Installing a doorbell in an existing building is a difficult task as it involves wiring that can look shabby if not concealed properly. Presented here is a circuit that does not require external wiring and can be placed in small enclosures. This circuit can be used as a call bell in office, too.
Circuit and working
This circuit consists of transmitter and receiver units.
Transmitter. Circuit diagram of the transmitter is shown in Fig. 1. The transmitter circuit is built around 5V voltage regulator 7805 (IC1), encoder HT12E (IC2), a DIP switch (DIP1) and a few other components. IC2 converts 12-bit (8-bit address and 4-bit data) parallel data to serial data, which is available at its DOUT pin.
DIP1 is used to set the address bit either high or low. All 4-bit data pins (AD8 through AD11) are connected to ground to reduce power consumption because 433MHz RF transmitter module (TX1) uses on-off key (OOK) modulation. When the doorbell pushbutton switch (S2) is pressed, data along with the address is sent serially through wireless transmitter module TX1.
OOK modulation is the binary form of amplitude modulation. When data being sent is low, the transmitter is fully off, suppressing the carrier. In this state TX1 consumes very low current of about 1mA.
When data being sent is high, the transmitter is fully on. In this state current consumption of TX1 is high of about 11mA with 3V power supply.
Receiver. Circuit diagram of the receiver is shown in Fig. 2. The receiver circuit is built around 5V voltage regulator 7805 (IC3), decoder HT12D (IC4), NE555 timer (IC5), melody generator UM66 (IC6), audio amplifier LM386 (IC7) and a few other components.
Serial data transmitted through TX1 is received by RF receiver module RX1. It is fed to pin 14 of the decoder. IC4 converts the 12-bit data into 8-bit address and 4-bit data. DIP2 is used to set the address of the decoder.
The 8-bit address of the decoder must match with the encoder to receive the information. The decoder checks the serial input three times continuously. If address bits of transmitter and receiver match, data is decoded and valid transmission VT pin of IC4 goes high. This triggers NE555 configured in monostable mode.
NE555 generates a high pulse for about five seconds, whose period is determined by resistor R5 and capacitor C6. Time period of NE555 is determined by the relationship:
Time period (in seconds) = 1.1×R5×C6
It means, when S2 is pressed momentarily, provided S1 and S3 are closed, output pin 3 of IC5 goes high for about five seconds. This output pulse activates the melody generator (IC6) and so the melody sounds up to about five seconds.
Zener diode ZD1 regulates the output of IC5 to 3.3V, which drives IC6. Output of IC6 is given to IC7 through potmeter VR1. Gain of the audio amplifier is set to 200. VR1 controls the volume of the sound before amplification.
Construction and testing
An actual-size, single-side PCB layout for the transmitter is shown in Fig. 3 and its component layout in Fig. 4. Similarly, an actual-size, single-side PCB layout for the receiver is shown in Fig. 5 and its component layout in Fig. 6.
Download PCB and component layout PDFs: click here
After assembling the circuits on two separate PCBs, enclose these in suitable plastic boxes. Use approximately 17cm long single-strand hook-up wire antenna for the transmitter and the receiver each. The receiver unit requires a well-regulated 9V DC power supply for low noise and efficient operation. Alternatively, a 9V battery each for the transmitter and the receiver may be used as power supplies.
A. Samiuddhin is B.Tech in electrical and electronics engineering. His interests include LED lighting, power electronics, microcontrollers and Arduino programming