Created: Aug 27, 2014
No description available.
Doorbell is a signaling device used to call the attention of the occupant inside the building when there is a visitor, this can be used on the entrance of a house or office. This project is a wireless doorbell so we can get rid of the hustle on the wiring job.
This project uses RF (radio frequency) for the transmitter and the receiver circuits to communicate. The transmitter circuit is made up of 303MHz RF oscillator and 32kHz crystal controlled oscillator. The 303MHz oscillator consists of a self-oscillating circuit made of a coil and a 9pF capacitor. A 7cm length of tinned copper wire is connected to the collector of the transistor to increase the range of the transmission. When the push button is pressed, the circuit produces a 303MHz carrier with a 32kHz tone. The receiver circuit detects the 32kHz signal to turn the sound chip on.
In the receiver circuit, a low impedance path is provided for the emitter by using a 82µH choke and a 2.2nF capacitor across a 560Ω resistor. This low impedance path is needed so that the 9018 transistor will have high gain. The signal on the collector of the transistor is fed to first inverter of the 74ABT04D hex inverter through 5.6kΩ resistor and 1nF capacitor. The second and third inverters amplify the signal. The 32kHz crystal only allows the 32kHz signal to be inputted to the base of 9014 transistor, any other frequencies will not be detected. The signal is further amplified through the fourth and fifth inverters of 74ABT04D. The signal is then passed to a diode pump that charges a 47µ capacitor. Normally, the capacitor is uncharged and the sixth inverter is high, the 8050 transistor is off and the sound chip is off.