The circuit uses two 555 timers configured as mono-stable oscillators. The first mono-stable is triggered by the doorbell switch, S1 and this mono-stable controls the time the doorbell will ring for. The on period is set by R5 and C3. The formula is below:On-Time = 1.1 * R5 * C3
With values shown this gives about 1 second ring time. The mono-stable trigger requires a single negative going pulse. This is derived from the standard doorbell switch S1. C1 is discharged by R1 and R2, when S1 is pressed, C1 quickly charges via R2. This provides a brief negative pulse filtered via C2 and R3 and applied to the trigger input. C2 also prevents spurious operation (triggering) by noise pulses . The output of the first 555 is fed via R7 and switches on Q1. Q1 in turn switches on Q2 which provides sufficient current to drive the door bell. As the doorbell, is an electro-magnet containing inductance, when power is removed there will be a large back EMF. D2 prevents the nack EMF from destroying transistor Q2.
Now for the second 555 Timer. When the doorbell switch is pressed, it simultaneously triggers both 555 mono-stables, the second circuit receives the negative going trigger via R4. This pulse controls the de-activation time, (or time before the doorbell can be rung again). The deactivation or off time is set by R6 and C6 and is given by:Off-Time = 1.1 * R6 * C6
With values shown this is about 10 seconds before the doorbell can be rung again. The output pulse is fed via D1 back to the trigger input of the first 555. As the output pulse id high, it prevents the first mono-stable being triggered again, until the timing period has ended.
The power supply is from a single step down transformer with a secondary output of 10V RMS and rated 0/5 Amp/ For protection, this transformer is thermally fused. The primary should be chosen for local mains, 230V for UK or 120V for North America. The AC output is full wave rectified by the bridge rectifier and smoothed by C7. The DC supply voltage will be around 13 Volts DC and it is recommended that C3,C6 and C7 be rated 16 Vdc or higher,Links
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