The timer is used in typical configuration, except that the timing resistor is replaced with the current source based on the op-amp DA1 (741). This solution allowed to obtain a good linearity (better than 3%). With the value of components used in this circuit the output frequency will increase linearly from 0 to 21kHz while the input voltage incre
ases from 0 to 5 V (the input voltage controls the output frequency with the relationship 4. 2 kHz/V).