I created a circuit containing one TLC5940 and one ADXL335 breakout board. The ADXL board comes configured with 0.1 uF capacitors to set the bandwidth of each axis to 50Hz.

It is wired to an Arduino Mega 2560 like this. The ground is shared between both the TLC and the ADXL. 5V and 3.3V are hooked up to the Arduino.

On a breadboard, there is no problem. With the exact same circuit a perfboard, the ADXL values seem to fluctuate when the TLC is updated. Also, when fiddling with the 3.3V wire to the ADXL (connecting/disconnecting), LEDs are turning on and off randomly.

I checked my soldering, and while not the best there is (it's my first perfboard) there aren't any shorts/mistakes as far as I can see.

Could it be because the power sources are 'dipping' when updating the LED's? If so, do I need decoupling capacitors to smoothen it out?

  • \$\begingroup\$ I do not get why there is a down vote? \$\endgroup\$ Jan 24, 2012 at 14:08
  • \$\begingroup\$ Online Circuit Builder \$\endgroup\$ Jan 24, 2012 at 14:12
  • \$\begingroup\$ You are asking a silly question, basically "I'm doing it wrong, why doesn't it work?". \$\endgroup\$
    – starblue
    Jan 24, 2012 at 20:14
  • \$\begingroup\$ Look, I'm new to electronics, I have very little background knowledge, and I'm just trying to grasp everything. In realized it was probably the lack of decoupling capacitors, and just wanted a confirmation before buying unnecessary parts or blowing up my IC's. \$\endgroup\$
    – Rengers
    Jan 24, 2012 at 23:40

2 Answers 2


Decoupling capacitors are absolutely essentialTM in any real world circuit.

You want a say 100 uF on the 5V supply to ground ON the perf board plus a 0.01 uF to 1 uF ceramic from 5V to ground at each IC as near to the supply pins as practical.

You also need to be ABSOLUTELY CERTAIN that all pins on all devices that should be connected to a defined level are connected. I'm not going to pore over avery pin description BUT YOU SHOULD.

5940 datasheet
ADXL335 datasheet

it's good to have a circuit diagram of what you are doing BUT what you have presented us with is very sloppy. eg the ADXL IC shows on its data sheet VERY CLEARLY that it needs a supply decoupling cap AND ALSO must have caps on its output lines WHICH CONTROL ITS BANDWIDTH. Either the caps are on your board but not on your diagram (very naughty) or not on your board (even naughtier). So, please to fix either board and/or diagram and report back. You may have decided to use no x y z caps and get max bandwidth - is that what you are now seeing as errors?

Coming back and asking more questions without fixing the board and or diagram and telling us what the reality is would be both pointless and rude - Olin will tell you this very loudly soon if I don't tell you now ;-).

BUT Olin will also vote to close your question and will influence others to do so too. I won't. I don't need to because others will do so quick enough if you don't fix these discrepancies. So, I suggest rapid action and we will all get a good result :-).

  • \$\begingroup\$ Thanks for your response. You are right about the ADXL decoupling, I didn't study the datasheet enough. The bandwidth caps are on the Sparkfun breakout board itself. The thing is, I never realized I needed decoupling caps because the circuit worked flawlessly on the breadboard. \$\endgroup\$
    – Rengers
    Jan 24, 2012 at 13:38
  • \$\begingroup\$ lol tm on "absolutely essential" :) \$\endgroup\$ Jan 24, 2012 at 14:08

Here is the image from the ADXL335 product page. Notice Cdc on the left of the part.

enter image description here

Here is the leading paragraph in the datasheet's "Application Notes" section:

enter image description here

Given these two pieces of information, why would you even attempt to build an ADX335 circuit without decoupling?

  • \$\begingroup\$ I'm kinda new to this stuff, and I didn't read through the datasheet properly. Thanks for pointing this out :). I'm still wondering though, why does this only happen on the perfboard and not on the breadboard? \$\endgroup\$
    – Rengers
    Jan 24, 2012 at 19:00

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