Electronic – arduino – Relay with arduino/microcontroller, diode needed

Per the specifications at the meb page linked in the question, the control input is a simple series curcuit: VCC is connected to a resistor (limits the current through the remainder of the series) which is connected in sereies with an optoisolator diode, which, in turn, is connected in series with a LED indicator. The optoisolator, (and consequently, the relay) and the LED will be turned on if 5V (more or less) is applied between VCC and INx.

So, yes, the device may be controlled by contact closures between INx and ground as you indicated. Alternately, the INx terminal could be connected to an open-collector or open-drain driver circuit to allow for uC control.

A coulple of things to keep in mind:

• the circuit driving the INx must be able to handle (sink) 20mA, per the device description.

• if you want to isolate the relay coil drive voltage from the optocoupler VCC, you will need to drive the node labelled JD-VCC with another 5V supply, connected between JD-VCC and ground. (In this case, the first 5V supply would connect to VCC only - this supply's ground would not be connected to the relay board. When the contact connected to the INx pin is closed, a return path will be formed through the INx pin.)

As Matt says, the first thing that jumps out is more capacitance needed.

Breadboards are terrible for stray inductance/capacitance so you really need to try and keep wires as short as possible, keep sensitive signals away from higher current/voltage/fast changing signals, and add plenty of bypass and bulk capacitance around the circuit.

I would start with a couple of >100uF electrolytics on each power rail and near the FET, with a few 100nF or higher ceramic caps on the power pins of the ICs.

Another thing to look at is using a separate rail for the relay, and routing the ground return separately from the uC's ground. Of course they need to be tied together, but you can ensure the high current return does not flow through the uC board's ground this way.