The Electronic Cat-Bird, by K4ICY

Practical Electronics Projects By Mike, K4ICY

Weekend Radio Click Here for More Electronics Projects and Tutorials By Mike Maynard, K4ICY

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The Electronic Cat-Bird - Try out this simple audio oscillator circuit, and annoy your family!
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Originally published in The Printed Circuit, Newsletter of the Tallahassee Amateur Radio Society, August 2013, page 14
[VISIT HERE] Edited/Updated January 2024
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Here's a fun little circuit your budding electronics-building youngsters should get a kick out of while annoying the rest of the family! It's a simple audio oscillator designed to make (rather unconvincing) animal noises; both a cat's 'meow' and a 'chirping' bird.
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Referring to the schematic [BELOW], the heart of this circuit is a Hartley-type oscillator which forms it's oscillation by use of two inductors, T1's transformer primary windings (with center tap) and a capacitor (C2), arranged together to form a resonant tank. Oscillators are fundamental circuits in analog electronics that generate periodic waveforms and Hartley is but one of many oscillator "topologies," or functional circuit designs. A Hartley tank circuit, like in this example, is resonant because capacitors, to some degree resist the flow of DC while inductors resist the flow of AC, and where combined, they have a mutual affinity for a certain oscillation frequency.

Feedback is provided via C1 (pulled through the center tap of T1) to the base of the PNP BJT transistor (Q1) that provides both oscillation and amplification. T1 also does double-duty while providing the resonant inductors for the Hartley tank circuit; it also matches the 8 ohm speaker's operational impedance to the oscillator circuit. As the base of the transistor 'sees' more positive current from C1, the collector-to-emitter path allows for more negative flowing current at a larger value. The implied sine wave signal is now amplified to a usable level, but it is thus also inverted. When the signal at the base increases, the collector's part decreases. It is through the transformer action of T1, or the total inductor series that flips the sine wave signal 180 degrees in phase. So a large signal value (in phase) is applied to the base and thus oscillation is allowed to be self-sustaining.
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Cat-Bird Hartley Audio Oscillator Circuit

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While the oscillator provides a 'musical tone', it's the other particular discrete components in this circuit that give the amplified result its characteristic sounds. When S2 is selected to "Cat", C3 charges and Q1 eventually is driven to 'shut-off' state. Oscillation is halted at this point. As the electrolytic capacitor begins to fully charge, current is restored to the transistor as the capacitor starts to block DC current. An audio signal is then allowed to be produced at a full level. This action almost sounds instantaneous, but as soon as S1 is released and the battery is taken off line, C3 performs a second action in providing a small amount of reserve power to the oscillator. As power is reduced, both the volume of the tone and even the frequency of the oscillation takes a dive, giving that characteristic cat 'MEOW' sound! R2+R3 allow the frequency of oscillation to change so you can vary the pitch of the sound. Just Push the button while spinning the dial to get your transistorized pet a-talkin’!

Things get a little interesting when S2 is switched to "Bird" mode. Capacitor C3 is then now charged more slowly through resistor R1, but once C3 is charged and current is reduced or stopped through Q1, C3 is now able to start a discharge cycle through the base-emitter junction of Q1. This is kind of a secondary oscillation in a way, but at a much slower rate, and yields that characteristic 'CHIRPING' sound! In this mode, R2 and R3 will affect the charge/discharge cycling of C3/R1 and the 'chirping' speed can be increased or decreased.

Some other notes to mention about this circuit: You can feel free to change any part of this circuit to experiment with the "zoo" of possibilities. However, you should not reduce R2's value at 22k or reduce the value of the 0.05 micro-Farad capacitor C1 or you will likely destroy the transistor (Q1). Q1 can be any type of PNP Bipolar Junction Transistor. If a lower battery voltage is used or the battery’s internal resistance is reduced, the oscillation may destabilize. If this is the case, as noted in the bird-chirp configuration, a 100 ohm resistor could be put in series with the battery.

This circuit has more productive uses than making unrealistic animal noises. As a 'sound-factory', other electronics applications may benefit from unique audio cues, such as alarms. Most parts should be available from any online electronics supplier including Digi-Key, Mouser, Amazon, AliExpress and eBay, but the audio output transformer used in this example was a Radio Shack part no.: [273-1380] but may now be sold for many times over its worth due to the demise of Radio Shack. But when searching online, look for any audio isolation or output transformer with an 8 ohm secondary and a center-tapped primary, measuring anywhere from 500 ohms to 3000 ohms. The wattage type can be very small. And be careful not to spend too much! Any type of voice-coil type tweeter speaker or low wattage PA can be tried for the output speaker.

If you want another interesting effect to experiment with, try gently pressing your finger into the speaker cone to restrict its vibrations to various degrees and you'll notice that the bird chirping repetitions may change its timing frequency! Doing so changes the characteristic impedance of TI's secondary (output) and, through transformer action, the double-inductor portion of the Hartley oscillator tank, consisting of T1's primary coil (with its center tap) and C2. This action changes the inductance within that section and thus changes the overall oscillation frequency.
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by Krishnavedala, CC BY-SA 4.0

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Understanding the characteristics and usages of each major type of basic oscillator topology (and variant circuit arrangements) will help give you better understanding of analog electronics and radio dynamics. Start off by reading more about the different varieties of oscillators by clicking HERE. Summarized from above, the Hartley oscillator employs an inductor-capacitor (LC) resonant circuit with a tapped coil, creating a feedback loop for oscillation. The feedback network is comprised of the inductor and capacitor in parallel, and the tapped coil provides the necessary phase shift for sustained oscillations. As for other types, the Colpitts oscillator, on the other hand, uses two capacitors and an inductor in its resonant tank circuit. The capacitors form a voltage divider across the inductor, producing the required phase shift for oscillation. Both the Hartley and Colpitts oscillators are commonly used in radio frequency (RF) applications due to their simplicity and versatility.

In contrast, the Pierce oscillator is frequently employed in generating stable clock signals for microcontrollers and digital systems as well as in some Local Oscillator and PLL tuning circuits used in radios. It employs an amplifier with a crystal resonator, usually operating at a fundamental frequency. The crystal provides the necessary feedback and frequency stability, making it advantageous for applications requiring precise and stable oscillations. Unlike the LC tank circuits of Hartley and Colpitts oscillators, the Pierce oscillator relies on the resonance of the crystal to maintain its oscillations, offering superior frequency accuracy and stability in various digital systems. And those are only three types I'll mention here for example. Oscillators literally form the beating "heart" of many electronics applications including that of little electronic cats and birds, and should be a point of study for every electronics enthusiast and homebrewer.

Go ahead and invite your youngsters up to the workbench and have a little fun with this circuit. Drive your spouse and pets nuts!
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The weekend is here, so go and build something!
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73! DE Mike, K4ICY MikeK4ICY@gmail.com