Computing stuff tied to the physical world

No more diode!

In Hardware on Dec 6, 2010 at 00:01

The JeeNode v5 introduced a change, which I now regret – due to the amount of confusion it has generated, and the limited utility: an extra protection diode in the PWR line on the FTDI connector (and only there!).

Here’s the PCB layout, as seen in EAGLE:

Screen Shot 2010 12 05 at 17.44.49

The diode is marked “D1”, with a solder bridge on the back side of the board (red lines are on top, blue lines on the bottom, and plated-through holes are in green).

Here’s a close-up:

Dsc 2074

What it does, is block the power when it’s connected the wrong way around. But that only protects the FTDI connector, which is actually quite well protected agains reversal already (if you flip the connector, GND will go to the reset cap, which blocks all DC current anyway).

The drawback of this design is that it lowers the voltage by about 0.7V, so instead of 5V, you get about 4.3V on the remaining PWR pins, such as on all the port headers.

Worse still, is that it causes the JeeNode to stop working from a 3.3V power source, such as the AA Power Board, because it will drop the voltage to about 2.5V – which is too low to run at 16 MHz!

I did design a workaround into the JeeNode v5: there’s a solder jumper on the other side of the board, to bypass that diode. Where “bypass” means: short out the diode, as if it wasn’t part of the circuit in the first place.

The irony is that I’ve been closing this solder jumper on every new JeeNode I’ve built so far.

Add to that the fact that the holes for that diode are almost too small to push the thick diode wires through, and it becomes clear that this new “feature” causes more problems than it solves.

Starting now, once the stock of kits is exhausted, new kits will be shipped WITHOUT the diode!

You will now have to always add a drop of solder to close the jumper on the back:

Screen Shot 2010 12 05 at 18.04.03

If you don’t, there will no power connection from the FTDI header to the JeeNode.

It’s an important change, but it removes the nasty suprise that sometimes things work and sometimes they don’t!

Oh, and if you’re into this tradition – I hope you had a nice Sinterklaas!