Remote hand sets of TV, VCD player, Home Theater system etc uses Infrared rays pulsating at 38 kHz. So in the receiver section, there is an Infrared receiver to receive the coded IR pulses. In most cases the 38 kHz IR module TSOP 1738 is used as receiver which is specific for 38bkHz IR rays. This module has a PIN photodiode and a pre amplifier stage enclosed in an epoxy case which acts as a filter and passes only IR rays. Its output is active low and gives + 5 V DC in the stand by state. The circuitry inside of the module amplifies the coded pulses from the IR transmitter. From the PIN Photodiode, the coded signals pass into an Automatic Gain Control (AGC) stage and then into a Band pass filter. Finally a Demodulator demodulates the signals and sends to the output transistor which forms the output. So when the module gets coded signals, the output transistor sinks current.
Circuit is simple and can be enclosed in a small box. Normally, the output of the module is high and the PNP transistor remains off. When the remote handset is focused on to the module and any one key is pressed, transistor T1 conducts and a chirping sound will be heard. The buttons in the remote is used to send varying signals, so the chirping sound will vary. This is helpful to check all the buttons in the remote hand set.