Switching Basics and More 55Experiment 7: Relay-Driven LEDsExperiment 7: Relay-Driven LEDs
You will need:
- AC adapter, wire cutters and strippers.
- DPDT relay. Quantity: 2.
- LEDs. Quantity: 2.
- Resistor, 680Ω approx. Quantity: 1.
- Pushbutton, SPST. Quantity: 1.
- Hookup wire, 22 gauge, or patch cords.
- Alligator clips. Quantity: 8.
- Utility knife.
The next step in our exploration of switching is to use a remote-controlled
switch. By “remote-controlled,” I mean one to which you can send a signal to
turn it on or off. This kind of switch is known as a relay, because it relays an
instruction from one part of a circuit to another. Often a relay is controlled by
a low voltage or small current, and switches a larger voltage or higher current.
This setup can be cost-effective. When you start your car, for instance, a rela-
tively small, cheap switch sends a small signal down a relatively long, thin,
inexpensive piece of wire, to a relay that is near the starter motor. The relay
activates the motor through a shorter, thicker, more expensive piece of wire,
capable of carrying as much as 100 amps.
Similarly, when you raise the lid on an a top-loading washing machine during
its spin cycle, you close a lightweight switch that sends a small signal down a
thin wire to a relay. The relay handles the bigger task of switching off the large
motor spinning the drum full of wet clothes.
Before you begin this experiment, you need to upgrade your power supply.
We’re not going to use batteries anymore, because most relays require more
than 6 volts, and in any case you should have a power supply that can give you
a variety of voltages on demand. The simplest way to achieve this is by using
an AC adapter.
First you’ll set up the AC adapter. After you have it running, you’ll use it to
power the relay. Initially the relay will just switch between two LEDs, but then
you’ll modify the circuit to make the LEDs flash automatically. Finally you’ll re-
build the circuit on a breadboard, and say goodbye to alligator clips, for most
of the time at least.
Preparing Your AC Adapter
An AC adapter plugs into the wall and converts the high-voltage AC supply in
your home into a safe, low DC voltage for electronic devices. Any charger that
you use with your cell phone, or iPod, or laptop computer is a special-purpose
AC adapter that delivers only one voltage via a specific type of plug. I’ve asked
you to buy a general-purpose adapter that delivers many different voltages,
and we’re going to begin by getting rid of its plug.