A common piezoelectric alarm, such as the Murata (Smyrna, GA) PKB5-3A shown, has many valuable attributes. It is compact, lightweight, efficient and reliable. However, the loud, high-pitched sound output can be quite irritating in many applications. The simple and inexpensive circuit provided here transforms this obnoxious little buzzer into a pleasantsounding bell (see the figure).
At the heart of the circuit is the popular 555 timer U1, which is configured as an astable multivibrator. The output low time is a short pulse that initiates the bell strike. The strike time is 0.693 ↔ R2 ↔ C1. For the component values shown, the strike time is 6.93 ms. The output high time is the sustain time during which the amplitude of the bell tone decays continuously. The high time is 0.693 ↔ (R1 + R2) ↔ (C1). For this circuit, the high time is 236 ms.
When the output of U1 is low, the series transistor switch Q1 turns on through bias resistor R3 to energize the piezoelectric alarm AL1 and charge capacitor C3. When the output of U1 is high, Q1 is off and the output of AL1 decays until C3 is discharged. When C3 is discharged again, the cycle is repeated.
The values of timing components R1, R2, and C1, along with decay capacitor C3, aren’t critical; various sound effects can be produced by experimenting with them.