In the particular embodiment of the invention described in the specification, a variable resistor is connected in potentiometer fashion to a DC power source and a time interval is selected according to the position of the movable contact. The ratio of the partial voltage provided by the movable contact to the full voltage applied to the resistor is determined and an oscillator signal target count for the selected time interval is computed from a count corresponding to the total voltage. The arrangement permits accurate time interval determination which is independent of variations in the supply voltage, variations in resistance value of the variable resistor, or environmental changes.