Feeling chirpy? Attract
new friends with this modified hartley oscillator. You could
also use it as a replacement doorbell.
Notes:
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.
