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I used this circuit to ring my doorbell
rapidly three times when a car pulled into my driveway. R2 and C2 set the output
frequency. Use a diode and resistor to change the duty cycle, if desired. R1
and C1 set the cycle time which sets the number of pulses. R3 can be deleted if you
have other loads on the power supply. The circuit is activated by applying power to
it. The first pulse with be longer than the others.