Transducers and I/O Devices

Contents of this page:

1. Switches
2. Variable Resistors
3. Electronically Thrown Switch
4. Moving Things
5. Piezos
6. Midi Stuff
7. Off the Shelf I/O devices
8. Catalogs and Magazines

See Catagories of Contact
MAIN PHYSICAL COMPUTING SCREEN

Switches

Switches make it easy to check the state of something in the physical world. Suppose you want to know when someone picks up a coffee cup. You usually have to rig the situation so that in one state a circuit is complete (closed) and in the other the circuit is broken (open). In this case you could put a small momentary switch under the cup and the circuit would open when you lift the cup. A momentary switch is one which springs back into the off position when you release it. There are many types of binary switches but all of them have two states, they are conducting a current or the are not. This makes them easy to make and easy to use with a computer. After you decide on your switch you will want to build it into a digital input circuit and connect it to your microcontroller. You would then program your microcontroller to watch the switch and perhaps program your authoring software to react to the microcontroller using serial communication.

• Garden Variety Undisguised Switch. When you want your switch to look like a switch you can simple go to a hardware store and buy a wall switch or doorbell push button. Radio Shack also has a wide variety of regular switches.
• Make your own switch. In many installations you will want the person to throw a switch without knowing it. This requires building the switch into some normal looking object. A switch is really just two pieces of metal or "contacts" that touch or don't touch. The current going through these contacts is so little that it is safe to have people touch it. If you can engineer a way for two contacts to touch or not touch depending on the user's actions, you have made a switch. For instance, if you take a thin slice of foam rubber with holes in it and sandwitch it between two metal plates and attach one lead from the switch in the digital in circut to each plate, then you have a switch. I someone steps on the metal plates, the foam rubber will compress and the plates will touch wher there are holes in the foam rubber.
• Foot Switch. The H&R Cataloge has many nice foot switches. (215) 788-5583
• Roller Switch. This is a switch that you can get at Radio Shack that requires very light pressure to close it.
• Mercury Switch. These open and close depending on how they are oriented. They work by having a drop of mercury in a glass tube with two contacts at the end. When the mercury rolls to one end of the tube, the mercury conducts current between the leads and the switch is closes. When the tube is tilted the other way, the contacts are not connected.
• Burgular Alarm Switch. If you are trying to make the switches in your installation unobtrusive, then burgalar alarms systems may offer just what you need. Radio Shack carries serveral intersting burgalar alarm switches including one that is thrown when a beam of light is broken and another that is thrown when a magnet comes near.

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Variable Resistors

When you want to measure varying amounts (analog measurement) of energy in the physical world, one of the easiest ways is to use variable resistors. Resistance is the measure of how easy it is for current to flow through a circuit. Suppose you want to know how hard someone is pressing down with their thumb on a picture on the wall. You would find something to convert the pressure from the thumb into an electrical resitance, a force sensative resistor. You would put the force sensative resistor under the picture on the wall and send a current to the resistor and depending on how much current flows through, you know how hard the person was pressing. To measure how much current came back, you will need a mircorcontroler with A/D conversion capabilities. After you decide on your variable resistor you will want to install it into a analog input circuit with your microcontroller. You would then program your microcontroller to watch the variable resistor and perhaps program your authoring software to react to the microcontroller using serial communication.

• Thermistor These resist current to differing degrees depending on how hot or cold it is. (Radio Shack#...)
• PhotoResistor These resist current to differing degrees depending on how light or dark it is.(Radio Shack#...)
• Potentiometer This is like the volume knob on your stereo and resist current to differing degrees depending on far it has been turned. Esentially they measure angular position. Radio Shack sells lots of cheap ones and they are the easiest analog device to get started with. You will need something between 5-50k to use with the Basic Stamp. You can buy precision potentiometers, continuous turn potentiometers, and multiturn potentiometers from catalogs. If you need a very rough continuous turn potentiometer just pop the back off of a Radio Shack model.
• Linear Potentiometer Same as the above except is slides instead of turning.
• Flex Sensors These resist to different degrees depending on how much you bend them. They were originally used in the fingers of PowerGlove, a cheap VR glove so they are about as long as a finger

