Charlieplexing 7 segment displays

Step 2 Disclaimer for bad design practices

Normally you must connect leds via a current limiting resistor in order to keep them alive for more than a few milliseconds.

Here I'm not using any resistors since the display is connected to the output pins of a microcontroller that normally can't sink or source more than a few times the current the leds/displays can handle continuously. Since we're multiplexing the displays they will only be on for 1/6'th of the time and can handle much more current than if they were on all the time.

In the following schematics and in my construction I've left out the current limiting resistors, both for the leds themselves and also the base resistor for the transistors.

Please note the following
It is a really bad construction practice to skip the current limiting and rely on luck and that duty cycle combined with the relatively low sink capability of the processor and the probable underrating of the leds maximum specs.

In anything that you do professionally, or just want to keep running for an extended period of time (longer than your first test run) one should adhere to good construction practices, read the datasheets and to the maths.

And read rgbphils comment about running leds over the rated current in the comments section of this instrucatble.

Comments

[circuitmage]

FYI on the current limiting; I just completed my first Charlieplex of 6 7-seg displays. I thought I would need current limiting, but after testing and measurments they were not needed so I took those Rs out. The LEDs as multiplexed are abit dim with only a few mA on each segment. It seems the code ensures power limits are in place! My project to be posted soon.