This three-tone bell can be enlarged for more sounds and even a small electronic toy organ. Powered by 2 or 4 batteries, it has a negligible consumption in the standby condition.
The three trimpots allow three different tones to be set in the drive of 3 pressure switches, allowing identification of the origin of a call.
With more trimpots and more switches we can extend this bell to a larger call system. In figure 1 we have the complete diagram of this apparatus.
In figure 2 we have the assembly that uses as a base a terminal strip.
The transistors allow equivalents and the resistors are 1/8 W. Values of 22 nF to 220 nF can be used for C1. Pressure switches may be remote.
The power can be made with 2 or 4 small batteries. S4 serves to disarm the circuit, being optional, because the consumption is very low when the circuit does not emit sound.
For higher sound intensities, change Q2 to a BD136 and power the circuit with 9 or 12 V source.
Material list:
Q1- BC548 - general purpose NPN transistor
Q2- BC558 - PNP general purpose transistor
FTE- 4 or 8 ohm- common speaker
B1 - 3 or 6 V- 2 or 4 batteries
S2 to S4 - Pressure switches (buzzer button)
R1 - 4.7 k ohm x 1/8 W- resistor - yellow, violet, red
R2 - 1k ohm x 1/8 W- resistor - brown, black, red
P1, P2 P3 - 100k ohm trimpots
Miscellaneous:
Terminal bridge, battery holder. Box for mounting, wires, welding, etc.
