Tuesday, 1 September 2009

Tuesday trepidation

I have started assembling and soldering the first opto endstop. These boards are made solely with through-hole components. My first soldering atempt on this PCB left a scorch mark on the board - note Nophead's comment about how the solder pads on the board could do with being bigger.
So I switched off the soldering iron, let it cool and swapped for a finer tip. I'm glad I had bought a set of tip replacements for my iron.

When I got to the LED, I had to ask on the forum what the instructions meant for how to get the orientation correct. The phrase was "Insert the short leg (negative) into the hole closest to the flat side of the silkscreen" - I don't know what that means, nor what a silkscreen is, in this context!
The replies came back that "silkscreen" means the diagram printed on the board (if only they'd written "diagram" instead) so the "flat side" refers to one side of the LED's almost circular diagram, ie the picture isn't completely round - there is a subtle flattening on one side. I'm hoping not all boards are so subtle....

Further information came back that I should not assume that the short lead on the LED is the negative one - it isn't universally true! But that this can be checked by using the lowest resistance range on a multimeter, and touching its black lead against the short LED lead (and the red against the other), causing the LED to light up if the short lead is indeed the negative one.
However, I found this didn't work..... but I found that it did work if I used the triangle (
now I know it means a diode) symbol on the multimeter. I only considered trying this setting because I had earlier seen the triangle symbol, labelled LED1, on the circuit diagram for this board.
Looking at information on multimeters, I found that a digital multimeter may not have the right resistance range to check diodes, hence the separate diode-testing setting.
I shall have to try to remember this test before connecting up my other brand LEDs.

My first soldering looked (generally) O.K. on the rear of the board, ie the solder itself looked fairly shiny (I'm using lead-free solder, which doesn't make as shiny a join as the traditional leaded type), concave, although not exactly neat, but it looked dull on the top, and I had used too much solder in places. As far as I can tell, there is, unfortunately, no way of checking the finished board independently of any other boards.


  1. If you connect 5V and GND to the right pins it will work stand alone. I.e. if you block and unblock the slot the LED will go on and off.

    A few people report that they don't work and give up and replace them with switches. I think the reason is that they must have lighting with a high IR content. E.g. bright halogen spotlights. The Darwin instructions say to make an opaque tab with AL foil. That is a bad idea as you want it non-reflective to IR as well as opaque to prevent stray light getting reflected into the sensor.

  2. It sounds like you've made a good start. I was very nervous soldering my boards, but it quickly starts to build confidence once you've done one or two.

  3. Thanks guys!
    And - I've posted up a new photo' of that first board.