what resistors or other ancillary components would I need? I found in a few places that people put a 1k resistor between the Arduino and the gate of the FET, would I also need a pull-down 10k resistor between the Arduino and the source?
The gate resistor is to limit the current that wants to rush in to charge the gate capacitance. The necessity of the resistor has been debated, but I would use one. You want to use the lowest value possible, though, to turn the MOSFET on as quickly as possible to lower the power dissipation (keep it out of the active region). For a 5V Arduino around 180 ohms. The 10K from the Arduino output (MOSFET gate) and ground is to pull the gate low while the Arduino is in reset to keep the MOSFET turned off until the Arduino is running.
Yes that device seems OK. Only one diode needed for each group of three relays.
A standard 1N4001 will do the trick, not fast but fast enough. At switch on, the gate charges up and this initial charge current can be tens of milliamps. The gate resistor just lowers the peak inrush current so that it doesn't damage the driving circuitry (in your case the Arduino output). The higher the value of gate resistor the slower the turn on time of the FET will be, so it's a trade off.
For this job a dirt cheap BC337 with a 470 ohm base resistor would be adequate.
NPN transistor - would I simply exchange the 180Ohm gate resistor suggested by Groundfungus for your 470Ohm base resistor and that's it? Does the NPN solution need any other bits and pieces? Also, how you calculate size of the 470Ohm base resistor?
See the enclosed. You need to drive the BC337 hard to turn it fully on, and 8mA is reasonable - hence the 470.
Your major problem will be the 5v psu - you can only take up to about 500mA from the arduino internal psu, depending the dissipation of it's regulator, and the USBN supply is llimited to a similar amouint, so you may well need an external +5v psu to drive lots of relays.
rldr.pdf (17.6 KB)
The arduino can drive lots of 8mA loads - no problem. But the internal psu will run out of puff trying to drive many relays at perhaps 100mA each.
You could always buy a multi-relay shiield to do what you need
Something like one of these which will probably cost less than the parts required
I would take @jackrae's advice, only problem is those cheap Songle relays, I've replaced all of mine with this Omron brand, they are only US $1.50 each.
Of course, they might be made in the Songle factory anyway.
The first Omron that fails, I'm going to open it up and compare it to a Songle's innards just for curiosity.