I connected a MAX7219 to a 8x8 LED Matrix. From the datasheet I determined the resistor to be 24.5kOhm
I'm not sure how you calculated that. Generally, the resistor value to use is 10k for an 8 by 8 matrix. Do not alter the "Scan Limit" from 8 however. 40 mA drive is fine for these matrices as of course, it averages to 5 mA as long as
you are multiplexing eight ways.
After some tests it turned out, that the LEDs are way darker than the one I just supply with 5V + pre-resistor, no MAX7219.
But - just what tests were these?
I don't know how you measured that but you can't connect a multimeter to a pulsed output (which is what your LEDs are receiving).
Wouldn't say "can't", more a problem of making any sense of it and it would entirely
depend on the nature of the DMM. I suspect if you get a sensible (stable) reading then it might be measuring the average
, given the 800 Hz multiplex frequency is an order of magnitude in excess of the DMMs integration frequency. Measuring current, a 10 or 100µF capacitor across the meter terminals should sort the average out pretty well and the same plus a 1K series resistor for voltage should give a good approximation anyway. (If you doubt this, measure a battery voltage with and without the 1k - made no
difference to mine.)
Just tried this (no capacitors or resistors) on my (fully lit, un-initialised) matrix and I get a stable value about 0.9V at 4.5V Vcc (USB doesn't like a fully-lit matrix) which seems very optimistic.