ASK RF Receiver receives the data transmitted from ASK RF Transmitter. The input code set to address pins A0-A7 at HT12E should match with the address pins of HT12D. If it is valid, the LED connected to the below circuit (fig 2) will glow and then transmission of data get starts. The positive and negative of the power supply is fed into HT12D via VDD and VSS respectively.
HT12D is a 212 series decoder, most commonly used in remote control applications. By using the HT12E encoder and HT12D decoder, we can transmit 12 bits of parallel data serially. HT12D converts serial data to its input to 12 bit parallel data. These 12 bit parallel data is divided in to 8 address bits and 4 data bits.
For the internal oscillator in HT12D an external resistance is connected in between OSC1 and OSC2 finally it forms an RC oscillator.
The serial data input is given to DIN from the output of RF receiver. Initially, HT12D decoder will be in standby mode. As a result, oscillator is disabled and when a high input is given to DIN pin activates the oscillator (you can also understand the overall operation of HT12D decoder by following the flow chart).