Musical Touch Bell

At the heart of the Musical Touch Bell lies the melody generator IC BT66T (or UM66). The circuit employs two transistors for sensing the finger touch and switching on the melody generator IC. The circuit connections are as follows: the emitter of transistor T1 (BC548) is shorted to the ground while that of transistor T2 (BC558) is connected to the positive terminal of the supply. Also, the collector of transistor T1 is connected to the base of T2, further the base of T1 is connected to the washer. 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. The underlying mechanism is very simple. When the touch-plate gap is bridged with a finger, the emitter-collector junction of transistor T1 starts conducting. Consequently, the emitter-base junction of transistor T2 also starts conducting. As a result of this, the collector of T2 is pulled towards the positive rail, which thus activates the melody generator IC1 (BT66T) to produce musical tunes at its output pin in the form of a varying electrical signal. The transistor BEL187 (or BC547) receives these variations at its base and then amplifies them in order to feed them to the loudspeaker. The loudspeaker converts these amplified electrical variations into audible sound, thereby generating the melodious musical tone. 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 screw head. 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 soldering. Application of the melody generator IC BT66 (or UM66). Application of PNP and NPN type transistors for switching purposes. Brand: Kits'n'spares, Marketed by: Kits'n'spares