Introduction: Battery-Powered Makey Makey
If you're new to electronics, you may wonder how you can power your Makey Makey with a battery so you can detach it from the USB cable for portable or wearable projects. I could simply tell you how to wire it up to the right battery pack, but I would like to walk through the basic exercise of looking at the schematic, choosing an appropriate battery power source, and testing it out.
You will need:
- Makey Makey kit
- Battery Case Holder for three AA batteries (Like this one from Amazon.)
- Piezo buzzer, 5V DC input at ~20mA (This one from Adafruit or this one from Radio Shack will work.)
- Makey Makey schematic diagram (Download it here at Sparkfun.com.)
- Atmega328 Microcontroller Data Sheet (On Atmel website.)
- Digital multi-meter (optional)
- Soldering iron and solder or 15-20 gauge wire (optional)
This Instructable assumes that you have already set up and tested the Makey Makey's basic operation while attached to a PC with the provided USB cable.
Thanks to Instructables for providing the Tech Valley Center of Gravity Maker Space in Troy, NY with Makey Makey samples through their Build Night program!
Step 1: Determine the Appropriate Input Voltage
First, let's look at the schematic to figure out where we can apply power, and how much. In other words, what battery voltage is safe to use, and where do we connect it?
There is a schematic for the Makey Makey in PDF format posted at Sparkfun. The power and ground input pins are shown on the left hand side of the processor (circled in the enlarged photo attached here).
Per the Atmega328 Datasheet (page 2), the processor's operating voltage is 1.8 to 5.5V. If the input voltage is too low, it won't run properly. An input voltage that is too high can damage the device. We can see in the schematic that there is no other circuitry to protect or regulate the source voltage (as is the case with some Arduino boards that contain a voltage regulator), so we need to stay within that range.
On the left hand side of the PDF schematic are the connector headers that are on the underside of the Makey Makey board (shown here in the second enlargement). With the Makey Makey lying connector side up, JP14 represents the connector at the top center. We will connect our battery holder wires to the 5V and GND pins of this connector.
(NOTE: The Sparkfun schematic shows JP14 as a 6-pin connector, while the actual connector on the board has seven pins. Refer to the printing on the board to determine the correct pins to attach the battery holder wires.)
Step 2: Choose a Battery Source
Next let's decide on an appropriate battery configuration. We can rule out a 9V battery because that exceeds our limit of 5.5V, and could damage the board. AA batteries output 1.5V each. Two batteries would give us 3V, which is within the microprocessor's input power range. A third battery provides 4.5V, which is closer to 5V and typical of external components that we might want to power. Four AA batteries supply 6V, which exceeds our limit. Three AA batteries best meets our requirement. If you are lucky, you might be able to cannibalize a broken electronic toy or other item for a battery holder, but since I didn't have one, I purchased one on Amazon. Mine happened to have an on/off switch, which is a nice feature.
I inserted the batteries in the holder and measured the power with my multimeter. It registered 4.8V with new batteries.
Since the wires on my battery holder were multi-stranded and thin, I used my soldering iron to apply some solder to the ends of the wires so they make good contact with the connector when they plug in. If you aren't comfortable soldering, or don't have a soldering iron, you could use the banana clips provided with the Makey Makey to attach the battery holder wires to the jumper wires that are also provided. You could also twist the battery holder wire together with a solid jumper wire (15-20 gauge), and secure the twisted wires with electrical tape.
Step 3: Connect the Circuit and Test
With the Makey Makey connector-side up, connect the inputs, outputs and power as follows:
Output: Connect the piezoelectric buzzer by connecting the keyboard output pin on the top center connector to the positive (+) terminal on the buzzer. Unless your buzzer has power and ground wires attached, use one of the white jumper wires and an alligator clip. Connect the buzzer's remaining terminal to one of the GND/Earth pins on the bottom center connector.
Input: Insert one white jumper wire into the GND/Earth connector, and a second jumper wire into the W pin of the left hand connector. These will serve as your controls. When you touch them together, they complete the circuit and cause the buzzer to sound.
Power: With the battery power off, insert the black wire on the battery pack into the GND pin on the top center connector. Insert the red wire into the 5V pin on the same connector.
Turn the battery holder on, and either pinch or touch together the input wires to complete the connection between the W pin and ground. (See photo or embedded video.) You should hear the buzzer sound. If it doesn't work, make sure that you have wired correctly, and that everything is making a solid connection.
Now that your Makey Makey is battery-powered, what can you do with it? With the buzzer, you could make it into a 'secret knock' doorbell that sounds when someone touches two conductors at the same time. Now that it's portable, you could hang it right on the inside doorknob (being careful to keep the wire connections intact.). You could catch a cookie thief by connecting a wire between the Earth connector and a piece of aluminum foil on the floor, and connecting the input wire to another piece of foil on the cookie jar lid. When the cookie pilferer steps on the foil (in bare feet, of course) and lifts the lid off the cookie jar, he or she completes the connection and sets off the buzzer!
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