Radio 1972, 11
The circuit of the multivibrator is shown in figure 1. This circuit produces pulses and the frequency of them can be changed in wide range while the duty-cycle remains constant.
When transistor T1 is switched off, the capacitor C2 is discharging through the diode D3 and resistor R4, and resistor R3. The same, when transistor T2 is switched off, the capacitor C1 is discharging through the diode D2 and resistors R4 and R5.
Fig. 1. T1-T2 - 2N3705, D1-D4 - 1N6940;
R1, R2, R6, R7 - 1K; R3, R5 - 470K; R4 - 5..50K; C1, C2 - 0.1 mF.
Because in this circuit C1=C2 and resistance of R4 is much less then resistance of R3 and R5, then famous formula for frequency can be written this way:
where C - capacitance of the coupling capacitor (C1, C2);
R - resistance of the potentiometer R4.
From this formula we can see that the pulse repetition frequency can be adjusted in wide range only by potentiometer R4.
Astable Multivibrator with this values of RC works on frequency of 140..1400 Hz.
Radio, Television, Electronics 1972, 2