Arduino animatronics- make your awesome costumes more awesome!

Step 4 Making connections- motors, LEDs and transistors

 
Inputs and outputs

Now we need to connect a few devices like servos, sensors and LEDs to our controller. The controller has inputs and outputs. Things like sensors and switches are input devices, while servos, LEDs and motors are output devices. The inputs and outputs are both analog and digital- a digital input is like a switch, so it's either on or off. Analog inputs are variable- it's more like a dimmer switch that gives you a range of values.
 

Digital outputs are similar- if the controller output pin is set HIGH then it's on. If it's set LOW, then it's off. This is great if you want to turn on a motor or LED. If you want to change the brightness of an LED or make a servo motor move then you want to make the controller output pin an analog output. This is done using PWM (pulsewidth modulation.) PWM simply allows the controller to fake an analog voltage output by setting the output pin HIGH and then setting the output pin LOW within a few microseconds or milliseconds of each other. If you pulse the pin HIGH for the same length of time you pulse it LOW you would get an average voltage of half the total voltage so the output pin would give you 1.6V instead of 3.3V. The amount of time the pin stays HIGH is called pulsewidth. The ratio of time for the pin to go from LOW to HIGH to LOW is called duty cycle. If you shorten the amount of time the pin stays HIGH relative to the amount of time it stays LOW you will effectively lower the output pin voltage. It really sounds more complicated than it is but this will come in really handy later on when you want make LEDs dim or make a servo move. Fortunately most of this complex stuff is done for you in the Arduino code libraries but it's still really good to know.
 

Sensors

There are all kinds of sensors- bend sensors, force sensitive resistors, accelerometers, potentiometers, joysticks, etc.

These analog sensors change their output voltage according to how you use them. In the examples we'll use button switches to turn things on and off and we'll use joysticks (potentiometers), bend sensors and accelerometers to make servos move.

When designing an animatronic system for costuming I try to match the type of sensor used with a specific body motion. Think about how the person wearing the costume is going to use it. Bend sensors are great if you want to make a LED dim or servo move by bending your finger. For even more control I can place a small joystick on a fingertip and use that to make a servo move. For a head tracking system that makes servos follow your head movement I use an accelerometer (from a Wii nunchuck) and I use fingertip switches to trigger sound effects. You'll see how these work in the examples.

Sparkfun has a good size momentary push button switch that is breadboard friendly-
http://www.sparkfun.com/products/9190
Here's the smaller version-
http://www.sparkfun.com/products/97
 

All of the sensors we'll use are connected to the Arduino input pins. A potentiometer is a device commonly used in an application like a stereo volume knob- it's a type of variable resistor. If you supply the potentiometer with 3.3V when you turn the knob the output voltage will range from 0 to 3.3V. A joystick is simply two potentiometers in a common housing- one for the X axis and one for the Y axis.

Sparkfun has a 10K potentiometer-
http://www.sparkfun.com/products/9939

They also have a couple of small joysticks-
http://www.sparkfun.com/products/9032
http://www.sparkfun.com/products/9426

A bend sensor is a resistor that changes its resistance value according to how much you bend it. By adding another resistor and creating a voltage divider, we can change the output voltage of the bend sensor to match the degree of bend. The only real drawback to bend sensors is that they don't have the wide range that a potentiometer has.

Sparkfun sells a bend sensor here-
http://www.sparkfun.com/products/8606

Accelerometers work by sensing a change in acceleration and then they alter their output relative to the change in acceleration. When you tilt an accelerometer it measures acceleration due to gravity- the more you tilt it the greater the change in output. Accelerometers are commonly used in video game controllers and cell phones.

A Wii nunchuck has a 3 axis accelerometer, joystick and two pushbuttons for $20.

Motors

Servos

Hobby servos are small geared motors that have a circuit board and potentiometer to control their rotation. This allows them to be able to move to an exact position relative to your input sensor signal. Most servos can move nearly 180 degrees and some can even do multiple rotations as well as continuous rotation. Servos have three wires- ground, power and signal. The signal wire (usually yellow or white) is connected to the Arduino output pin. The power and ground wires are connected to a separate power source, usually ranging anywhere from 4.8V to 6V. The reason for connecting servos to their own power supply is that motors generate a fair bit of electrical noise, which can cause glitches or a stuttering effect in their movement.
 

If you have an input sensor that generates an input voltage from 0-3.3V the Arduino takes that analog voltage and assigns it a value from 0-1023 using an analog to digital converter (ADC.) The code on the Arduino then tells the servo how far to move based upon the converted value. So if your sensor outputs 1.65V then you would get a reading of 511 and your servo would move half of its rotation. Many Arduino boards operate on 5V so the same sensor at the same position would read 2.5V and the servo would still rotate half way. A continuous rotation servo would rotate in one direction, stop as the sensor gave a 1.65V reading and then reverse direction as you caused to sensor to raise the input voltage.
 

