The novel circuit light and touch operated music bell used a photo transistor (T1) with an npn transistor in emitter-follower mode to increase sensitivity of the circuit. So even if very little light fall on the photo transistor, it gives a triggering pulse to SCR gate and the SCR starts conducting.
Circuit Description of Light and Touch Operated Music Bell
When SCR conducts, IC UM66 starts producing music. After the music has halted, no current flows through the SCR and the SCR switches off till the light falls again on the photo transistor.
If the touch plate is inserted in parallel to switch SW1, as shown in circuit diagram of light and touch operated music bell. Whenever the plate is touched by someone it gives a triggering pulse to the SCR which again produces music through the IC. So the same circuit can be operated either by light or touch. Push button switch SW1 permits the circuit to be operated in the normal way, i.e. by pushing the bell switch.
PARTS LIST OF LIGHT AND TOUCH OPERATED MUSIC BELL
Resistor (all ¼-watt, ± 5% Carbon)
R1 = 2.2 KΩ
R2 = 220 Ω
C1 = 1000 µF/10V
IC1 = UM66
T1 = 2N5777
T2 = BC104
T3 = SL100
SCR1 = SN204
SW1 = Push-To-On Switch
LS1 = 8 Ω speaker
B1 = 3V to 5V DC