We’ve all been here: you operate the TV remote and nothing happens. Is the device broken or do you just need to renew the battery? Eyesight alone is no help for testing the operation of the infrared LED. Help is at hand, however, and this circuit will produce a useful gadget that can test all infrared (IR) remote controls simply and rapidly.
The circuit comprises basically a Darlington amplifier stage with three transistors, the first of which is a phototransistor. The combination is very insensitive to ambient light levels. The IR-transistor will also react to (be turned on by) constant (unmodulated) infrared light (incidental light generates base current in the transistor) but in this case, a constant DC current flows across R3 and R2. The voltage divider created by these two (high-impedance) resistors ensures that the Darlington amplifier of T1 and T2 that follows cannot be turned on. Using the low supply voltage of 3 V means the voltage on R2 remains below the threshold voltage of the Darlington stage of around 1.2 V (2 x UBE) when the phototransistor conducts.
Events take a totally different course when the phototransistor receives a pulse-modulated IR signal of the kind transmitted by the IR-LED of an infrared remote control. This pulse train with a frequency of 35 to 40 kHz passes straight through capacitor C1, without attenuation, to the base of the Darlington amplifier where it is amplified substantially. The LED illuminates as a result, thus indicating the remote control is working.
Capacitor C2 integrates the amplified pulse train so that the LED lights visibly even for short bursts of the modulated IR-signal. It’s worth emphasizing the minute quiescent current of under 500 nA drawn by the circuit, meaning that the battery is assured a long life even without an off switch.
The selection of components is not at all critical. For T3 you can use virtually any IR-phototransistor, whilst for T1 and T2 all standard NPN small-signal transistors are suitable.
To simplify construction the author has devised a printed circuit board complete with the milling data files (GBR and HPGL) available for download  free at the Elektor website. www.elektor.com/110088