I have a 10 character x 16 segment display that I'm driving using multiplexing off a PIC. The problem is, however, that with each display only being on 1/10th of the time, the display appears much dimmer than it normally would. Currently the LED's are getting their recommended voltage – 2.2 volts across each red segment. Would increasing the voltage across the display be a safe way to make it brighter? Is there a way to do this in software?
Electronic – Is upping the voltage on a multiplexed LED display an appropriate response for dimness
displayledmultiplexerpicvoltage
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Best Answer
You can overdrive an LED with more than the rated current. The destruction of an overdriven LED is caused (mostly) by the heat generated in the junction (P=IV, and I scales exponentially with forward voltage, V, so power is a higher exponential in V). If you can contrive to keep the heat within reasonable boundaries, probably by overdriving for only short periods to allow heat to dissipate, you can put as much current though an LED as you want. @Anindo Ghosh's answer has a link describing the limits of this.
Another solution, which you can do if you have the ability to add components to the design, is to use shift register/LED driver. This means you can push the data in to a chain of ICs and have all the LEDs constantly driven at a constant current (some driver ICs also include things like dimming controls). When you have pushed in new data, you flip all the chips to the new values at once. Many driver chips have 16 outputs, which sounds like a good match for your displays.
Additionally, you only need 2 pins from your MCU for an I2C device (4 for an SPI), do you can potentially downside on the MCU. You might need another couple for things like output enable and latch strobe if this is not part of the serial protocol used, but it should still be less than the "outputs * log2(devices)" for a basic multiplexer. Also, the time taken to write out to the displays is very short (you only need a few cycles per LED to toggle the lines, and if you can use an interrupt-drive hardware module, it becomes even easier!), freeing processor cycles for other things. You can also ditch whatever multiplexers you currently have.
An example part is the Texas Instruments TLC5925, which has 16 constant-current outputs up to 45mA with analogue dimming control. You need 2 lines to clock data in, one line to en/disable the output and one line to strobe the register latches. This part costs about £1 in single-unit quantities, half that in bulk. It is an SOIC, so soldering is easy, but you can also get much more compact packages if space if limited and your hand is steady.