• Force Sensative Resistors. These resist current to differing degrees depending on how hard they are being pressed. You can make cheap force sensative resistors by simply sandwiching conductive foam (sold at Radio Shack) between two copper plates. Look in the "Science Projects" Mini-Notebook at Radio Shack for mor info. You can also buy force sensative resistors from Interlink Electronics, phone number is 805 484-1331 fax 805 484-8999

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Electronically Thrown Switches

These are just like regular switches which turn things on and off except the computer can control them. Computers control switches all the time because transistors are just little switches that pass current or not depending on whether they are supplied with the tiny current that the computer circuitry provides. In the case of a relay the computer flips a more mechanical switch using an electromagnet. You should learn what is the resting position for your relays (normally open, normally closed) or for your transistors (NPN, PNP). After you decide on your electronically thrown switch you will want to install it into a digital output circuit with your microcontroller. You would then program your microcontroller to activate or deactivate the switches and perhaps program your authoring software to react to the microcontroller using serial communication

CAUTION: SERIOUS INJURY FROM ELECTRICAL SHOCK AND DESTRUCTION OF EQUIPMENT CAN OCCUR IF YOU ARE NOT CAREFUL WHEN USING NORMAL AC HOUSE CURRENT. THE CIRCUITS WE ARE MAKING ARE USUALLY USING 5 VOLTS DC. THE CURRENT COMING OUT OF THE WALL IS 120V AC. ONLY THE RELAYS AND THEN ONLY THE LEADS ON THE RELAY FOR THE AC LOAD SHOULD TOUCH ANY AC WIRES. FIRST TEST YOUR CIRCUITS WITHOUT ATTATCHING AC CURRENT, LISTEN TO HEAR YOUR RELAYS (they click audably) THROW AND USE A VOLT METER FOR CONTINUITY ON TRANSISTORS. AFTER CAREFULLY ATTATCHING THE VOLTAGE FIRST TEST WITHOUT A SERIAL CONNECTION TO A REGULAR COMPUTER BY MANUALLY APPLYING THE 5 VOLT SIGNAL VOLTAGE TO THE APPROPRIATE PINS.

• Reed Relays (easiest)
• Solid State Relays (only works with AC)
• 9V Relays
• Transistors
• SCR
• Triacs

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Moving Things

Strong analog motions might require amplifying the the 0-5 V signal put out by most microcontrollers. However there are some ways to play with analog out. After you find something to control, you will want to install it into a analog output circuit with your microcontroller. You would then program your microcontroller to control the output voltage and perhaps program your authoring software to react to the microcontroller using serial communication.

• Servo Motors The easiest thing to work with are the little servo motors made for Radio Control Airplanes. They can be powered right off your microcontoller. The down side is that they only travel in a 180 degree arc. The great news is that they go to a given position for a give length of pulse out of your microcontroller. Any hobby store sell them starting around $17. --America's Hobby Center 146 W 22nd 675-8922
• Regular Big motors
• Regular little DC Motors
• Stepper Motors
• Muscle Wires These are wires that expand and contract depending on whether current is passed throught them. Mondo Tronics Inc. San Anselmo CA 1 800 374-5764

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Piezos

Piezos are great for registering vibration and also for accelleration. When the piezo material is deformed it generates a voltage. This is like how guitar pick ups and drum machines work. Piezos are also commonly used for output like a speaker. The voltage can get very high and the current if very low so you need a circuit to protect your microcontroller and also to amplify the current. In addition to the massaging, you will have to always be watching for the signal because it is transient which will require a fast processor (faster than the Basic Stamp). You will also need authentic A/D Conversion because Piezo's create a voltage rather than a resistance (which is what an RC circuit does on the stamp). There are midi devices that convert these tiny voltages into midi information which is a lot easier.

• AMP puts out a great kit full of peizos and notes for how to use them. They also sell a circuit for massaging the signal into something usable.
• A music store with a good selection of electronic stuff will sell piezo triggers for around $25. You can do it more inexpensively using the piezos (the very flat kind) from radio shack. They are meant for use with something like the Yamaha DTX2U which might convert the voltage to midi for you but you are also paying for a drum sounds which you may not use.

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Midi Stuff

Musicians and dancers have have naturally been attracted to physical computing. As a result there is a lot of good transducers made to fit into a midi configuration. Unfortunately I don't know much about it so I am looking for people to send in info or links to info.The Troika Ranch has some good info on making your own sensors.

