These are handy for running LEDs
and other low load electrical devices. What I use is 12V
DPDT PCB (printed circuit board) relays. These can be
purchased on eBay or from online electronics retailers like
Newark or Mouser.
On some of them the "trigger"
terminals are polarized, on some they aren't. You can
test this (and the input/output terminals) using a 9V
battery. Once I've determined the polarity of the
trigger terminals and which terminals I want to use
(normally open or normally closed,) I snip off the pins I
won't be using and solder wires to the terminals I will be
Then I wrap it in electrical
tape, hang it upside down and fill the electrical tape with
either five minute epoxy or JB Weld.
Here's a couple of examples of
relays I've made. The one on the left is a two amp relay and
the one on the right is an eight amp relay.
About PCB Relays:
PCB relays function like
automotive "Bosch" type relays. The schematic below
shows the pin arrangement for a typical 12V DPDT PCB relay.
And here's a terminal pinout for
a typical Bosch 5-pin relay:
"Trigger" Terminals: There are two terminals
labeled "-" and "+" at the top of the PCB relay shown above.
When the "-" pin is connected to ground and the "+" is
connected to 12V+ the relay is activated. These
correspond respectively to terminals 85 and 86 on a Bosch
The design of these two pins can
vary from relay to relay. The easiest way to determine
which pin is positive and which is negative is to touch the
two pins to the contacts on the top of a 9V "transistor"
battery. You will hear the relay click when the
polarity is correct.
N, C & O
are two sets of N(input), C(closed) and O(open) pins, These
are respectively equivalent to the 30, 87a and 87 terminals
on a Bosch relay. The three pins on each side of the relay
are independent circuits. I usually just use one side.
The configuration of these pin
cans vary by relay but by using a continuity tester on the
lower pins and triggering the relay with a 9V battery it is
easy to determine which pin is which. When the relay
isn't activated there should be continuity between the N
terminal and the C terminal. When the relay is
activated by the 9V battery there should be continuity
between the N terminal and the O terminal. As you'll see on
the yellow relay in the picture above, I usually label them
N, C and O accordingly.
On the yellow relay above, the
blue wire is the 12V trigger, the brown wire is the trigger
ground. They gray wire is the input to the N terminal
and the green wire is output to the C terminal, making it a
"normally closed" relay.
On a Bosch relay the blue wire
would go to the 86 terminal, the brown relay would go to the
85 terminal, the gray wire would go to the 30 terminal and
the green wire would go to the 87a terminal.
To make this a "normally open"
relay instead, the green wire would go to the O terminal -
which is the equivalent of Bosch terminal 87.
© 2013 Drake Smith - Please do
not use or reproduce this elsewhere. Feel free to link
to it though.