Computing stuff tied to the physical world

Let’s find out what’s going on

While testing the FTDI serial connection, it can be quite useful to instantly see what’s going on with each of the pins. Here’s a little custom “plug-through” circuit we can use for that:

Screen Shot 2015 02 11 at 09 20 53

The idea is: 5V is reduced to approximately 3V via three diodes. These are common 1N4148 diodes, with each about 0.65V drop when connected in the forward (conducting) direction.

The rest is simple: an LED tied to a signal pin, with a 1 kΩ resistor in series. Some math to explain that resistor value: 5V – 3x 0.65 – 1.8V = 1.25V. So a 1 kΩ resistor will pass 1.25 mA, which is also the current through the LED: enough to light it up, but not too high to load down the signal line too much (it can probably handle somewhere from 3 to 10 mA).

Note that all signal lines use inverted logic, so we want the LED to light up when the signal is low. If you follow the path from 5V -> diodes => resistor => LED => signal, you’ll see that this will only pass current when the signal is a “0”. On a “1”, it’ll be almost 3.3V, and then there will not be enough voltage on the LED to light it up. Simple!

Here is the above circuit, placed on a little protoype board:

DSC 4936 DSC 4937

This is the complete setup, with an FTDI BUB and the LPC810 programmer hooked up:

DSC 4935

And sure enough: this little tool was essential to solving some issues. Coming up next…

[Back to article index]