This is a circuit of RC5 Infrared Remote Control Transmitter. This circuit uses The PCA8521 that when a key pressed, the PCA8521 generates It generates output pulses, in accordance with the RC5 protocol. Although the IC doesn’t contain a software programmable processor, it contain a ROM which can be used to store the codes that have to be transmitted. Here is the circuit:
The optimal oscillator frequency for this circuit is 432kHz or 4MHz. For a 4MHz oscillator, the capacitor is integrated in the circuit. But we must add additional external capacitors for 432KHz oscillator. The oscillator will start, when a key in the key matrix is pressed because a sense line and a drive line are connected. So, it will generate the corresponding code to the RC5 protocol.
This device has eight sense lines (SN0 to SN7) and Seven drive lines (DR0to DR6). There is no transmission will take place when two or more keys are activated simultaneously. [Source: NXP Application Note]