Controlling multiple strings of leds

brianandrus

All,

Pardon the ignorance, I am more software than hardware these days...

I have the propeller demo bard and want to hook it up to control several strings of G35 lights.
I think I have the software ready to play with, but I am a little nervous about hooking up the strings. They have 3 inputs (V, GND and Data).
Since I want to control several strings, I seems sensible to let each string use it's own power supply and just connect each data wire to a digital out pin on the propeller (please correct me if that is not a good plan).
Now my concern is the GND connector, which I need connected to the propeller as well so I can send the data signals down the data connection.
Is there any concern with combining the GND from the power supply for the lights with the GND off the propeller? I am guessing I could put a diode there top be safe but it would be ok.

Input/suggestions?

Brian

Mark_T

One normally commons grounds unless there's a reason not to. If there was a reason not too then connecting logic signals via an opto-isolator would be a good choice.

Reasons not too might include power supplies running from different mains phases. If everythings from the same wall socket I don't forsee problems.

Beau Schwabe

In the thread below I was controlling 25 LEDs from one Propeller ... but I was doing it so that only 5 LEDs were ever on at any given time. Each LED had a 100 Ohm resistor to a Propeller Pin, with a common to all of the LEDs ... After the I/O resistance of the Propeller pin (about 40 Ohms), and the 1.7V forward drop of the LED, that comes out to about 11mA per LED.

Iled = ( Vsupply - Vled ) / ( Rled + R_IO )
Iled = ( 3.3 - 1.7 ) / ( 100 + 40 )
Iled = 1.6 / 140
Iled = 11.4 mA


Reference:
http://forums.parallax.com/showthread.php?142225-Post-your-2012-Halloween-Projects-here&p=1131799&viewfull=1#post1131799