The presented circuit of a musical greeting card circuit was requested by one of the keen readers of this blog, so I designed this interesting little circuit, which is simple and easily gets embedded inside any standard greeting card fold.
Looking at the given circuit diagram, we see a design consisting of a very few number of components, rigged via connecting wires.
The design may be understood with the following explanation:
The main music generating component is the IC UM66, which has a digitally processed music data embedded inside it.
The IC just requires a 3 volt supply for reproducing the content over an appropriate speaker.
Since the output from the above IC is pretty low, it requires some kind of amplification before it can be practically heard, a transistor stage needs to be introduced.
The BC547 transistor performs this function well and is therefore positioned for amplifying the tiny music info inside the IC.
To keep things extremely thin and slim, a piezo is incorporated as the speaker as these devices require hardly any space to sit and also operate with nominal signal outputs.
The amplified music from the transistor is though not too powerful, is significant enough to drive the connected piezo element so that the user is able to hear it at reasonably good volume.
Now the main criterion with a musical greeting card is that, the music should play only when the card is opened and should shut off when it's closed.
In order to implement this we need some kind of trigger arrangement, a leaf switch could have been a good option, however employing an LDR looked more technical and solid state.
An LDR here becomes the base bias resistor for the transistor. When the card is opened, ambient light falls over the LDR lowering its resistance value which in turn switches the transistor and music starts playing.
The moment the card is closed, LDR is inhibited from the ambient light, it's resistance shoots up in Mega Ohms choking up the transistor conduction and instantly switches OFF the music from the piezo.
A 3V button cell becomes enough for sustaining the functioning of the system almost forever.
The entire shown circuit should be carefully fixed inside a good quality greeting card, making sure that the LDR is able to "see" the ambient light whenever the card is opened.