Tachometer kit construction project

February 26, 2011

As mentioned previously I bought myself a small electronics kit tachometer only to find that it was all surface mount components.

The supplier ikalogic send me some more tips on construction.

http://www.bdmicro.com/smt/
http://electronic4beginner.wordpress.com/

There were several recommendations to use a temperature controlled iron but my 23W Antex XS soldering iron has never caused problems in the last 25 years. The modern equivalent gives you an extra couple of watts but I’m sure that would make little difference. As with any such project you need to be careful not to overheat the components and board. I’ve been soldering since I was about 8 so am fairly confident in these things.

As well as my regular tools, a couple that I’d not normally need are the scalpel and magnifying glass. The scalpel is essential for getting the components out of their tiny packages but I also used it for positioning the items. Perhaps something like a metal work scriber might have worked well. I just used the magnifying glass to check the results afterwards rather than using it whilst soldering. The “good lighting” recommended by all is a double edged sword as it shines off the solder and can make inspection with the magnifier a little difficult.

The kit is very professionally packaged with all the components clearly labelled in separate bags. The assembly instructions are clear and brief. The LCD sub assembly comes pre-made and the crystal and micro-controller are already soldered onto the main board.

I tinned all the pads on the board and used the desolder braid to ensure that they were flat not spiky or domed.

The first components I put on were the resistors, no real problems with those. These resistors are numbered rather than having colour codes so make sure you don’t take them out of their separate packets before you are ready.

The capacitors were even smaller (and have no labelling), these took me a couple of attempts to get in position correctly and flush to the board. I thought the transistors would cause a problem but held in place with the scapel I just gave each leg and quick prod with the soldering iron and the solder melted locking them into place. The button that activates the tacho was not so easy and it took me a couple of attempts, there’s a little notch in the board and you should align the button to that. Having looked at the schematic, the button has 4 legs for just for support as it’s a single pole, single throw switch.

There’s only a couple of components on the back, the LED cathode is indicated with green dots. As per old-school LEDs this is the large piece of metal in the package, i.e. left in the picture below.

The IR sensor is as challenging as the button on side one, I put a little more solder onto the pads and then heated the legs as I’d done with the transistors and that seemed to work.

Lastly the power connector, I went with the provided 3 AA cell battery holder but will be swapping this with a 4 cell one so I can get it running reliably on rechargables (another handy tip from the Ikalogic team).

I pressed the button and wiggled my finger back and forth. Not a particularly scientific test but it does produce different counts for different speeds of finger wiggling. Next step is to workshop proof it and get it to measure some real rotation.

By Andy from Workshopshed One Comment

  Posted in: Electronics Tags: pcb, smd, tachometer