Controlling a servo is done by PWM. You send a send a pulse to the servo on the servo signal line every 20 milliseconds. The pulsewidth tells the servo what position to move to. Most servos operate within a 1 to 2 millisecond pulse range so a 1 millisecond pulse tells the servo to move to the 0 degree position and a 2 millisecond pulse tells the servo to move to the 180 degree position. Any pulse between 1 and 2 milliseconds tells the servo to move to a position that is proportionate between 0 and 180 degrees.

I get all my servos here-
http://www.servocity.com
 

DC motors
 

Unlike most servo motors DC motors are best used when you need continuous rotation, especially when you want high RPM. Since DC motors can draw a fair amount of power they are connected to the Arduino output pin using a transistor or a PWM speed controller.

Pololu sells a large variety of small DC motors-
http://www.pololu.com/catalog/category/22
 

Stepper motors
 

I don't usually use stepper motors in my animatronic projects (at least not yet!) but I felt they are worth mentioning. Stepper motors allow for precise positioning as well as continuous rotation and speed control. The drawback to them is that they require a fair bit of electrical power and they're usually significantly larger and heavier than a servo of equal torque rating. Small stepper motors can be salvaged from old printers and scanners. Unlike DC motors stepper motors have multiple individual coils inside that must be activated in a proper sequence in order to get the motor to move. The Arduino controller is able to drive stepper motors using a specific driver chip or transistor array that is capable of energizing each individual coil in the motor. For more information about steppers have a look in the reference section.
 

LEDs
 

Small LEDs are pretty simple to connect to the Arduino- just remember to use a resistor between the Arduino output pin and the resistor cathode to limit the current flow. You can put a resistor on either the anode or cathode of the LED- either way will work. Most of the small 3.3v LEDs will have a forward current of around 20mA so a resistor value around 100 Ohms works pretty well. For accurate resistor value calculations have a look here-
http://led.linear1.org/1led.wiz

For my Iron Man repulsor I made a small 2" diameter LED board that has 24 PLCC-2 LEDs. You can get the bare PCB here-
http://www.batchpcb.com/index.php/Products/41872

The board uses 24 1206 package SMD 100 Ohm resistors-
http://us.element-14.com/vishay-dale/crcw1206100rjnea/resistor-thick-film-100ohm-250mw/dp/59M6948

I frequently buy PLCC-2 super bright LEDs on eBay at good prices-
http://stores.ebay.com/bestshop2008hk

 

High power Luxeon LEDs have a much higher current rating and will work best using some type of constant current source to drive them (there are several instructables on this.) A 1 Watt Luxeon LED will have a forward current of 350mA so you cannot connect it directly to an Arduino output pin. Much like a DC motor you will need to connect it to the output pin using a transistor.

Sparkfun sells Luxeon LEDs and a constant current driver-
http://www.sparkfun.com/search/results?term=Luxeon&what=products
http://www.sparkfun.com/products/9642
 

Transistors
 

A transistor is basically just an electronic switch. Each Arduino output pin is limited to 40mA output current so we'll use a particular type of transistor known as an NPN Darlington transistor to turn on high current devices. These transistors have three pins- the collector, emitter and base. The base pin is connected to the Arduino output pin using a 1K Ohm resistor. The collector pin is attached to the high power device and the emitter pin is connected to ground. When the Arduino output pin is set HIGH the transistor turns on and allows electricity to complete a circuit.

For applications that do not have power requirements over 1 Amp I designed a small transistor board that connects to digital out pins 10-13 using ribbon cable and two eight pin IDC connectors. This uses four SOT-23 package SMD transistors and four 1206 package 1k Ohm SMD resistors. The board is really easy to solder.

Transistor board PCB-
http://batchpcb.com/index.php/Products/41936

SOT-23 NPN Darlington transistors 4 ea-
http://us.element-14.com/fairchild-semiconductor/mmbt6427/bipolar-transistor-npn-40v/dp/58K1891

1206 SMD 1K Ohm resistors 4 ea-
http://us.element-14.com/yageo/rc1206jr-071kl/resistor-thick-film-1kohm-250mw/dp/68R0298

2x4 pin IDC connector 2ea-
http://www.surplusgizmos.com/8-Pin-2x4-IDC-Ribbon-Cable-COnnector_p_1879.html

For loads up to 5A I use a TIP 120 transistor in the TO-220 package. These are great for small DC motors and servos. Use a 1K Ohm resistor to connect the transistor base pin to the Arduino output pin.

I usually buy TIP 120 transistors from my local Radio Shack. They're very easy to get online as well.
 

Power supply
 

To power the Arduino servo board and servos you need two separate power sources- one single cell LiPo battery for the controller and a small 4.8V- 6V battery pack (4AA batteries work just fine) to power servos. The servo board has an additional socket that provides power from the LiPo cell to power low voltage devices like LEDs.