Monday, January 13, 2014

Raspberry Pi: Another Homebrew Prototyping Breadboard

I modified a prototyping breadboard that I had originally build for prototyping Microchip PIC devices by adding a Raspberry Pi. As what I have done may give people some ideas for homebrewing their own out of junk box parts, I will describe it a bit more.

As described in my discussion of another prototyping board, I like to have things securely fastened down so that you don't accidentally knock something out of whack, and you can pick the whole thing up and move it around. The only "moving parts" are the wires and components plugged into the solderless Breadboard / Proto Board units.

You can see where I mounted the rPI to a small aluminum plate (made from some scrapped electronic device chassis) using insulated grooved washers (to keep from shorting connections on the bottom). The plate is in turn screwed to the front bezel of the prototyping board. I used an Adafruit Pi Cobbler Breakout + Cable to get from the rPI GPIO pins to the proto board.

In my normal manner of scrounge and re-use, the main chassis is made from an old CD player that I found on the side of the road.

In the lower left front corner is the original power switch, which switches the original power cord to a +/- power supply inside consisting of a center tapped transformer, bridge rectifier, electrolytic filter capacitors. I use 3 terminal regulators to produce +3.3, +5, +12 and -12 voltages (most commonly needed for circuits). On the lower right front panel are 6 LEDS monitoring the unregulated and regulated voltages. You can find schematics for this type of regulated analog power supply all over the internet. Just be VERY careful working on the 120 volt primary side, heat shrink or tape all exposed connections as 120 volts can KILL YOU INSTANTLY. If you do not have the knowledge and skills to work with high voltages, buy pre-build power supplies to be safe.

The raspberry PI is actually powered by a separate +5 volt power supply out of the picture. Make sure that since the rPI GPIO pins as 3.3 volts, that you use all 3.3. volt circuits, OR do proper level conversion.

On the left hand top from the bottom are 8 leds that can be used to monitor a byte on the proto board. Above that are 4 LEDS to monitor the regulated +5 (2 supplies), +12 and -12 switched power to the board, with the switch just above that.

Above the power switch are a couple of switches and a connector that used to be used for a PIC programmer (no longer supported so removed). In it's place on the upper left corner is a Parallax 2 x 16 Serial LCD, very handy for monitoring and troubleshooting 5 volt TTL serial interfaces.

Along the top edge are 4 uncommitted binding posts, good from bringing leads from test equipment, etc. on board.

On the upper right is an LCD display using the parallel Hitachi HD44780 LCD controller (pretty much an "industry standard"). It is easy to interface this with a rPI or PIC/PICAXE.

Below that is a couple of knobs for LCD contrast, and speaker volume (small speaker on the side).

Below that are two knobs with 10K pots from +V to gnd to generate analog voltages.

Below that are 12 toggle switches and 2 push button switches to generate digital test signals.

Down both sides of the large white/blue proto board in the middle are proto pins on paddle boards that connect to the signals coming to/from underneath from the power, LCD, pots, switches, etc. These paddle boards were cut from a surplus proto board that plugged into the ancient IBM PC bus, so this would be hard to find. You could fabricate something similar with multiple  Adafruit Pi Cobbler Breakout + Cable, I suppose.

Note also that I have lots of Electrolytic and Ceramic bypass/decoupling capacitors plugged in on the power busses of the proto boards to provide transient suppression, this is VERY important for circuit stability. A lot of newbies leave these out, and end up with flaky operation of their circuits.

So as you can see, this prototyping breadboard provides a good solid base with lots of I/O devices for different experiments, without a ton of loose wires and parts lying all over the workbench. That makes if very hard to troubleshoot problems with your circuit.

Eric Pierce VA3EP - See the Disclaimer in the Introduction

© Eric Pierce and "VA3EP Amateur Radio And Other Geek Pursuits", 1952-2099. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Eric Pierce and "VA3EP Amateur Radio And Other Geek Pursuits", with appropriate and specific direction to the original content.

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