This was a Custom Built Product for Converting a 10-100 V Signal from a Motor Tacho to pulses, to indicate the distance moved by Conveyor belt with Job Material. The Digital Indicator would indicate the speed and the Mechanical Counter would Log Quantity of Materials Moved.
The voltage was first brought down to low levels with an attenuator and zener protection. A VCO and a monostable created a train of pulses. These pulses drive a darlington transistor pair which operates the Electromechanical Counter that has a Solenoid. The voltage is also scaled, calibrated and displayed on a Digital Voltmeter. – Process Control Instrumentation
The circuits are partially shown here, two versions.
Voltage to Frequency Converter ICL8038
In this the VCO chip is 8038, it is hard to procure, but it is an excellent chip.
VCO – Voltage Controlled Oscillator using Opamps
This is using opamps, hard to configure and build, but a proven design.
The VCO can also be built by a Microcontroller chip which has an analog input. Any I/O port can be the Oscillator output. This is easily possible as this system is within the 1 MHz range.