mandag 3. august 2015

Led indicator ring driver

I intend to have rings of LEDs around the potentiometers on my synth to show the current value of the parameter, and have been thinking about ways of doing it. One can take the normal approach and do a full matrix that runs through all the pots and have a single controlling circuit, but that requires a lot of data lines running across the PCB. Another option is to use a 16bit shift register per dial and connect each led to a pin, which reduces the number of lines necessary to six - two power lines, serial in-out and clock, latch.

I have been considering having even more LEDs per dial. For 32 LEDs this last method requires either two 16bit or one 32bit shift register, and things start being costly and take up a lot of board space.

Thus, a combination may be a good solution. If one uses a 12bit shift register, one can control 6 rows x 6 columns for a total of 36 diodes, while still only using 6 transmission lines (16 diodes may be controlled by a 4x4 matrix, requiring an 8 bit register only). It means you have to do row scanning instead of just updating the shift registers whenever the dials change, so there are pros and cons of each method. The scanning requires more (but cheaper) components - transistors and resistors. It also requires a more complex controlling algorithm which takes up time in the microcontroller, and ultimately the number of controllable LEDs and the refresh ratio is limited by the time it takes to fill the shift registers.

A possible circuit may look like this:


The shift register must be able to sink around 20mA per pin which means a normal CMOS chip won't do. A constant current sink led driver will be a good option, it may even do away with the resistors in the schematic.

I've found 16bit led driver shift registers at prices of just above $1in quantities of 50, for example the STP16CPC26 at mouser.com.

PS: a 32 LED dial may be controlled from a 12 bit shift register using 8 bits for rows and only 4 for columns. This reduces the necessary transistors from 6 to 4.

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