how to switch triacs ?

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Triacs cannot be fired without doing zero detection. Triacs doesn't behaves like a transistor. When you trigger the gate, M1 and M2 terminals will be closed until the AC waveform reaches to zero volt. No matter you pull the gate low before the zero crossing, the triac will be still on. So you have to detect when the waveform crosses zero and fire the triac with a specific delay as needed.
 

I am trying to refer to in AN392 data sheet. He had shorted four pin of microcontroller and connected to gate and traic firing is done as per the figure 3 in data sheet.
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ST6210 can source 15mA by any pin, thus 3 fused pins can source around 45mA which is enough to drive BTA16. This circuit can work, but the fact is there should always be an isolation between high voltage side and low voltage side. Or it will not be safe to operate. Your MCU can be blown out if any short circuit happens, and also any human operator can get a nasty shock. So an optoisolator is always recommended. If you don't want to use any optoisolator for zero detection, you can use a resistor divider, followed by a zener diode.
 

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