A simple microcontroller like an 8-pin PICAXE can be used to read voltage in a circuit, but its capabilities depend on whether it has analog-to-digital conversion (ADC) and digital-to-analog conversion (DAC) resources. The specific microcontroller mentioned has built-in ADC for measuring current but lacks DAC for outputting varying voltages. However, it does feature digital outputs, allowing for control through pulse-width modulation (PWM), which can effectively manage power to devices like solenoids or motors based on the current measured.