Connecting 240V live mains to PIC microcontroller safely

[arisu1love]

Hi,

For my project, I need to connect a 240V mains live connection wire to a PIC input pin to detect the zero crossing of the sinusoidal waveform. Is there a safe way to do the connection since it is 240V?

 

[malik_zohaib]

use opto-coupler.

 

[ujjwol77]

also the signal must be full wave rectified as the micro-controller will not recognise negative voltages!

 

[malik_zohaib]

use a diode to clip the negative half then feed this to optocoupler (dont forget to use resistor). Now you can interface it with ucontroller. That is all.

 

[exnol]

Microchip has documented this pretty well in their application note for X10 home automation : **broken link removed**

Although it's not isolated, its pretty good.

 

[DXNewcastle]

If you are using a linear power supply for your PIC then you can use the low voltage AC on the secondary of the transformer.
This will not require an opto-isolator, just a resistor to a zenner diode. The zenner will clamp the positive half cycles and remove the negative half cycles. You can add a small capacitor to avoid spurious transitions.
You do NOT need to use full-wave rectified AC, but if you do, remember that the frequency will be doubled.

 

[ecnbom]

Hi, connect IO through 1M resistor to 240VAC directly, it is OK for MCU since there are 2 diodes built inside, which can limit the voltage at IO from -0.7 to VDD+0.7, however, the system is hot, and can not touch directly.
it is a wide use solution, let me know any doubts.

 

[garg29]

better use a comparator & get directly a sqare wave

 

[prmurthy]

Use IN4007 and 33k and feed to opto-coupler(CYN17 or 4N35) the output of the opto-coupler is DC level <=5.0V

Remember this should be live wire. If you reverse the connection the Opto-Coupler is no more

Thankyou