A very simply off line motor drive can be built using a triac and a control IC. This circuit can control the speed of a universal motor. A universal motor is a series wound DC motor. The circuit uses phase angle control to vary the effective motor voltage.

A micro controller can also be used to control a triac. A PNP of BRT may be used to drive the triac. As shown, the MCU ground is connected to the AC line. The gate trigger current is lower if instead the MCU 5V supply is connected to the AC line.

The MCU must have some means of detecting zero crossing and a timer which can control the triac firing. A general purpose timer with one input capture and one output compare makes an ideal phase angle control.

In some cases it is necessary to isolate the MCU from the AC line for safety or due to electrical noise suseptability. An opto triac driver can be used to drive a large traic. Small motors may be driven directly by an opto triac.