Mon Sep 5 14:26:23 EDT 2016

12V relays

Received the wrong kind (12V instead of 5v).  But it looks like these
can still be driven from 5V/3V, it just needs a 12V supply to drive
the relay.

They use opto couplers to drive a transistor that drives the relay
current from the provided voltage JD-VCC voltage.

library://4_channel_relay_schematic.png md5://f1281597e5e8a8b0b721f9bc3808caf5
library://4_channel_relay_PCB.png       md5://d1444c443d1b708481cd635566f3b6e6

So it's quite easy to change the driving voltage by connecting VCC to
e.g. 3V, and JD-VCC to 5V, 12V, ...

This schematic is inverting and corresponds to the 12V boards (marked HW-316).

The first one I had (unmarked) is non-inverting.  It has
IN-R-OPTO-LED-GND on the input side.