The post discusses a 2 tone bell chime circuit which can be used as a door bell for getting a twin tone melodious sound, whenever someone presses the given bell push switch
Constructing an two tone electronic chime is easy due to its inexpensive parts.
When you press the doorbell button, S2, T1 passes a logic low level to NAND gate N1.
Then, it returns with a logic high level at its output causing the oscillator which comprises N2 and N3 to switch around 1 Hz.
As the buffer capacitor C1 stays charged for a while after S2 has been freed, the oscillator will stay and provides the 1 Hz pulses to C4 and C5, in addition to a second oscillator section consisting of N4 and several related parts through R6.
At pin 10 of inverter N3, a logic high-level permits T2 to connect preset P2 similarly with frequency-determining parts R7 to P1.
The twin overlapping frequencies can be attuned with P1 and P2 depending on individual preference.
On top of controlling the tone frequencies of the ring, the 1 Hz pulses dictate the packet shape of the subsequent chime sound through T4 to T5 and its related components.
To establish the required decay characteristics for the two tone sound of the chime, preset P3 is set. Moreover, the emitter follower T6 works like a simple voltage-controlled amplifier that powers the single-chip AF output amplifier LM386.
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