This circuit can accept positive or negative or differential control voltages. The output frequency is zero when the control voltage is zero. The 741 op amp forms a current source controlled by the voltage Ec to charge the timing capacitor C1linearly. NE555 is connected in the astable mode. so that the capacitor charges and discharges between 113 Vee and 213 VeeÂ·
The offset is adjusted bY the 10-K potentiometer so that the frequency is zero when the input is zero. For the component values shown: f = 4.2 Ee kHz. If two de voltages are applied to the ends of R1 and R4, the output frequency will be proportional to the difference between the two voltages.