This bird sound generator circuit was one of the first circuits I built as a child. It is a fairly simple and small circuit made up of only a transistor and some common components like resistors, capacitors, an audio transformer, and a few others.
The function of this circuit is straightforward: when pressing the switch, it will emit a baby bird or a chick sound. It also consumes only 20mA of power, allowing us to power it with a 9V battery. However, using a 12V battery will result in a slightly louder sound.
Because this circuit is relatively simple, it is an excellent way to learn about oscillator circuits. In the case of this circuit, it uses an LC oscillator. Which is a type of feedback oscillator that functions with an inductor and a capacitor.
How It Works
From the complete circuit diagram below, the oscillator part of the circuit includes Q1, C1, C2, and T1.
To better understand how the oscillator circuit works, we will ignore the rest of the circuit for the time being. The interesting bit here is the transformer, which is an audio transformer designed for audio circuits, and in this case, we are using it to drive a speaker.
We can divide the audio transformer into L1, L2, and L3 inductors. L1 and L2 are from the primary coil, with a center tap in between them. L3 is from the secondary coil.
(1) It begins with current flowing through R1 to bias Q1, (2) which allows current to flow through the center tap through L2 and Q1’s C-E to ground. (3) Some current also flows through R1 to charge C1.
When a current flows through L1 and L2 (the primary coil), a magnetic field is formed in T1. Which then induces electrical energy into L3 (the secondary coil), resulting in an increase, or ‘high’ phase, in the output signal.
Then, the magnetic field inside T1 collapses, creating opposing electrical energy or reverse polarity that flows out from L1. This negative current flowing through C1 to B of Q1 causes Q1 to stop working and the output to drop to zero.
When C1 is completely discharged, the current begins to flow through R1, and the process repeats. This cycle occurs repeatedly, resulting in oscillation with an output waveform as shown on the oscilloscope below.
Changing the capacitance of C1 to a lower value will change the output frequency as shown here:
Or, in other words, an increase in pitch.
Next, we add C3 and R2 to B of Q1; they cause Q1 to stop conducting rhythmically. The oscillation stops while C3 is charging. It restarts again when it discharges through Q1’s base-emitter.
Therefore, if we want the sound to output less frequently, we can increase the capacitance of C3 until we get the rhythm we like. But the sound might not sound similar to real birds. We can solve that by increasing or decreasing the C1’s capacitance for lower pitch and higher pitch, respectively.
C2 helps improve the frequency stability when the transformer is operating. C4 also helps with frequency stability when using a 9V battery.
How To Build
This circuit only uses a few components so we can easily assemble them on perforated board and wire them together in the same layout shown below.
Components list
Resistors 0.25W 5%
R1: 47K
R2: 1K
Ceramic Capacitors, Mylar Capacitors
C1: 0.039µF 50V
C2: 0.33µF 50V
Electrolytic Capacitors
C3: 100µF 25V
C4: 470µF 16V
Semiconductors
Q1: BC548 or BC337 or equivalent
Others
S1: Pushbutton Switch
SP1: 8Ω 0.25W Speaker
B1: 9V Battery and a snap connector
T1: 1K Centre Tapped: 8 Audio Transformer
Conclusion
This sound generator circuit is a relatively simple oscillator circuit, with its frequency determined by two components: C1 and R1. Furthermore, the input voltage level also affects the sound. This circuit can take an input range of 6V to 12V.
However, one downside of this circuit is that the audio transformer is quite hard to come by these days, so switching to an IC-based circuit appears to be a better option. But, with that said, old and simple circuits like this always make us feel nostalgic about the past; plus, they always are an excellent electronic teacher for everyone.
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I love electronics. I have been learning about them through creating simple electronic circuits or small projects. And now I am also having my children do the same. Nevertheless, I hope you found the experiences we shared on this site useful and fulfilling.