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    First published on: 1-6-2006



    Here is a mains-operated door-bell that produces parrot-like sweet voice without requiring any musical IC. The circuit is cheap and easy to construct. The AC mains is fed to the circuit without using any step-down transformer.

    The complete circuit is shown in Fig. 1. The main components of the circuit are a resistor-capacitor network, transistor BC337 and audio output transformer X1. The oscillation frequency depends on the combination of resistors R4 and R5 and capacitors C3, C4 and C5. When switch S1 is closed, the audio signal generated due to oscillations is amplified by transistor BC337 and parrot-like sound is reproduced from loudspeaker LS1 connected across the secondary of transformer X1.

    Fig. 1: Circuit of parrot-sounding doorbell

    Here we have used an 8-ohm, 0.5W loudspeaker. The audio output transformer (X1) is normally used in transistor radio. The function of the audio output transformer is to transform the high impedance of the output amplifier to match the much lower impedance of the speaker. This is necessary to get an efficient transfer of the audio signal to the speaker. If a wrong audio transformer is used, the result can be low output and loss of tone quality. The audio frequency tone across the speaker terminal is about 3 kHz. The dimensions of the audio transformer used in the experimental setup are shown in Fig. 2.

    Fig. 2: Dimensions of audio transformer

    The circuit is powered directly from 220V AC mains. The operating DC voltage obtained at the cathode of diode D1 is about 6V. However, if you press switch S1 continuously for a few seconds, the maximum voltage developed at this point may go up to 20 volts, which must be avoided to prolong the life of the circuit. R1 limits surge current in the circuit. The parallel combination of resistor R1 and capacitor C1 limits the circuit current to a safe level for circuit operation. R2 across C1 provides DC path for the current as well as a discharge path when the circuit is switched off. This is to prevent a possible shock to the operator by charged capacitor C1.

    Fig. 3: Pin configuration of BC337



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    commentPost Comment | 7 Comments

    khushboo thakare   1181 days ago

    what is the transformer rating?
    akshay vedpathak   1148 days ago

    i do it.
    Hashmi   1120 days ago

    I made the circuit but no sound is heared...... Plz... give the correct information of the circuit.
    commentReply   1 Reply
    Efy Admin   1117 days ago

    Please check the connections properly especially transformer connections. You need to use the exact component values as mentioned in the circuit otherwise you will not get oscillation and sound. The prototype available with us is working well.
    richard   1038 days ago

    This circuit looks Dangerous the only sound your going to hear from this are the tormented screams of agony as whoever presses the push to make switch electrocutes themselves
    abhijeet   614 days ago

    Ckt working fine but now sound is coming as low ocilation like batt. Low so tell me which part to be replace. I think both electrolyte capacitor to be changed pl.tell me.
    Kaushik Mondal   604 days ago

    I have a driver transformer used in audio output section in my old radio BEL 7 transistor with AC 128. It has two input line and four output lines. can I use the transformer in this circuit by joining two output lines ?

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