capacitor as a reservoir that reduces the effect of any
spikes or dips in voltage. Finally, we’ll add an LED to show
when the board is powered up.
A Power Supply in Five Steps
Step 1: Voltage Regulator
Place the voltage regulator on the breadboard in the
third row, leaving room for capacitors and leads on either
side. You’ll need to refer to the datasheet for your
particular regulator in order to identify the ground, input,
and output pins. Figure 5 shows a typical example.
Step 2: Power In
Place the leads that you’ll connect to your power
source in the first row of the breadboard. I soldered a
couple of jumpers onto a 9V battery clip, but you could
also solder a jack connector that accepts the connector
from a wall wart. Make sure that your voltage regulator’s
input and ground pins are next to each other, and connect
the source so that negative goes to ground and positive
goes to input.
Step 3: Capacitors
Next, we need capacitors to help maintain a stable
voltage. The voltage regulator’s datasheet will usually
advise on what capacitance these should be in their
application circuits. Usually these are polarized, so make
sure the leads are connected to GND and V+ correctly.
Insert one capacitor on either side of the regulator as
shown in Figure 6.
Step 4: Regulated Output
We have now created a regulated supply. Insert
jumper leads in the same row as the GND and output
pins of the regulator, and connect them to the power rails
of the breadboard.
Step 5: Turn On the Light
Finally, add an LED and a resistor on the opposite side
of the breadboard divider, connecting the LED’s anode to
the positive power rail, and the cathode to the resistor
(which, in turn, connects to the ground rail).
It’s now time to test! Connect the input leads to the
battery/wall wart and see if the LED lights up. If it doesn’t
light up, then disconnect immediately and check the
connections. I once connected my regulator back to front,
and ended up nearly melting my breadboard!
Improving the Power Supply
There are two really useful items that you can add to
the power supply to improve it. I haven’t added these here
in order to maximize breadboard usage and to keep things
simple. Firstly, a protection diode: This ensures that
nothing gets damaged if you mix up the polarity on your
input. After my first mistake with switching the polarity, I
learned the value of adding a protection diode between
the positive input and the first capacitor.
Secondly, a PTC fuse can be useful to ensure
that you don’t try to draw too much current
through the power supply. A PTC fuse (PolyFuse)
is a resettable fuse that trips once its current rating
is exceeded, then “resets” once it cools down and
allows the current to flow again. I usually place
this between my protection diode and the
Let’s bring the microcontroller on board. To
ensure compatibility with the Uno, we’ll connect
an external 16 MHz crystal — in later projects,
you’ll see this isn’t really necessary as the
ATmega328P can operate on an internal (slightly
less accurate) oscillator. Before continuing, make
sure the power is disconnected from the board —
40 March 2015
FIGURE 5. Identifying the pins of a voltage
regulator from the datasheet.
FIGURE 6. Building the regulated power supply
on the breadboard.