Audio Circuit DiagramsZener Diode

EMD-immune Electronic Doorbell Schematic Circuit Diagram

Whenever an antediluvian electric doorbell is used in an apartment building, the rain of sparks that are generated when the Wagnerian hammer pounds against the ‘sounding body’ infests the bell network with interference pulses. These can significantly disturb electronic doorbells, or even cause them to give up the ghost. If you cannot convince your neighbor to convert to something more modern, or at least to build in a noise suppression network, you can use the electronic doorbell described here, since it is immune to EMD.

EMD-immune Electronic Doorbell Schematic Circuit Diagram

This circuit is based on a simple multivibrator stage to which a loudspeaker is connected. Capacitor C4 provides dc isolation between the multivibrator and the loudspeaker (8 Ω, 0.25 W). The frequency is determined by the RC networks R2/C2 and R3/C3; it lies at around 0.7 RC = 2 kHz. The multivibrator stage receives its supply voltage from the bell transformer. For this purpose, the ac voltage must be rectified by D3-D6, and Zener diode D7 prevents the voltage from rising above approximately 18 V. EMD immunity is provided by the lowpass network R5/C1. The bell can also be silenced using switch S2. In this case, the only thing that happens when someone presses the bell button is that D1 lights up.

A multivibrator is an electronic circuit used to implement a variety of simple two-state devices such as relaxation oscillators, timers, and flip-flops. The basic principle of the Astable multivibrator is a slight variation in the electrical properties or characteristics of the transistor. This difference causes one transistor to turn ON fast than the other when power is applied for the first time, thereby triggering oscillations.

Astable Multivibrators find their applications in delay and timing circuits and in the transmission and reception of radio signals. Monostable multivibrators are majorly used in analog systems in order to control the frequency of the signal at the output. One of the major advantages is that they can work continuously. They are also the simple type of multivibrator. Also, they are quite cheap so they can be helpful financially.


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