Sunday, November 2, 2008

Project: The Prototyping Box

The first major project I built in late 2000 was a basic self-contained prototyping center integrated into a box that contains a power supply, an extensive prototyping area, and an array of common panel mount components such as potentiometers, switches, and connectors. The front panel of the prototyping area is show to the left.

The front panel has:
  • 7.25" x 7.5" 3220 point solderless breadboard
  • Power on/off toggle switch at the top
  • 5 way binding posts in various colors for the different power supply voltages (-12v, -3.3v, -5v, Common Ground, 3.3v, 5v, 12v
  • 2.1mm and 2.5mm coaxial DC power jacks
  • 8 red T1 LEDs
  • 8 toggle switches
  • On the left side: SPDT toggle switch.
  • Momentary pushbutton switch.
  • Rotary 2 pole 6 throw switch.
  • 1k, 10k, 100k, and 1M ohm linear taper potentiometers
  • 8 ohm speaker
  • On the right side,
  • Male and Female DB25 connectors wired in parallel
  • Male and Female DB9 connectors wired in parallel
  • 2.5mm Stereo headset jack
  • 3.5mm stereo headset jack
  • On the bottom I later added an embedded LCD digital voltmeter (DVM) with positive and negative 5 way binding posts, a switch to switch the negative leg between the black binding post or the power supply common, and a rotary range switch to select a range (20.0v, 2.00v, 200mv, 20.0mv). The power switch for the ATX supply also controls the DVM, so when the power is on the DVM is also on.
Almost all of the parts are wired to single or double row female .100 pitch headers which allow the jumpers to be directly plugged into the components from the prototyping area. The exceptions are the power supply binding posts and the DVM (in retrospect, that was a mistake - the power supply voltages should also have been wired to headers)

I used Insulation Displacement Connectors (IDC) and ribbon cable wherever possible in the construction, all the rest of the connectors were wired point to point and soldered (because at this point I was not willing to create printed circuit boards)

The bottom of the hinged top looks like this:
You will notice the 9V battery that is stuck to the underside of the panel. That is used to power the integrated DVM. By using a battery, I avoided common mode issues when measuring the voltages of the power supply. I have been using this for better than 5 years, and have yet to replace that battery.

At the bottom of the photo, you will see colored wires that go off the bottom of the photo. These are the power supply wires that connect to the ATX power supply via a female connector. Rather than cut the connector off the ATX supply and directly connect the wires, I purchased a 20 pin ATX power supply extension cable and cut it in the middle to get a female connector with wires I could connect to the binding posts. I removed the unused pins from the adapter to eliminate the risk of unused power wires inside the box. This removable connector allows me to easily swap out the power supply. Most of the rest of the bottom picture is uninteresting expect that you can see where I used ribbon cables and IDC connectors vs point to point wiring. In most cases where I used point to point wiring, I used crimp pins and connector housings from Jameco on the connectors.

The mounting panel is a sheet of fairly thick (0.062") aluminum sheet that I machined using various drill bits, a dremel tool, and a nibbler tool (which I strongly recommend). The panel mount parts are all mounted normally, while the parts that are not normally panel mounted are all epoxied in place from the bottom.

The panel is inset flush into a frame made of 1" x 2" x 1/2" pine. The aluminum panel is mounted to the pine frame via 8 wood screws, one at each corner and one in the middle of each side. The top of the pine frame is hinged to the bottom box section.

The box portion is made out of 1/2" pine and 1/8" hardboard (for the bottom). I created it large enough to house the ATX computer power supply and sloped the sides towards the front to make it easier to work with. The extra space is used for storage of various parts, connectors, etc.

The back of the box is routed and drilled to mount the ATX power supply so access to the fan power switch and connectors is available on the back of the box. When not in use, the power cord gets stowed inside the box.

I used an ATX power supply because I happened to have a couple of spares lying around, and the fact that it supplies a number of useful voltages at fairly significant current ratings. While I have seen many references to using a resistor to draw enough power to force the switching supply into regulation, I have not found that to be necessary for this particular supply. In fact, I found it unnecessary with every single ATX power supply I ever tested.

The wiring for the ATX connector is:
  • The green power on wire is connected to the power on switch on the front panel, Turning the switch on shorts this wire to ground which causes the power supply to turn on.
  • The grey power good signal is connected to an LED via a 220 ohm resistor to provide a visual indication of correct power on (and properly regulated power)
  • The black wires are common/ground. One is connected to the power good LED, one is connected to the power on switch, and one is connected to the black binding post.
  • The yellow wire is connected to the +12v Yellow binding post
  • The red wire is connected to the +5v Red binding post
  • The orange wire is connected to the +3.3v Brown binding post
  • The white wire is connected to the -5v White binding post
  • The blue wire is connected to the -12v Blue binding post.
  • The green binding - not sure what I was thinking when I bought it and put it on the board.
What I would do differently:

If I was to build this again, I would have made it smaller and lighter, with fewer fixed parts. Realistically very few of the permanently mounted positions ever get used. The potentiometers are used,as is the speaker, because they are more convenient to use as mounted.
  • The switches are rarely used because when prototyping a circuit I am much more likely to either use a small pcb mount switch directly on the breadboard, or just plug and unplug a wire.
  • The permanently mounted LEDs are never used, nor are the permanent switches. It is easier to plug a LED directly into the board and replace the switches as described above. Because of the need for a limiting resistor on LEDs and usually a pull up or pull down resistor on a switch, they just sit there unused. I would replace these with a breadboard module PCB that could be plugged directly into the board (more to come about these)
  • The rotary switch and DB9 and DB25 connectors are never used. It is easier to strip and tin the wires from a DB9/25 cable and plug right into the breadboard in most cases, and I just haven't designed anything that uses a rotary switch.
  • The same is true for the two coax power supply jacks and the stereo headset jacks. It is easier to strip and tin wires from a spare cable.
  • I would go with a more modular approach based on small single function PCBs that plug directly into the breadboard. (although honestly I only consider that now that making PCBs with my CNC PCB Milling machine is so easy)
  • I probably would have built a much smaller enclosure with regulated and unregulated battery driven power rather than the ATX supply. I would still include binding posts for connecting external power from a bench power supply. I intend to create a good dual/triple bench power supply in the future after which the ATX supply will be retired.
  • That said, the ATX power supply has served me very well. This project box also inevitably serves as the power for my projects until I get around to building a standalone power supply or finding an adequate wall wart in my collection. The reason to build the bench power supply and shrink the project box to a more reasonable size comes down to needing the desk space.

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