Signal generating circuit with variable rhythm

The signal generating circuit with variable rhythm can realize different types of alarms and can also be used in toys, which is very interesting. The circuit principle is shown in the figure. This signal generating circuit includes a relaxation oscillator composed of a single junction transistor VT1, a multivibrator composed of VT2 and VT3, and a class B push-pull power amplifier composed of VT4 and VT5.

Component selection:

VTl uses a double base diode BT33C, and the DC resistance RBB between the bl and b2 poles of the tube should generally be 3~10kΩ. VT2 and VT3 use 9013, 50≤β≤85. VT4 and VT5 use 3DGl28, 50≤β≤65. The electrical parameters of tubes VT2 and VT3, VT4 and VT5 can be as close to the same as possible, which is conducive to the accuracy of the frequency. Power switch Sl: KNX (1×1). Selector switch S2: KZX-1-3W1D (band switch). Audio transformer T: MTT22 pot type ferrite, L1-2ΦO. 13mm high-strength enameled wire, wound 380 turns, L2-3ΦO.13mm high-strength enameled wire, wound 380 turns, L4-5Φ0.19mm high-strength enameled wire, wound 46 turns. Except for resistor R12 which is 1/2WRJ type resistor, the other resistors are all 1/8WRJ type resistors.