Delayed Audible Alarm

In this circuit, three multivibrator-type oscillators and a ripple counter are configured to produce an interesting, alarming sound, using two inexpensive integrated circuits plus a few other components.

Operation is as follows:  U1A and U1B and their associated components are configured as multivibrators, each of a selected audio frequency. Their square-wave outputs are combined via CR1 and CR2, which are two inputs of a three-input AND gate that produces the modulated product waveform of the two frequencies, present at their common anodes. This waveform is fed to the base of transistor Q1 via RN2C and RN2B. Q1 drives loudspeaker SP1 via R4. The waveform can be inhibited by the third AND gate input, the cathode of diode CR3, which is controlled by the output of binary ripple counter IC U2.

Upon setting switch SW1 to the ENABLE position, power is applied to the circuit. Binary ripple counter IC U2 is immediately reset by C4 charging through RN2G at U2's pin 11, initially placing output Q12 in a low state, inhibiting the waveform. The multivibrator consisting of op amp U1C and associated components provides a square wave through diode CR4 that clocks the U2 counter at approximately 2.6Hz. After 2¹² counts,  equating to about 14 minutes, U2's Q12 output goes from a low to high logic state, turning off diode CR3, and permitting the waveform to appear at the base of transistor Q1, driving loudspeaker SP1. When counter output Q12 transitions to a high state, it also turns on diode CR5, which jams the counter's clock input to a high state, preventing further counting. Therefore, the audible waveform will remain present after the delay, as long as power is applied to the circuit.

Resistors RN2B, C, and D form an attenuation network that is controlled by U1C's 2.6Hz squarewave output. The voltage waveform presented to the loudspeaker is accordingly modulated between two amplitude levels, providing further complexity to the sound as a rudimentary but effective echo effect.

Op amp section U1D and its two associated resistors provide a half-supply reference for the three multivibrator oscillators. Frequency-determining resistors R1, R2, and R3 may be selected per preference, but note that R3 will control both the delay and the echo modulation rate. The delay may be conveniently shortened by halves simply by substituting U2's Q12 output with its Q11 output (for about 7 minutes), Q10 (for about 3.5 minutes), etc. Capacitors C5 and C6 provide decoupling for the ICs, and should be connected close to their packages. Capacitors C1, 2, and 3 should be plastic film types for good temperature stability. Two dual-inline-packaged (DIP) resistor networks were employed to facilitate ease of construction, however, standard discrete resistors may be used in substitution. Loudspeaker SP1 is a 45 Ohm type for compatibility with the small transistor driver, with R4 providing current limiting and isolation from reactive effects at the transistor's emitter. CR6 is a protection rectifier to prevent damage to the circuit in the event of reversed battery polarity. Since all three multivibrators exhibit exceptional power-supply independence, no voltage regulation is required for this circuit. Regardless of the low power, an inexpensive, 3 or 4-inch speaker still provides ample volume in most situations. It is important to note that this circuit will not stop automatically; once the delay time has elapsed, the only way to silence it is to set switch SW1 back to the OFF position.

July 31, 2010
Text and images ©2010 by Arthur Harrison

Source documents circa 1991

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