Microcontrollers require a DC operating power supply in the range of 2V to 5.5V. It is easy to supply such a range of voltages from a battery or secondary power supply. However, in certain cases, microcontroller-based products must operate directly from a 120V or 220V AC outlet without a step-down transformer or a resistor that generates a step-down resistor. As an alternative, a polyester/polypropylene film capacitor specified for AC line service can act as a no-dissipation reactance (Figure 1). Capacitor C1 is a 2mF AVX FFB16C0205K rated at 150V rms, providing a significant AC voltage drop that reduces the voltage applied to diode bridge rectifier D1. The flame-resistant metal film resistor R1 limits the current spikes and transient voltages in the AC power line caused by lightning and sudden load changes. In this application, the AC current does not exceed 100 mA rms, and the 51Ω, 1W resistor provides sufficient current limiting capability. R2 is a 5W, 160Ω Yageo J-type resistor, and D2 is a 1N4733A Zener diode that provides a 5V stable power supply for the Freescale C68HC908QT2 microcontroller. 
The schematic shows a representative circuit of a microcontroller-based fan governor where the thermistor senses air temperature while the microcontroller drives the fan motor. Figure 2 shows a light intensity regulator based on an inexpensive dual diode rectifier and a triac controller that share ground. IC2 is a Fairchild MOC3021-M triac thyristor opto-isolator that isolates the lamp return path from the ground return of the microcontroller (Figure 3). In each of these three circuits, the Kingbright W934GD5V0 LED indicator includes a built-in current limiting resistor (not shown).
Shenzhen Innovative Cloud Computer Co., Ltd. , https://www.xcypc.com