Article : Andy Collinson
An electronic version of a chirping canary. May be used as an alarm, a sound effects generator or perhaps a replacement doorbell.
This circuit is a modified hartley oscillator with a couple of extra components included. The transformer is a small audio transformer, type LT700. The primary is center
tapped with an impedance of 1Kohms at 1KHz . The secondary has an impedance of 8 ohms. The inclusion of R1 and C1 give this oscillator its characteristic "chirp". As
the 100u capacitor charges via the 4.7K resistor, R1 the bias for the transistor is cut off. This causes the oscillation to stop, the capacitor discharges through the base
emitter circuit of the transistor and oscillations start again. Altering these components alters the frequency of the chirp. The chirp is also voltage dependent. When
the push button switch is operated the 100u capacitor is charged. When its released, the oscillation decays and the chirp becomes faster.
Click here to listen. The download size is 164kb.