The circuit shown below is a typical circuit used for AC load control from microcontroller, one LED can be connected in series with MOC3021, LED to indicate when high is given from micro controller such that we can know that current is flowing in internal LED of the opto-coupler.The idea is to use a power lamp whose activation requires mains AC as opposed to a DC voltage. That s way, the mains AC power that we are trying to switch the lamp and no external power supply is required. To switch the AC current to the lamp, we have to use an opto-coupled triac, lamp and a diac is shown in circuit below. A triac is said to be as an AC controlled switch. It has three terminals M1, M2 and gate. A TRIAC, lamp load and a supply voltage are connected in series. When supply is ON at positive cycle then the current flows through lamp, resistors, diac, and gate and reaches the supply and then only lamp glows for that half cycle directly through the M2 and M1 terminal of the triac. In negative half cycle the same thing repeats. Thus the lamp glows in both the cycles in a controlled manner depending upon the triggering pulses at the opto isolator as seen on the graph below. If this is given to a motor instead of lamp the power is controlled resulting in speed control.
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