent to an amplified current that drives a standard LED (Figure 1). This approach provides a visual feedback to indicate that the transmitter is working. The circuit can be enclosed in a small plastic or metal box and requires just a 9V transistor battery for operation. Diode D₁ is a basic Everlight PD333-3C/H0/L2 or equivalent IR photodiode in a T1¾ package.

You can configure amplifier IC_1A as a photovoltaic amplifier. When the IR-light energy impinges on photodiode D₁, it generates a small photocurrent that tries to pull the inverting input negative. Meanwhile, the output of IC_1A goes positive, maintaining the virtual-ground node on Pin 2 of the amplifier at 0V. The transfer function for the circuit is V_OUT=I×R₁. If you set the gain high, IC_1A goes to the power-supply rail when the circuit detects light. Analog Devices’ AD823AR JFET-input amplifier directly drives the LED through a 750Ω current-limiting resistor. C₁ compensates the amplifier, preventing it from oscillating due to capacitive load from D₁ and the input parasitic capacitance.

If the output of IC₁ oscillates, you may need to increase the value of C₁. You can determine the value of C₁ by using the following equation for a 45° phase margin: where F_C is the unity-gain-crossover frequency of IC_1A—typically, 16 MHz for the AD823—and C_D is D₁’s 0V junction capacitance, including any parasitic capacitance on that node. Adjust R₁ for optimum gain. For testing, the remote-control transmitter window should be as close as possible to photodiode D₁ for maximum signal transfer.