Electronic toy "Morning Bird" circuit

Photodiode is also called photodiode, and its circuit symbol is
"Reed 0". The characteristic of photosensitive diodes is that when exposed to light, the
reverse impedance of the diode will change accordingly (as the light irradiation increases, the reverse impedance
will change from large to small). Taking advantage of this characteristic, photosensitive diodes are often
used as Photoelectric sensor devices are used.
    The picture shows the circuit of the electronic toy "Morning Bird", which is a light-controlled oscillation
circuit. Place it in the window, and it will emit pleasant
sounds of birdsong at dawn.
    In the figure, VT1 and VT2 form a complementary self-oscillation turtle circuit, using
the charge and discharge of RC to simulate the chirping of birds. Since a photosensitive element is connected to the bias circuit of VT1
, the whine is controlled by light. When there is no light, the impedance
of the photosensitive element is very large, VT1 is cut off and the circuit does not work; when there is light, the impedance of the photosensitive element decreases, VT1 works, and the speaker sounds. R2 can adjust the light control sensitivity, whine tone and interval time. The capacitance in the circuit is related to the tone of the whine. As the values ​​of cl and C2 decrease, the sound becomes sharper and the interval shortens; as the values ​​of cl and C2 increase, the pitch decreases and the interval becomes longer; appropriately increasing C3 can lengthen the interval between whines.