A very simple musical door bell circuit can be built and installed in homes, the design features a replaceable music chip option, and an adjustable ringtone duration, as per the user preference, we'll learn the entire circuit procedure through the following article.
Today we can find a huge range of door bells in the market in all shapes, sizes and specs, and we have all the options of choosing the right one for our home.
Finding the Right Doorbell
However searching for the right door bell that would have the most pleasing sound according to our preference could be quite tedious and normally we tend to succumb to the retailer's views and choice and finally buy the one that the shopkeepers endorses us to buy.
The idea of the musical doorbell presented here is simple and yet provides the user with some very useful features which are normally absent even in the most sophisticated of the door bell models available ready made in the market.
The proposed circuit allows the user to select the musical chip as per his/her own choice which may be replaced with another tune whenever the user needs it, just for a change.
Another useful feature is the duration for which the bell sounds, which is adjustable here, and the user has the facility to determine for how much long the door bell should play once the door bell button is pushed and released.
The following diagram shows how a UM66 IC musical tone generator can be used for door bell application
The diagram above depicts the proposed musical door bell circuit, the various stages may be understood with the following explanation.
The BC547 transistor along with the associated preset and the 100uF capacitor form a simple delay OFF timer circuit, where the preset and the capacitor determines the delay for which the sound output may be sustained once the indicated push button is pressed momentarily.
The BC557 transistor functions like a switch and triggers ON/OFF in response to the conduction of the BC547 stage.
The collector of the BC557 can be seen linked with a COB which is the acronym for Chip On Board device, which is an embedded musical tune IC. This IC is programmed internally to produce the specified tune as soon as a 3V potential is applied across its supply terminals. The sound signal is acquired from the extreme right copper layout strip of the chip.
Since the output from the COB could be very low in current, it needs an amplification before the programmed sound can become loud and audible across a give area.
The third transistor 2N2222 is positioned to accept the weak sound signals from the COB and amplify it over the connected 8 ohm speaker.
How COB (Chip on Board) Works
These COBs are plentifully available in the electronic spare part market today, and these come with a diverse range of different tunes such as Christmas melodies, birthday songs, new year tunes, congratulation wishing tune, animal sounds, and many other customized speech forms depending upon the users specifications.
In case you are unable to find these chips, a decent alternative could be in the form of the IC UM66 which are much popular and easily available in the market anywhere across the globe or could be also procured from any online electronic store.
The shown push button attached with the circuit is supposed to replace the home bell push button and may be extended to any distance, this will not affect the circuit performance.
When the push button is pressed, the BC547 switches ON and continues to conduct even after the switch is released due to the stored energy inside the 100uF capacitor.
The BC557 transistor responds to this and also switches ON supplying the required 3V potential to the attached COB, which now begins buzzing with the embedded piece of programmed tune.
The musical signal from the COB is forwarded to the next amplifier power transistor which instantly amplifies the music signals driving the connected speaker with a loud musical door bell sound.
The music continues only as long as the 100uF capacitor is able to sustain, and the music stops as soon as the 100uF is completely discharged.
The 100k preset may be set as per the user preference for enabling the desired length of the musical tune in response to every push of the bell button.
Door Buzzer Circuit
The above door buzzer circuit will produce a sharp buzzing sound whenever the push button is pressed by any guest at the door.
This design is suitable for users who do not wish to have a musical sound, instead prefer a buzzer kind of sound which sustains only as long as the button remain depressed.
The circuit is basically a square wave oscillator built around the op amp LM351. You can actually use any op amp instead of the specified one.
The parts C3, R7 decide the oscillation frequency which in turn produces the required buzzer door bell sound on the connected loudspeaker.
TR1 can be also replaced by any 1 amp NPN transistor if the specified number is not available.
Door Bell Using UJT and IC 555
IC1 is configured as the tone generator, and this is a common 555 employed as a free-running multivibrator. Capacitor C4 is charged up to 2/3 of the supply voltage through R5 and R6, and subsequently discharged down to roughly 1/3rd supply voltage through R6 and IC1. This charge/discharge course of action keeps repeating non-stop. During this process the output pin 3 of ICI turns high as C4 charges, and turns low as it discharges.
The waveform generated at pin3 is used to operate a loudspeaker which as a result produces the door bell audio tone. The 1/3rd supply voltage limit where C4 begins discharging is influenced externally by implementing a control voltage to pin 5 of IC 555 through a UJT oscillator circuit. When this external UJT frequency at pin5 voltage increases, the charge and discharge periods of C4 are extended, creating a reduced working frequency. As the UJT frequency at pin5 decreases, the charge and discharge periods of C4 likewise decreases, causing a higher operating frequency to be generated. The door bell audio tone generated by the second frequency generator is as a result frequency modulated through a control voltage fed at pin#5 of IC1.
The warbling sound effect is generated by applying a control voltage which increases and decreases several times each second. The nature of the output audio tone relies greatly on the waveform of the modulating signal from the UJT circuit, which is identical to a sawtooth signal. This kind of waveform voltage goes up relatively slowly after which all of a sudden drops down to its lowest level.
This in fact provides a continuous decrease in output frequency accompanied by a fast resuming of the original high frequency even though this process happens very quickly to be distinctly recognized, allowing a pleasurable warbling effect to be heard on the speaker.
A unijunction relaxation oscillator is employed to build the modulating signal. Capacitor C2 is charged by means of resistor R3 right up until a charge potential of approximately 7 V is reached, after which C2 starts discharging extremely fast by means of Q1 and R2. Transistor Q1 subsequently shuts down, C2 begins to charge yet again, and so forth. R4 is used to connect the Q1 output with pin 5 of IC1.