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multiplexing LEDs how do I keep the LEDs bright shining? — Parallax Forums

multiplexing LEDs how do I keep the LEDs bright shining?

StefanL38StefanL38 Posts: 2,292
edited 2012-11-24 04:42 in General Discussion
Hi,

A friend of mine wnats to build a 6x6x6 LED-cube. To keep the number of IO-Pins smaller we try to realise the concept of multiplexing the LEDs.
Switching On/Off columms and rows of the LEDs. Now this means the ON-time of the LED is just a fraction of 100%. As there are a lot of rows and columms
the voltage must be increased to get an averaged current that still makes the LED shining bright.

Is my conclusion right if the multiplexing should stop accidentically a much too high current would destroy the LED?

So one idea to overcome this is to add a transistor and a RC-component to each channel. the multiplexing refreshes the energy stored in the capacitor
to keep the transistor "switched on" until the next refreshing pulse comes in. This means the transistor would stay switched on all the time as long as refreshing pulses come in.

This means the voltage and current can be set to the continous operation and then if the multiplexing of the matrix stops accidently the opposite will happen.
The energy of the capacitor will go down to zero switching off the transistor and the current through the LED.


What do you think about this solution? will this work? Are there better solutions with less effort?

Please comment on this idea.

best regards
Stefan

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2012-11-22 12:34
    You can certainly do what you suggest. Most commonly designers ignore the problem or decide that the risk is low, then design the program so the risk is indeed very low. You can build a timeout into the program so, if the program doesn't reset the timeout, the power to the display gets cut. This is one of those inherently dangerous situations with a cost (of repair / replacement) if the program fails to execute properly. You have to estimate the risk and compare that to the potential cost to see what measures make sense financially. If you have a high cost of repair (like in a spacecraft), you may opt for some completely different design choices ... like avoiding multiplexing unless you can guarantee that the device (LED) can handle the peak current without degradation.
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-11-22 12:37
    Yes, if your design is based on 10% duty and your are giving it a mA rate in the upper limit.
    but while playing around with code it stops multiplexing, you could burn something.

    Could use higher resistor values temporary until code is done,
    or go with many 595 ICs that eliminates the need to multiplex.
  • Invent-O-DocInvent-O-Doc Posts: 768
    edited 2012-11-23 05:47
    Just keep in mind that the capacitor has to charge in addition to discharge, so you may be limited on how fast you can refresh the LEDS. For cool light 3D displays, that might be fine....
  • jmgjmg Posts: 15,183
    edited 2012-11-23 22:08
    StefanL38 wrote: »
    Is my conclusion right if the multiplexing should stop accidentically a much too high current would destroy the LED?

    Yes.
    StefanL38 wrote: »
    So one idea to overcome this is to add a transistor and a RC-component to each channel.....

    What you describe is a local watch dog, and they are quite common.
    You can make one from a Monostable, or a charge pump which needs a certain clock-rate to overcome some discharge.

    You can do it on every channel, but often just a presence of clock/Data is decided as 'I have scan', and then the OK is given to drive.
    Usually you can gate in some common way, so can avoid adding a lot of components to implement this.
  • Erik FriesenErik Friesen Posts: 1,071
    edited 2012-11-24 04:42
    http://www.ti.com/product/tlc5917
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