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16-TONE DOOR BELL PROJECT
This is a very easy to construct DIY project that you can do on a weekend. With only a few components, you can build a sophisticated selectable 16-tone doorbell.
The circuit consists mainly of four (4) sections, first is the sound generator section, second is the audio amplifier section, third is the power supply section and lastly, the regulator section.
The Sound Generator Section
This section produces the sound output that you will hear once the push button PB1 were pressed (see Schematic 1). The sound generator IC (U2), with part no. EDC1901-A - a product of Electro-Digital Crafts, is basically a microcontroller unit (MCU) programmed to produce sound output. It accepts four (4) digital inputs coming from the DIP switch (SW1) to select desired tones. The audio signal from the "audio out" terminal can be fed to the audio amplifier section thru the 10K Ohms variable resistor/potentiometer (VR1).
The Audio Amplifier Section
The amplifier section is where the weak signal from the sound generator IC is being strengthened to drive a loud speaker. The heart of this section is the LM386 IC (U1) which happen to be a complete 1-watt audio amplifier. With very few components we can wire this IC to perform its function. Loudness can be controlled using the variable resistor/potentiometer VR1 and 10K Ohms limiting resistor R1. The 8 Ohms loud speaker (LS1) were coupled to the output of the amplifier IC using a 1000uf/16v electrolytic capacitor (C1).
The Power Supply Section
This section provides power to the circuit coming from the AC mains (220VAC). It converts the alternating current to direct current which is needed to power up the circuit. The transformer TR1 reduces the 220VAC by stepping it down to 12VAC. The 1A fuse FU1 act as a protection for accidental short circuit and the switch SW2 cuts/closes the current path going to the transformer - power on/off switch. The 1N4001 diodes D1 and D2 connected to provide full wave rectification of the alternating current. Rectification means allowing the positive cycle to pass through. After the rectifier stage, you cannot get a clean direct current, so you will need a filter to straighten the ripple. The 1000uf/16v Capacitor C2 performs the filtering to minimize the ripple voltage.
The Regulator Section
This section regulates the voltage coming out of the power supply section, that from time to time, the output voltage fluctuates. This fluctuation is very harmful to the entire circuit. The 7805 regulator IC (U3) performs the function of minimizing fluctuation together with capacitor C3 and C4. The light emitting diode (LED) D3 act as a power indicator together with the 330 ohm limiting resistor R3.
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