Friday, July 5, 2013

Small Build, Big Execuition


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.


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.



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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  1. lid removable without wires on it OK
    battery removable OK
    circuit provided OK
    instructions clear and provided OK
    repairable OK
    casing strong enough OK

    good example. i've seen to many rats nests in cardboard boxes that fell apart when touched

  2. You could probably have labelled or marked the headers such that they could not be inserted backwards accidentally. (especially the ribbon cable)

    1. I decided not to this time because the cables had to be awkwardly twisted to fit them in the wrong way so it was easy to see a mistake. A trick I like to use it to remove a pin from the male side and glue full the matching hole on the female side.

  3. How long does the battery last? I expect you only energize the relay during the 25 to 100mS.

    1. 80-100 presses. Or many months of use.

    2. I just heard back from my coworker. He said he uses it about 50-100 times a day, 5 days a week for over a month and it is still going strong... So Thousands!

  4. I would attach the schematic on the underside of the lid. don't need to look at schematic every time I use this device. Good build and tips.

  5. Looks good. It's funny but I was thinking about this same thing this week. Some of the key areas that the hacker, maker, F/OSS, etc. communities are weak are the logging and documenting areas. The reason being that those are things that suck to do and generally professionals only do them because they are required and paid to do so. Anyone designing and making things on their own time, for fun, isn't inclined to do this.
    I think that developing at least basic habits like in this post is not only a great idea for building marketable skills, but for your own sense of accomplishment and future sanity.
    Having to fix or upgrade something you built for your own use a few years/months/whatever after you built it will be a WHOLE lot more fun if you've developed these kinds of habits.
    Nice post.

  6. I always use a extra large housing to put my projects in. On purpose. So that I know that it will fit. You are very good at making everything fit in a neat small case, but I know that I'm not.

    I'm wondering if the inside of your box won't become very warm.

    1. Thank you for the compliment. I wanted to make this hand held so I had to make it small.

      None of the circuitry generates any significant heat. It's also only used 5-10 an hour so very low duty cycle.

  7. Somewhat off-topic: what kind of glue gun and heat gun do you recommend buying?

    1. Depends on what your using it for. The minimum hot glue gun I recommend is a 40 watt 2 temp model like the Wal-Mart [Ad tech 2-Temp] which is the one I use. Don't buy one of those tiny single temp ones with the little glue sticks, they are just crap IMHO

      For heat gun I use the Harbor Freight [12 Interval Heat Gun With LED Temperature Settings]. It last a lot longer then the smaller two temp model and has a nice cool down feature. You can usually pick one up for US$20-30

    2. I use Bison transparent contact glue for strain relief, because hot glue makes a mess and does not hold.
      Yes, it needs several hours to dry, but stays flexible forever and holds really really well.

  8. Dude, lose the perf board and the hot glue. Make a decent PCB and use screws. This is not a "build", is more like a "stack" or "pile"... WTF is wrong with you?!

    1. I appreciate your feedback. I made this from being asked, to delivery in about 4 hours so no time for a proper PCB. I did cut the perf board to fit in the slots in the case so it's not free floating. I will add this to the description above so others don't become confused.

    2. Seriously?
      That's your idea of constructive criticism?
      WTF is wrong with you?!

    3. OK where are th links to what you have shared with the DIY-hacker-maker community?

  9. You should look through and choose something like these for your high current side cabling, such as the output end of the relay board, rather than header pins:

    1. Those are sweet. Thanks for the link!

      This was a one day build so I could only use what I had on hand. I put 4 on each side and used rather beefy header pins rated at 2 amps max. The relay is only rated for 7 so it will start giving problems first.

  10. Caleb, first I think I need to clarify something about my previous post... The thing is I couldn't understand why someone with skills and knowledge would choose to use perf board and hot glue. It is a good project, and I'm quite impressed you managed to put it all together in just 4 hours (I just wish you had mentioned the 4 hours part in the post).
    The best part is that you shared it with all of us. That is something(I must admit) I would never do... And that's not because I'm selfish, but I like to keep my DIY projects to myself (I don't have the patience to document / explain all the steps for everyone, plus to take pictures, etc. Let's not mention all the wise guys - like yours truly- who would let all sorts of comments).
    Anyway, you're cool.