RasPi and PWM controlled LED strip

Easiest thing I can recommend is to use an ULN2003 8 channel driver chip, quite cheap and available everywhere

you can gang channels together for more current handling and each channel will handle about 500mA (So 4A total)

look here for details http://www.newark.com/stmicroelectronics/uln2003a/darlington-transistor-array-npn/dp/89K1130

you may need to search you local branch, just search for ULN2003

directly logic compatible and will handle well over 12 volts for the supply of the devices being driven and much simpler to interface then individual transistors (No resistors except perhaps to limit the LED current

power the PI from a separate adapter than the LEDs and other loads, it will be much more stable and less lightly to blow the PI and then you do not need a capacitor on that line.

only have the grounds of the PI and LED supply connected together.

You probably can use any available GPIO for the ON/OFF and there is one GPIO that supports PWM, this too can directly drive the 2003 chip

see http://raspberrypi.stackexchange.com/questions/298/can-i-use-the-gpio-for-pulse-width-modulation-pwm for details on PWM

if you still want to use the TIP120 then put a 2K2 resistor (Approx.) between the output of the PI to the Base of the TIP120, connect the emitter to ground and the collector to the LED's low side, connect the high side of the LEDs to the 12V. Make sure there is something to limit the LED current if there not pre wired to sun on 12V supply (I don't know the brand or configuration you using) The ULN2003 contains 8 channels of drivers and there all Darlington pairs so very high gain, plus they contain all resistors etc to drive the load from a simple logic output so you wont need resistors between GPIO and the 2003 for instance (Simpler to use)

Easiest thing I can recommend is to use an ULN2003 8 channel driver chip, quite cheap and available everywhere

you can gang channels together for more current handling and each channel will handle about 500mA (So 4A total)

look here for details http://www.newark.com/stmicroelectronics/uln2003a/darlington-transistor-array-npn/dp/89K1130

you may need to search you local branch, just search for ULN2003

directly logic compatible and will handle well over 12 volts for the supply of the devices being driven and much simpler to interface then individual transistors (No resistors except perhaps to limit the LED current

power the PI from a separate adapter than the LEDs and other loads, it will be much more stable and less lightly to blow the PI and then you do not need a capacitor on that line.

only have the grounds of the PI and LED supply connected together.

You probably can use any available GPIO for the ON/OFF and there is one GPIO that supports PWM, this too can directly drive the 2003 chip

see http://raspberrypi.stackexchange.com/questions/298/can-i-use-the-gpio-for-pulse-width-modulation-pwm for details on PWM

if you still want to use the TIP120 then put a 2K2 resistor (Approx.) between the output of the PI to the Base of the TIP120, connect the emitter to ground and the collector to the LED's low side, connect the high side of the LEDs to the 12V. Make sure there is something to limit the LED current if there not pre wired to sun on 12V supply (I don't know the brand or configuration you using) The ULN2003 contains 8 channels of drivers and there all Darlington pairs so very high gain, plus they contain all resistors etc to drive the load from a simple logic output so you wont need resistors between GPIO and the 2003 for instance (Simpler to use)

I was thinking of using the cap to protect the RasPi from voltage spikes at startup, I have measured a slight spike on the 5v rail, is this necessary? (it reached 5.3 and pretty much immediately set itself to 5.01v)

When you say high and low, do you mean positive/negative? (coming from a purely computerbased background, I can only think of 1 and 0 when hearing that )

I was thinking of using the TIP120 as I have quite limited space (also because I have a couple lying around, hehe), and I would need to rethink the cabling a little, if I should choose to use the 2003 chip.

But thanks alot for giving me some solid advice on resistor values and such, and thanks for the link on the PWM functionality. And it is good to know that it is not too many questions

One last question: do I need to take care of what type of wire I use, or is that not so important? (thinking of using solidcore wire, unsure of thickness yet)

Cap for the PI supply, not necessary but make sure you have a good power adapter (a 5V one, NOT 5.25V one) and a good USB cable to the PI (Or a short one), see http://www.element14.com/community/community/raspberry-pi/blog/2014/03/07/raspberry-pi-usb-power-cables-crashing-and-other-problemsfor details and background

solid wire is ok on bread boards and proto boards, not so good if the device attached can move around

I would suggest something like 24AWG or better multi-strand wire for the run to the LEDs, available from home depot and most hardware stores. Here depending on how far away they are you could get a significant voltage drop across the cable if it is too thin (28AWG for instance) and the LEDs will therefor be dimmer.

Using the TIP120 is ok, it is already a darlington pair (Like one of the channels in the 2003 but without the base 2.2K resistor) but more current capable so it will work with no problems in your application

The LEDs your mentioning should have markings on them to indicate + and -. high refers to the + end, low refers to the - end.

As said, the PSU is a Corsair CX, it is using the 5v rail on that, meaning a relatively stable ATX PSU.

The device will not move around much (it will be put in place, and moved slightly when opening the case door).

They have the markings, the prototype is now wired up with everything in a bit of an ugly way, and now are only missing the paintjob to reach a "proper" casemod procedure, thanks alot for the help

no problem, post a pic when complete, would be nice to see

Peter,

The ULN2003 is only a seven channels power darlington.

For an 8 channels, there was an ULN2083 most likely discontinued now.

Or better look for MOSFET low side drivers that are now higher current and fairly high voltage.

Jean-Paul

AC9GH

Thanks for the correction, either way in this case more than enough for the job

Peter

Peter I would note the manufacturer's de-rating curves for temperature/Duty Cycle/number of outputs and I think you'll find it's current switching capacity for this application to be much much less than the optimal 4A.

Also he does  not specify the number of light strips which are normally at 1A+ per Metre.

Larger descrete FETs may well be the way to go !

this is all for illuminating the inside of a computer case for a mod hack, so not too many LEDs and stuff the devices recommended should be ok for this purpose

I was working on the "several strips" at 700ma ea and mapping on to the curves in the Data Sheet

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