To modify the Gertboard, I added a 4.7k pulldown resistor to each of the lower (lower according to the schematics above) buffers to keep their LEDs normally off and to provide anti-static protection. 100k pullup resistors were added to each upper (upper according to the schematics above) buffers for anti-static protection.
The modifications were made by soldering surface mount resistors to the underside of the Gertboard. The resistors were of the 1206 size - 0.12" x 0.07" x 0.02" thick. These are about the smallest parts I can solder by hand. The 4.7k resistors (pulldowns to lower buffers) are soldered between adjacent pins of J10. This was fairly easy as the distance between these pins was 0.1". Refer to the photograph above this paragraph. You can see these resistors along the bottom edge.
Soldering the 100k resistors was a little more difficult since this was done at the ICs pins themselves. A copper wire was soldered to pins 20 of the ICs. The resistors were soldered between these copper wires and pins 2, 4, 6, and 8 of the ICs. You can see these connections in the photograph above. I suggest you use the single strand, 30 AWG, wire-wrap wire for this purpose. This is available from Sparkfun for about $9 for 100ft.
If you are using any of the three switches on the Gertboard, the 100K resistors are sufficient to pull the level up to 3.3V when the switch is not pressed. Therefore, it is not necessary to use the Raspberry Pi's internal pull-ups.
Hey, thanks for the post! I am having the same issue with my gertboard. On the photos it seems like you have used the 4.7k resistors for the upper buffers, but you mention 100k in both the schematic and the text, was that a soldering error on your end?
ReplyDeleteThanks Anonymous - You are right. I had a bunch of 4.7K resistors and no others. 100k should be adequate for the purpose though.
DeleteThanks for this. I have a gen 1 Gertboard. I will have a go at fixing the problem the way you described it.
ReplyDeleteGreat blog!
Sorry Marty, but you said "A copper wire was soldered to pins 20 of the ICs". Isn't that pin 10 of the 74HC244 hence GND?
ReplyDeleteThat sentence is talking about the 100k resistors which connect to 3.3V which is pin 20 of the IC.
DeleteI don't mean to be annoying here (although I realise I might becoming). I am honestly trying to understand how to connect the resistors! The picture above is the bottom of the Gertboard. The squared pad on the PCB where the 74HC244 is soldered on is pin 1 right? Pin 20 in just opposed to pin 1 on the other raw:
ReplyDelete1 2 3 4 5 ...
20 19 18 17 16 ...
So the wire to which you connected the resistors is soldered to pin 10 GND. Have I got this all wrong?
Sorry Francesco - you are correct. It's been a while since I did this post. The photo shows how my board is actually modified. When I did the drawing, I realized it was better to use pull-ups rather than pull down resistors. Perhaps you could add the modification to one of the twelve circuits and test that it indeed works OK and recomment. When I get a chance I will change my board to match the drawing and make a new photo.
DeleteFor all of you with sharp eyes like Francesco, who noted in my photo that I did not modify my Gertboard exactly like my schematics, my Gertboard is now modified EXACTLY like my schematics. You can see this in my photo above. I have extensively tested the board to make certain all functions work properly.
Delete