Voltage to Frequency Converter Circuit
Table 1.
| C1, C2, μF | F, kHz | R, kHz/V |
| 0,001 | 35 | 3,5 |
| 0,01 | 3,5 | 0,35 |
| 0,1 | 0,35 | 0,035 |
| 1,0 | 0,035 | 0,0035 |
| 10,0 | 0,0035 | 0,00035 |
A process of conversion of continuous signals into discrete values is widely used in digital voltmeters and other measurement tools. A simple device, that uses this principle, can be built according to the circuit diagram, shown on left side of this page. Every voltage value correspond to a frequency of pulses on the output of the circuit.
As we can see in the circuit diagram, a multivibrator, based on transistors T1, T4, has in its base networks two transistors T2, T3 (connected in common-collector configuration) instead of resistors. The frequency of pulses is determined by transistors T2, T3 and capacitors C1, C2. A discharge time of capacitors C1, C2 is defined by an internal resistance of transistors T2, T3. The internal resistance depends on a control voltage and its sign.
When the input voltage is increasing, the internal resistance is decreasing, the discharge time is decreasing too, therefore the frequency of pulses increases. When the input voltage is decreasing, all processes are going in opposite direction.
If the input voltage is in the range from -5 to +5 V, a conversion ratio will remain linear.
Transistors T1, T4 - V435; T2, T3 - C450. Diodes D1, D2 - EA403. It is possible to use modern transistors in this circuit, in this case it takes two resistors with a value about a hundred kohm, each of them have to be connected between a base of transistor T1, T2 and the +9V wire.
A frequency of pulses (F) at zero output voltage and the conversion ratio (R) is determined by capacitors C1 and C2. This parameters is shown in table 1.
"The Radio-Constructor", 1969, 07.