my background is physics and not electronics, so diagrams are not my forte
I was hoping to send 5V from the arduino to the fet on the main control board to trigger the relay.
Connecting electronics directly to the mains supply is always fraught with difficulties.... perhaps you could have a go at a circuit diagram?
How are you powering the Arduino? If you are in the USA, the hot tub MUST be on a GFI protected circuit, unless you wired it up yourself. So, the Arduino power must be on the GFI protected circuit, as well.
Can't see that follows, my background is in Physics also.So given all the concern above I will add another one. How do you know that the FET on your board will turn on with 5V? That voltage will only work if you have a logic level FET. If you have a normal FET you will need to drive this to 10V.Also by having two signals going to the same FET what happens if one is HIGH and the other is LOW? Something melts that is what happens.
The oscilloscope will see ac waveform due to stray capacitances with the mains. The multimeter seesjust the DC offset.If at least one supply is isolated, common the grounds. If not, you use opto-isolations.To test if a supply is isolated, disconnect, measure resistance between mains neutral and output ground,it should be inifinite if isolated.Note even an insolated supply can deliver a measurable AC current due to capacitive coupling, it can beenough to be easily felt.Since this is involving water I would expect all the supplies to be earthed and it seems sensible to earth yourlow voltage ground to the same point.
Knowing the difference between a FET and a JFET on sight is not easy.
To protect agains the HIGH and LOW being sent at the same time, I have two Schottky diodes in place to stop either the arduino's HIGH reaching the original IC's low, or vice versa.
I actually spend a good 30 mins drawing one on a web app, only to have it shutdown, delete my work and ask me for a paid account, before I could save the circuit.
Who mentioned JFETs? It was not me.New information that would have been good to know from the start.Using pen and paper minimizes the risk of the paper shutting down before you photograph it and post the results.
vaj : my worry is that there is a voltage offset between both circuit grounds. Since they share another common wire at some point (they are both running of the 120v mains) I'm afraid I might have significant ground loop current. Hence why I tried to measure the voltage between the two grounds on the scope. The very high scope impedance limits the current and allows me to see the voltage offset...that's when I measured 18v...and that's what worries me.
Here is a rough draft. D1 and D2 are the two diodes I inserted to avoid collisions between HIGH and LOW from the arduino and the microcontroller.
What concerns me is connecting the Arduino ground and the microcontroller ground.
While that will protect the microcontroller and the Arduino when both are off the FET gate is effectively floating. You need at least a 10K resistor pull down on the gate of the FET. Also the diodes will rob a bit of voltage from the gate. It would be good to know the exact part number to see if it will turn on at 4.6 or so volts.If the diagram is correct then it should be no problem both systems are isolated from the mains.Have you got a part number for the switching regulator feeding the Arduino. It is unlikely that it won't be isolated inside with an input rating of 120V AC.