Friday, November 8, 2013

HESA Works!

I was working on the Home Environmental Sensor Array at the Milwaukee Makerspace today.  (I took the day off so I was able to make some good progress.)

I had an idea to use an old computer power supply as a project box for the HESA.  It makes sense because I need an enclosure to put the parts in, 120v power, an on/off switch, and a fan.  The power supply box has all of that.  I hope to install all of the parts inside the power supply box and then mount it to my wall.  This includes the PowerSwitch Tail relay, the raspberry pi, and the circuit board.

After taking of the electrical components out of the power supply, I wanted a rectangular piece of wood to attach to the box where the old circuit board was.  This would allow me to attach parts to the wood and not the metal.  I also wanted to use the CNC router to cut the wood just so I could use the router again.  I had to design my own DWG file.  The tool I used to design the wood piece was LibreCAD.  LibreCAD is free and runs on Linux.

The wood piece is a simple rectangle with four holes to attach it to the power supply.  I spent about 2 1/2 hours designing the piece and then cutting it with the CNC router.  It takes a lot of time to measure everything precisely and then design it in the CAD software.  It does not help that I have not used LibreCAD and have to try and retry everything many times.

All that being said, the results were perfect.  The nice thing is that I can re-cut as many of these as I want with the exact same specifications.  In fact, I did cut six more before I left just because I could.

After cutting the wood and mounting it in the power supply, I also installed the PowerSwitch tail in the power supply box.  When I turn the switch on for the power supply, the PST has power as well.  Cool!  Next, I need to figure out how to wire in my raspberry pi to power as well.

The fan that came with the power supply did not work so I replaced it with a different one from the parts rack at the Makerspace.  The fan says it requires 12v input but it will work down to 3v.  My raspberry pi has a 5v output so it does power the fan, albeit a bit slower than it could run.

Now that the enclosure is working a bit I started testing out the new circuit.  First, I wired up the A and B circuits.  Each circuit has two outputs and one input.  One of the outputs sends a signal through a 1k resistor and the other sends a signal through a 100k resistor (not at the same time).  The way it work is like this.  The program on the raspberry pi sends out a signal on circuit A's 1k section.  The input on circuit B looks for a signal.  Then, the pi turns off the first signal and sends another on circuit A's 100k section.  If circuit B does not see that signal, the pi shuts off the signal and tries the same tests sending on circuit B and listening on circuit A.

For whatever reason, this circuit design works perfectly.  The pi always detects a signal when it is sent over the part with the 1k resistor.  I never got any false readings.  I think the problem is solved.

Here is a link to the circuit schematic.  I made it in Digikey's free SchemeIt on-line tool.  By the way, I have tried a few of schematic designers.  Most of them are either not free or too complicated.  I just want to make simple schematics for electrical circuits.  Circuit Lab was nice but is no longer free.  I installed the Electric VLSI Design System on my Linux computer but it is too complex.  SchemeIt was pretty easy to use, and allowed me to easily create my schematic.  I can share the link on the internet and even save it to PDF if I want.  For now, I will stick with SchemeIt.

I also made some videos of the HESA in action that I will upload to YouTube and post a link to.

Another great day at the Makerspace.

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