Midibox SID LCD backlight

MIDIbox SID
1

hello all, i’ve just returned from my holiday to find a lovely package waiting to be torn open and the contents constructed!  ;D

now, the problem is: i’ve made up the first core, and it works fine, i’ve uploaded MIOS and the SID application, however, i cannot adjust the backlight with the left hand pot. the other adjusts the contrast just fine. what can i have done wrong? i have checked the pins of the pot with my multi meter, but there is no voltage any direction. any ideas? thanks.

also an unrelated (i think) problem, i have an lcd from SmashTV which works fine (apart from the backlight being on full power). however i also have an LCD from crystalfontz, but that backlight does not work at all. i applied the backlight voltage in reverse, and it worked! is this unrelated to the adjustment problem, or could it be related?

cheers in advance.

2

the brightness pot does work, but on my LCDs, it barely registers (this is true for the smash LCD)

3

Maybe it works better with a 50k pot instead of a 10k pot, could somebody try this out?

i applied the backlight voltage in reverse, and it worked! is this unrelated to the adjustment problem, or could it be related?

The backlight has a polarity, it’s a LED - therefore this is not related

Best Regards, Thorsten.

4

the brightness pot does work, but on my LCDs, it barely registers (this is true for the smash LCD)

i measured the output to the LCD though (B+ and B-) and there was no change in voltage. strange.

5

Maybe it works better with a 50k pot instead of a 10k pot, could somebody try this out?

i’ll try this out.

The backlight has a polarity, it’s a LED - therefore this is not related

Best Regards, Thorsten.

yes i understand this, i just couldn’t understand why they would be different if they both are compatible with the hitachi standard. i guess it’s just the other pins that have to comply, not the backlight. ah well, i’ll just have to switch them round.

6

You won’t register a change in voltage with the brightness control, it’s acting as a current limiter. LED’s don’t really respond well to changes in voltage, more current usually = more brightness. As TK said, i’d put in a bigger pot, maybe get hold of a 100K multi-turn pot, i’m sure that’ll solve your problem.

7

Yeh the HD44780 standard does not include the backlight, that’s left up to the manufacturer - although I never could figure out why they use strange voltages sometimes  ???

8

You won’t register a change in voltage with the brightness control, it’s acting as a current limiter. LED’s don’t really respond well to changes in voltage, more current usually = more brightness. As TK said, i’d put in a bigger pot, maybe get hold of a 100K multi-turn pot, i’m sure that’ll solve your problem.

ahh i see. thanks for the tip.