If your AVR or any microcontroller gives 3.3volt output on its GPIO,s then you are not able to operate a relay directly because the current value is least and its not able to operate a relay(5volt and 12 volt properly).
If you want to operate a relay module for industrial purpose using microcontroller you have to boost the voltage value and current value of operating relay input.you can do it with this circuit.this circuit first boost the current and work as voltage shifter(3.3 volt to 5 volt) which can be easily sensed by a relay.if you are using 5 volt relay then it can directly connected to output of level shifter transistor circuit.if you are using 12 volt relay you have to operate this module with relay driver ic i.e ULN2003.in industries most common operating voltage and relays are 12v and 24 volt.i am describing here for 12volt relay module.there are using optocoupler for the electrical isolation from the field side to MCU. 8 relay module working on AVR controller working properly and we are using it in industries to control heavy motors ,generators,machines,blowers..
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