You could use a 7805 with its input and output capacitors - which you have not shown on that diagram so your 7809 may be oscillating - but you would need a proper heatsink. You need a regulator to supply 5 V to the Arduino via the '5V' pin as well as the relay module - which latter requires about 90 mA to operate. Going to check this in a couple of days, but i have a hunch that indeed the voltage drops and it is not able to power the relay propperly through the arduino. What do you see if you monitor the 12V supply with a DMM while the project works? Is the 12V steady or does it drop out when the motor turns on? should have either explained it more propperly or use the right parts. couldnt find all of the parts right away.
Yeah sorry about that, just used it for the first time yesterday. So we waste time while we wait to find out what is actually being used. People put parts in that look good but do not represent what is actually there. That is one reason that we do not like Fritzing.
