Experiment 7: Relay-Driven LEDs
56 Chapter 2
- It’s important to make sure that your AC adapter is not plugged into the
wall! - Chop off the little plug at the end of its wire. See Figure 2-49.
- Use a box cutter or utility knife or scissors to make a half-inch cut between
the two conductors, and then pull the conductors apart a couple of inches. - Use wire cutters to trim one of the conductors shorter than the other,
so that after you strip away a little of the insulation, the exposed copper
wires cannot easily touch each other. This is a precaution against short-
circuiting your AC adapter and burning it out. - Strip the two conductors using your wire strippers. Twist the copper
strands between finger and thumb so that there are no loose strands
sticking out. See Figure 2-50. - Make sure that the two wires are not touching each other, and plug your
AC adapter into a wall outlet. Set your meter to DC volts and apply the me-
ter probes to the wires from the adapter. If the voltage is preceded with
a minus sign, you have the probes the wrong way around. Reverse them
and the minus sign should go away. This tells you which wire is positive. - Mark the positive wire from the adapter. If the wire has white insulation,
you can mark it with a red marker. If it has black insulation, you can tag it
with a label. The positive wire will remain positive regardless of which way
up you plug the AC adapter into a wall outlet.
The Relay
The type of relay that I want you to use has little spiky legs on the bottom, in
a standardized layout. If you buy some other kind of relay, you will have to
figure out for yourself which pins are connected to the coil inside, which pins
go to the poles of the switch inside it, and which go to the normally closed
and normally open contacts. You can check the manufacturer’s data sheet for
this purpose, but I strongly suggest you use one of the relays mentioned in the
shopping list, so that you can follow the instructions here more easily.
I asked you to buy two relays so that you can use one for investigational pur-
poses—meaning that you can break it open and take a look inside. If you do
this very, very carefully, the relay should still be usable afterward. If not, well,
you still have a spare.
The easiest way to open the relay is with a box cutter or utility knife. Figures
2-52, 2-53, and 2-54 show the technique. Shave the edges of the plastic shell
containing the relay, beveling them until you see just a hair-thin opening.
Don’t go any farther; the parts inside are very, very close to its housing. Now
pop the top off. You can use needle-nosed pliers to nibble the rest of the shell
away. Read the following section, “Fundamentals: Inside a relay,” and then ap-
ply power to the relay to see how it works.Figure 2-49. Preparing an AC adapter. First,
cut off the little low-voltage plug and throw
it away.
Figure 2-50. Second, strip the wires, mak-
ing one shorter than the other to reduce
the risk of them touching. Color one of the
adapter wires red with a marker, to identify
it as the positive one.