Resistor R1, capacitor C1, and two inverters from a square wave generator, which produces the basic tone. The generator is followed by an inverter that acts as both a buffer and a driver for the speaker. Resistor R2, which has a minimum value of 100 ohms, limits the current and controls the volume. Diode D1, capacitor C2, resistors R3 and R4, and two inverters create the pulse generator that determines the turn-on and decay times of the chime. The decay circuit-formed by D2, C3, R5, and Q-reduces the amplitude of the chime tone exponentially as a function of time.