It’s now time to add a new component to our circuit – a pushbutton (or just “button”). A button is just a very simple switch – when it’s not pressed, electric current can't flow through the button and when it is pressed, electric current does flow through the button.
Let’s take a closer look at how a button works. This is a representation of most basic pushbuttons – they have four “legs” that can be attached to your breadboard and wired to the circuit and then one button in the center of the device that can be un-pushed (“open”) or pushed (“closed”):
Internally, the left side of the
button (the two legs on the
left) are connected together, and the right side of the button (the two
legs on
the right) are connected together.
The
left side and the right side are separated by a switch that opens and
closes as
the button is pressed or unpressed:
When we insert a button into a circuit, it has one of two operations:
When the button is left unpressed, it creates an open circuit, no electric current can flow through the button and nothing happens.
When the button is pressed, the button acts as a wire and closes the circuit – electric current can then flow through the button.
So, if we go back to our first
breadboard circuit containing
a simple resistor and LED that is always illuminated, we should be able
to add
a switch that we can then use to control the LED turning on and off (by
pressing the button).
Let’s build that circuit now…
Here’s what the schematic will look
like:
To build the circuit, we can start with the circuit we built in Raspberry Pi Power:
We’re going to be adding the button
into the circuit between
the LED and ground, so the first step will be to remove the wire that
connects
the LED to ground.
This is what the breadboard should
look like at this point:
The next step is to provide connectivity between the second lead of the LED and the button. While we could wire the button in the same connect row as the LED to provide this connectivity, the buttons take up a good bit of space in a connect row, and it would be hard to fit the LED, the button and the wires we'll need all in the same row. For that reason, we suggest running a wire (the half-inch green wires work great for this) from the connect strip that the LED is in to a new connect strip that we can use for the button.
Assuming you choose to do it this way, this is what the breadboard should
look like at this point:
Next, we’ll place the button on the
breadboard. Note
that the button should be oriented so
that the two sides of the button sit on different connect strips, as follows:
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CORRECT | WRONG!!! |
One side of the button should sit in the same connect row as the green wire from the LED. This will provide connectivity from the LED through the green wire and to the button, as follows:
Note: In our diagram above, you'll notice that the button sits across a notch in the breadboard. This isn't absolutely necessary -- the button could sit with both left legs in the same connect strip and both right legs in the same connect strip. But, because buttons take up a lot of space on the breadboard, we find it easier to position them across a notch in many cases. You can decide how you want to wire your buttons, but keep in mind that either way will work.
Finally,
we can
complete the circuit by connecting the other side of the button to
ground, like
this:
At this point, your LED should not be lit. Remember, while we have a potentially complete circuit, there is still a lack of physical connection through the button until the button is pressed. In other words, there is a break in the circuit inside the button (this is how a button works) and the way to complete the circuit is to press the button.
If you press the button, you should see the LED illuminate.
In this
project,
we’ve manipulated the circuit using a purely physical device. By inserting the button
into the circuit, we
can interrupt the flow of electric current through the button (and
therefore the circuit) by not pressing the button and then
restore the flow of electric current through the button (and the
circuit) by pressing the button.
Congratulations! You’ve just built the same
circuit used in
most simple flashlights…
In the next project, we’ll add software into the equation using GPIOs.