Here is the circuit of an ultra sensitive IR receiver that can be used to switch on various AC devices through an IR transmitter. It uses the Photo transistor L14F1 to detect IR signals from the transmitter.
Photo transistor L14F1 receives the IR signals from the IR transmitter. It is an NPN transistor and its base is exposed to receive light / IR signals. It has an inbuilt lens to focus light on to the semiconductor junction. When T1 receives IR signals, it conducts taking the base of T2 to ground potential. T2 is a PNP transistor so that it triggers when its base becomes negative.
The high output from the collector of T2 triggers IC1. CD4538 is a precision monostable multivibrator free from false triggering and is more reliable than the popular 555 IC. Its output pin 7 becomes high at power on and sinks current when the trigger input pin 5 gets a low to high transition pulse. When T2 conducts, diode D1 forward biases and triggers IC1 and its output goes low for a period of four minutes based on the values of R3 and C2. By increasing the vale of either R3 or C3, this time delay can be increased. When the IC1 gives low output, T3 conducts and activates the Relay.
Diode D2 give back e.m.f protection to T3.The circuit requires 9 to 12 volt 500 milli ampere power supply.