Power-on reset is provided by the combination of resistor R3 and capacitor C1. Switch S17 is used for manual reset. A 12MHz crystal along with two 33pF capacitors provides the basic clock frequency for operation.
The receiver address to be transmitted can be set with the help of 8-way DIP switch S18. Port pins P1.0 through P1.7 of the microcontroller are interfaced with the keypad. Pins P3.0 and P3.2 through P3.5 are interfaced with TE pin and data inputs AD8 through AD11 of encoder HT12E.
When all switches (S1 through S16) are opened on the keypad, the microcontroller pulls the TE pin as well as data input pins AD8 through AD11 to logic 1. If any switch is closed, the microcontroller pulls the corresponding data pin along with TE pin to logic 0. When switch S1 is closed, the microcontroller makes pin 10 (AD8) and pin 14 (TE) of encoder HT12E low, and logic 0 is transmitted through TX1. The other data pins of encoder HT12E will be in logic 1 state in this case. LED1 glows to indicate transmission enabled.
Fig. 3 shows the receiver circuit with SM RX-433 RF module. AT89C51 is a low-power, high-performance CMOS 8-bit microcontroller. It has 4 kB of Flash, 128 bytes of RAM, 32 I/O lines, two 16-bit timers/counters, five-vector two-level interrupt architecture, a full-duplex serial port, on-chip oscillator and clock circuitry.