Small Build, Big Execuition

GOOD PRACTICES

Sometimes its not the complexity of a project that makes it noteworthy, its the way in which it was built. A prime example of this is in a recent project of mine where I was asked by a coworker to build a small and simple device to act as a monetary off switch. It uses a 555 timer to give a momentary pulse to a relay that switches a high power load off for a fraction of a second. Because of the simplicity of this project it will be easy to see the good practices I used to make the device more reliable, easier to repair, modify and operate.

OUTSIDE IN

A rats nest of wires on a breadboard is not a finished project, it is an easily damaged mess. Things need to be permanently soldered together, glued to keep them from bending or breaking and protected by an enclosure. The enclosure has always been the hardest part for me so I like to start with it. I selected a small project box from radio shack (I keep some on hand) and designed everything to fit in it.

Source: beavishifi.com

NOT A BUILD LOG

I didn't take photos during the build of this project so I will show the photos of the finished item and tell you the good practices used.

Instructions for use and schematics for all boards

Turn pot accessible from outside. This allows adjustments to timing without dissembling the unit. It was recessed and designed to be only adjustable using a screwdriver so accidental changes were unlikely.

Proper use of space. Many different configurations were test fitted to determine what looked and worked best.

The battery is easy to remove without disrupting the other parts.

The 555 timer board has sockets and connectors to allow it to be removed without de-soldering or cutting anything. The board was also cut to size so it would fit nicely in the cases's vertical PCB slots.

The main IC (555 timer) is in a socket to it can be removed and replaced without de-soldering. The turn pot is hot glued down so the legs are not bearing the load

All wires are hot glued for strain relief.

The relay board uses header pins so it can be easily removed and replaced. There are better connectors but this is what I had on hand.

The relay board has the protection diode built in. The entire board needs to be replaced if the relay dies.

The external banana plugs are screwed and hot glued to provide extra support.

The button board is hot glued in so it is removable with a little heat. There is no easy way to screw it in so hot glue was the best option. The wire connector is hot glued on both sides to provide strain relief.

Don't forget to document your work! This blog is one of my ways of keeping a build log. I have many projects I've done that I have to relearn how I did them if I ever want to fix them. Keeping a log like this will help you in that process.

All in all a great little device that does its job and is easily modifiable and repairable by anyone, not just the designer.

Please leave any feedback on what I did wrong or what you would have done better :)

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Adding Capacity

I was recently given a wind up light key fob but unfortunately the internal battery was no longer accepting a charge so constant winding was required. I was unable to turn it at a constant RPM so an unacceptable strobe light effect was produced. I really liked the little light so I decided to crack it open and fix it.

Most little charge up lights I've seen, whether they are solar powered or crank style, don't have a rechargeable battery and simply use a primary cell. Four very small screws later I discovered that is the case here too.

I could have swapped the little lithium coin cell out for a new one and had a working light again but I really liked that the wind up part of this key fob was real. I wanted some way to give it the ability to be wound up for a few turns and hold power for several seconds. I also didn't want to spend any money so it had to be from my parts bin. Luckily I work on a project a while back where I needed some very small ultra-capacitors. I had two 0.47F 2.7V capacitors left over that fit my requirements perfectly.

Looking at the PCB I could see the a full wave diode bridge rectifier, yes 4 whole diodes! They spared no expense. They fed into the non rechargeable battery, then the switch made or broke connection to the two LEDs.

It was simply a matter of de-soldering the battery and replacing it with the two ultra-capacitors. I chose to put the two in series so a higher voltage was capable and thus a longer run time. I could have added a joule thief and only one cap but that would have been a lot to cram in there.

Before I soldered the caps in place I first checked to see if they would fit. The answer is no, they would not fit without removing the board and having everything free floating. I didn't want to do that and I have also been trying not to make my projects look like the Borg made it. Cutting a piece of the case away for the caps to hang out of was a no go as well. I decided to try heating the plastic and adding a depression for them to fit into. This wound't look too ugly and should be easy enough to do.

With the caps fitting it was time to screw everything back together and see if it works. Success! About 10-15 fast turns (the faster you turn it the more power is produced) would get about 10 seconds of light. You could also tell when it was fully charged b/c it would start to spin very easily when cranked.

Larger Boards in EagleCad Free

A few years ago I made a large flexible LED sign on some copper clad kapton. I used the toner transfer method to etch the kapton but I quickly ran into the dilemma of board size. I was using the free version of eagle cad and was limited to 100mm x 80mm. My boards were in the range of 160mm x 120mm. After a little trial and error I found a way to bypass the board size limitation without modifying eagle cad in some illegal way.

First let me say this works best for a simple repetitive design. I needed it for the 1 layer surface mount display boards which used a simple layout repeated multiple times.

Starting in the board layout window draw a component. For me it is the large LED. I used the rectangle tool to draw the pads in the TOP layer and the line tool to draw the chip indications in the TPLACE layer.

Figure 1

Next I copied it around to form my 4 LED pixel.

Figure 2

This part is a little tricky. I laid down the traces without any nets for guidance. Use the line tool to mark where you want the copper to stay.

Figure 3

Repeat until the pixel block is complete. Copy pixel block to form half column block. Add connection pads to bottom and connect pixels.

Figure 4 : Rotated Left for better fit

Copy this around to form full display.

Figure 5

Success! Eagle cad will gladly let you make this board and export to PDF or gerber. Side note, each line you draw counts as a new net so DRC will still work.