Cuckoo Song Generator: 667Hz & 545Hz

Description

This circuit generates a two-tone effect remarkably similar to a cuckoo’s call. It can be employed for applications such as doorbells or other sound effects due to its built-in audio amplifier and loudspeaker. Serving as a sound effect generator, it can be connected to external amplifiers, tape recorders, and other devices. In these configurations, the integrated audio amplifier and loudspeaker may be omitted, and the output can be taken across terminals C8 and ground. There are two operating modes available: free-running, when switch SW1 remains open, and one-shot, when SW1 is closed. In the one-shot mode, a distinct two-tone cuckoo song is produced with each press of button P1.

IC1 is configured as a square wave generator, producing both the higher and lower tones of the cuckoo song. The frequency of the higher tone (667Hz) is adjusted using trimmer resistor R2. When the output of IC2D goes low, a further trimmer resistor (R22) is added between IC1’s timing components via diode D6, generating the lower tone (545Hz). To accurately replicate the cuckoo song, the square wave output of IC1 is converted to a quasi-sinusoidal waveform utilizing resistors R3 and R4, capacitors C3 and C4, then mixed with the white noise produced by transistor Q1, connected through resistor R6.

Transistor Q2 has two functions: it mixes the two incoming signals and gates the resulting tone, shaping its attack and decay behavior through the components connected to its emitter. Integrated circuit IC4 is the audio power amplifier, driving the loudspeaker, and resistor R15 serves as the volume control. The various timing and pause durations for the circuit are provided by the clock generator IC2A, which drives the decade counter IC3. Certain output pins of this IC are gated by IC2C, IC2D, and related components, controlling the sound generator and the sound gate appropriately.

When switch SW1 is left open, the circuit operates in free-running mode, continuously generating the cuckoo song. When SW1 is closed, the circuit produces two tones followed by a stop, because a high-state signal appears at the last output pin (#11) of the decade counter IC; consequently, the count is inhibited by diode D1 feeding into pin #13. The circuit is reset by a positive pulse at pin #15 of IC3 when button P1 is pressed.

Circuit diagram:

Parts:

Circuit operation:

IC1 is wired as a square wave generator and produces both tones of the cuckoo song. The frequency of the higher one (667Hz) is set by means of trimmer R2. When IC2D output goes low, a further trimmer (R22) is added to IC1 timing components via D6, and the lower tone (545Hz) is generated. To imitate closely the cuckoo song, the square wave output of IC1 is converted to a quasi-sinusoidal wave form by R3, R4, C3 and C4, then mixed with the white noise generated by Q1, R6.

Q2 has two purposes: it mixes the two incoming signals and gates the resulting tone, shaping its attack and decay behavior by means of the parts wired around its Emitter. IC4 is the audio power amplifier driving the speaker and R15 is the volume control. The various sound and pause timings for the circuit are provided by the clock generator IC2A driving the decade counter IC3. Some output pins of this IC are gated by IC2C, IC2D and related components to drive appropriately the sound generator and the sound gate.

When SW1 is left open the circuit operates in the free-running mode and the cuckoo song is generated continuously. When SW1 is closed, the circuit generates two tones then stops, because a high state appears at the last output pin (#11) of the decade counter IC: therefore the count is inhibited by a diode D1 feeding pin #13. The circuit is reset by a positive pulse at pin #15 of IC3 when P1 is pressed.

Setup:

Best results will be obtained if the two tones frequencies are set precisely, i.e. 667Hz for the first tone and 545Hz for the second: in musical terms this interval is called a Minor Third. Obviously a digital frequency counter, if available, would be the best tool to setup R2 and R22, but you can use a musical instrument, e.g. a piano or guitar, tuning-up the notes accurately by ear.

Tuning by ear:

Notes: