The Electronic Replicant

Here’s something we haven’t done for a while, but someone did submit a good question, and it deserves a good answer.

Q. Where do you find the best parts to scavenge for robotics?

A. Just as the best food for fish is fish, the best parts for robots are robot parts. However, finding a dead robot from which to scavenge parts can be difficult under normal circumstances. So I have prepared this list, based on my own experience, of things one is likely to find in normal circumstances, and of what useful things one might find inside those things.

Computers

Power Supply

Not worth scavenging parts from as it contains large capacitors whichcan be quite dangerous. However, AT/ATX power supplies are quiteuseful to the experimenter as they deliver a useful range of voltages:+12, +5, -5, -12, and 3.3 volts. If you’re using an AT power supply,ensure that the switch is adequately insulated to avoid anelectrocution hazard. You will also need to supply a load for thepower supply to work properly, such as a large resistor. Perhaps youmight consider building an enclosure for the switch, load, and powersupply with spring-loaded terminals for each voltage. Although I donot recommend opening the power supply, this page is still helpful. And here are some other ways to get power from a PC.

Motherboard

Very old motherboards are treasure troves of 7400-series chips,kilobytes of memory, and some specialty chips like UARTs andprocessors. However, unless the board is bad, first consider whetherit might serve you better as the brain of your robot.

Expansion cards

As with motherboards, very old expansion cards are even bigger trovesof 7400-series chips. People are usually happy enough to get rid oftheir old modem cards, and these usually have relays, which are usefulfor controlling motors, especially larger ones.

Floppy disk

I don’t get too excited about 3.5" floppy disk drives as the steppermotors are pretty small and haven’t proven that useful when removed.On the other hand, 5.25" drives, if you can still find them, have muchlarger steppers. You may find a stepper motor driver on the board ifyou’re extremely lucky, but in my experience, the steppers are drivenby the drive’s CPU either directly or via a transistor array. Youmight also, with some research, be able to get the pancake motor (thatspins the disk) to work. On the oldest 5.25" drives, the flashing ofan infrared beam shining through a hole punched in the disk was usedas a timing signal to keep the motor spinning at 300 RPM. Otherpancake motors use magnets instead of a beam of light to accomplishthis.

CD-ROM

You can usually find two to three DC motors in a CD-ROM drive: one tospin the disk, one to move the laser, and one to move the tray. You’llalso find some gearing assemblies in conjunction with the tray thatmay or may not be useful in making a linear actuator. If you look onthe board, you could find a few audio amplifiers, a DC motorcontroller, or a microcontroller.

Hard disk

Unless it’s very old, there won’t be a lot of useful parts except forthe neodymium magnets. These have all sorts of uses, particularly inattaching heavy objects (like bulletin boards) to metal objects (likeyour refrigerator.)

Mouse

You’ll find a few microswitches, and either a pair of rotary encoderswith infrared emitters and phototransistors, or a tiny camera.

Printers, fax machines, copiers*

Here’s where you’ll find a few motors and loads of gears, cams, rods,springs, and pulleys. In inkjet and dot-matrix printers, you’ll alsofind a screw or belt that drives the print head back and forth. Therewill probably also be an encoder disc or even a strip that theprinter’s CPU uses to ensure that the print head is in the righthorizontal position.

Entertainment

Stereo*

Loads of analog components. If it’s got a CD player, see CD-ROM. Ifit’s got a tape player, there will be at least one DC motor andperhaps a belt system. You may also find a DC motor controller.

VCR*

You’ll probably find at least one DC motor and motor driver. You’llalso find a few gears and belts, and an infrared detector.

Televisions (and monitors)*

Not much besides a few analog components such as resistors andcapacitors. In fact, old televisions often have especially largecapacitors inside, which can be dangerous if not properly discharged.I would avoid these entirely.

* Take care when opening such devices. Unlike your friendly neighborhood PC, the power supply in one of these devices is unlikely to be shielded, and even if the device has been turned off and unplugged, large capacitors can store a hazardous amount of voltage for a surprising amount of time.

Welcome to the second installment of q+=a, The Electronic Replicant’s new question-and-answer feature. As nobody has actually submitted a question yet, I’m going to consult my referral logs again.

Q. How can I control a 120VAC motor with a BASIC stamp?

A . This should not be too difficult if all you need to do is turn the motor on and off. Now, it is important to remember that I am not an electronic engineer, nor do I play one on television. I am also not an electrician, with which one really ought to consider consulting before trying to build anything that interfaces directly with wall current. The following ought to work, but it could just as easily electrocute you, catch fire, or otherwise malfunction. You’re probably better off buying an X-10 module.

Still there? You are obviously an individual of unswayable fortitude, or perhaps you have simply fallen asleep at the keyboard. In either case, what you’ll need is something along these lines:

This is quite similar to the motor controllers I mentioned last time. A pulse from the BASIC Stamp (or other microcontroller, or your PC, or whatever else) causes the LED side of the optocoupler to emit light and thus trigger the phototransistor side of the optocoupler. The transistor energizes the relay coil, creating a magnetic field which closes the contacts, allowing AC to flow from the plug to the socket, thus powering your motor, Christmas lights, fan, pump, or whatever.

A nice thing about this circuit is that the optocoupler will protect the Stamp or PC from destruction by high voltage if the relay happens to short, which is not likely, but it could happen if the current flowing through the contacts is much higher than what the relay is rated for. That could also lead to the relay melting and/or bursting into flame, which is just not a good thing. Of course, the optocoupler is really effective only if the PC and relay are on separate power supplies.

That concludes this installment of q+=a. If you’ve got a question you’d like answered, or if you think my answers are utterly and completely useless, or both, or neither, please leave a comment at the prompt.