• Another update on the CNC.

    The interfaceboards are etched, soldered and tested.

    IO / Control Boards

    IO / Control Boards

    Step- and directionsignals are generated by an ATmega32 which is controlled over RS232.
    This is only for testing purpose. In the final version a PC will control the movement.

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  • For a long time I had a layout for this circuit, but could never build it, because the layout was to small to be made by tonertransfer.

    So it was the first layout I made with my new exposure unit. It’s quite small, so it fits under the LEDMatrix itself. Therefore only SMD parts are used.

    It took three trys to get it right. The first failed because the etching solution was to weak, took to long and caused heavy underetching. The second was not properly exposed.

    The third try worked perfectly. The alignment of the two layers was good and the etching took only 10 minutes.

    74HCT138

    74HCT138

    MAX6964

    MAX6964

    Soldering the SMD resistors and transistors was not very difficult using a magnifyer and tweezers.
    The 74HCT138 was more or less easy to solder, but the MAX6964 was a bit harder. (The pins have a pitch of only 0.635mm.) But with desoldering braid any superfluous solder can be removed.

    The MAX6964 is controlled via I²C. It has 2 8-bit registers which switch the 16 outputs on and off and 8 8-bit registers which controll the brightness (via PWM).

    To keep the layout easy and the number of vias low, i had to wire the pixels of the LED matrix in somewhat weird way. But this is easily compensated in software.

    The correct order for the rows is saved in a lookup-table (just an array).
    The data for one row is two bytes long. The first byte contains the data for the first four pixels. (Two bits per pixel, for red and green.) The first byte is left to right, the second byte is right to left.

    Most of the code is I²C communication and transforming pixel data to the correct format for the MAX6964.
    Layout and code can be downloaded here. Datasheet for MAX6964 here. (Maxim is very generous when it comes to shipping free samples…)

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  • Manufacturing circuitboards containing SMD-parts with toner-transfer is nearly impossible. The thin traces almost never get transferred completely to the copper.

    One solution to this problem is to use photoresist boards and expose them with UV light.

    The layout is printed onto a transparency, which is then used as a mask for the photoresist.
    I got the best results with inkjet-printed layouts. They seem to be more opaque than laserprinted ones.

    The cheapest UV exposure unit from Reichelt costs 200€. That’s far too much for what is essentially some UV-light source and a glassplate.

    UV Exposure Unit

    UV Exposure Unit

    A much cheaper way to get one, is to build it yourself.
    So i got a broken scanner (the older, the better (bigger housing)) and a face tanner (not broken, but old) from eBay.

    The tanner contained 4 UV tubes, 2 trafos and 4 starters.

    All this was glued into the gutted scanner with hot glue.

    About 90 seconds per layout seem to sufice for good exposure.

    The boards are then developed in a 10% sodiumhydroxide solution and etched in a mixture of hydrochloric acid and hydrogen peroxide. (More information on this etching solution in this great tutorial).

    The remaining photoresist can be removed with isopropanol.

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