Here is a musical call
bell that can be operated by just bridging the gap between the
touchplates with one’s fingertips. Thus there is no need for a
mechanical ‘on’/‘off’ switch because the touch-plates act as a switch.
Other features include low cost and low power consumption. The bell can
work on 1.5V or 3V, using one or two pencil cells, and can be used in
homes and offices. Two transistors are used for sensing the finger touch
and switching on a melody IC. Transistor BC148 is npn type while
transistor BC558 is pnp type. The emitter of transistor BC148 is shorted
to the ground, while that of transistor BC558 is connected to the
The collector of transistor BC148 is connected to the base of BC558.
The base of BC148 is connected to the washer (as shown in the figure).
The collector of BC558 is connected to pin 2 of musical IC UM66, and pin
3 of IC UM66 is shorted to the ground. The output from pin 1 is
connected to a transistor amplifier comprising BEL187 transistor for
feeding the loudspeaker. One end of 2.2-mega-ohm resistor R1 is
connected to the positive rail and the other to a screw (as shown in the
figure). The complete circuit is connected to a single pencil cell of
1.5V. When the touch-plate gap is bridged with a finger, the
emitter-collector junction of transistor BC148 starts conducting.
Simultaneously, the emitter-baser junction of transistor BC558 also
starts conducting. As a result, the collector of transistor BC558 is
pulled towards the positive rail, which thus activates melody generator
IC1 (UM66). The output of IC1 is amplified by transistor BEL187 and fed
to the speaker. So we hear a musical note just by touching the touch
points. The washer’s inner diameter should be 1 to 2 mm greater than
that of the screwhead. The washer could be fixed in the position by
using an adhesive, while the screw can be easily driven in a wooden
piece used for mounting the touch-plate. The use of brass washer and
screw is recommended for easy solderability.