The circuit shown in the figure converts an input voltage into pulses of proportional duration. An accuracy better than 1% in the conversion can be achieved thanks to the feedback system with a transistor. Note that the circuit requires a symmetrical power supply for the operational amplifier. An important feature of the circuit is that the pulse duration changes with the voltage, but the frequency remains stable without changing what may be a characteristic required for many practical applications. The pulse duration range and therefore frequency depends on the components connected to pin 6 and 7 of the 555 integrated circuit. The transistor can be the BC548.