• Pitch to Midi Converters.
• Midi Sound Mixers --The Yamaha Pro 1 is a great box
• Drum Trigger System Yamaha DTX2U.
• Midi Sound Effects Units-- The Yamaha VL70 is really cool
• Midi Lights Dimmer, I think PAVO makes one.
• PAVO Miditools Custom Instrument. This is like a microntroller especially for midi. Erector Set for midi. PAVO 10
• The book Electronic Projects for Musicians and the magazine IndivieVisions might be interesting.

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Off-the-Shelf Computer-Ready Input Devices

Although many I/O devices can be very expensive, the time you spend learning to build your own will probably be much more expensive. On the other hand you will learn a great deal in growing your own electronics.

• X-10 Devices
• UltrasSonic Range Finders -- Contaq (802) 453-3332 (expensive but plug and play).Poloroid (cheap but requiring extra circuitry).
• 3D Sensors-- These are expensive devices for locating an object in XYZ space and sometimes also in Pitch, Roll, and Yaw orientation.
  • Polheums makes an nice expesive tethered magnetic sensor with serial output. Colchester, VT 802 655-3159.
  • Acension makes an nice, less expesive tethered magnetic sensor with serial output. It is called "Flock of Birds." Burlington VT, ascension@std.world.com 802 860 6440 6439fax.
  • Fake Space makes a mechanical boom Menlo Park, CA 415 688-1940.
  • Origin Instruments make an untether tracker using infrared. Grand Prairie, TX 214 606-8740 The
  • Vivid Group makes a 2-D tracker using Video Scanning Toronto Canada 416 340-9290
  • David_Rokby also of Canada tracker using 2 Video Cameras for 3D. David Rokeby@intacc.web.net
• Modems There are lots of cheap modems out there and they are pretty easy to control. Most of them use the same commands. You need to get the commands for your modem to dial a number usually "ATDT ### ###-####", to put it into answering mode usually ATSO=1, and for hanging up.
• Laserdiscs
• Camcorders/Videodecks You can control most video decks and camcorders using "VISCA", CONTROL L, or CONTROL S protocols. Sony sells a V-Box for this but instead it is highly recommended that you use the Video Toolkit for Abbate Video Norfolk MA 02056-1416. Because Abbate Video use Apple Events to contol things, you will not be able to use this on a PC or on a microcontroller. Instead you would have to find a video deck that use ordinary RS232 serial communication.
• BrainWave Sensors Psychic Lab Inc 249 East 48th Street, Suite 15D, NY, NY 10017
• Body Sensors, -- BodySynth muscle tension to midi. Ed Seveign, Chris Van Raalle ed.s@sfnet.com or BioControl Systems 430 Cowper St, Palo Alto 94301 415 329 8494 8498fax. or WaveRider Box 4467 Berkeley CA 94704, 510 526-5881 waveridr@crl.com

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Catalogs, Books and Magazines

You can find lots of good weird stuff through these catalogs and magazines.

 

• Magazines
  • Circuit Cellar
  • Nuts and Volts Magazine (Get this first because many Suppliers advertise in this.)

• Surplus Catalogs
  • All Electronics 1800 826-5432
  • Alltronics 408 943-9773
  • H&R Herbach and Rademan 215 788-5583
  • American Science & Surplus 708 982-0870
  • C and H Sales 800 325-9465
  • Servo Systems 800 922-1103
  • Resources Un-Ltd 603 668-2499
  • GoldMine Electonics
  • Amazing Products
  • HSC
  • Robot Store
  • Hosfelt
  • Marlin P. Jones

• Catalogs
  • Jameco (this is my favorate)
  • Digi-Key 800 344-4539
  • Mouser Electronics 800 346-6873
  • SuperCircuits (for micro video)
  • Edmund Scientific expensive scientific stuff
  • Tech America Radio Shack on Steroids
  • Home Automation Systems X10
  • Small Parts mechanical

• Books
  • The Art of Electronics Horowitz and Hill
  • Radio Shack Engineer's Mini-Notebook series

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If you have suggestions or corrections please contact: dan.osullivan@nyu.edu