RasPi and PWM controlled LED strip

Question

Hello there people, I am currently working on a casemod, and I would like to use a RasPi as the controller for some LED strips (12v, pulling about 700ma each).

The RasPi will act as a webserver hosting a website for remotely controlling the strips, as well as viewing some information about the system (temp sensors, fan speed. don't care about these, they are up and running). But it will also control 2 12v LED strips, and these strips pull too much for the RasPi to control directly.

Now, I do have some TIP120's here, and as far as I can see they should work, but I am unsure of how to connect them.

Could someone here please help me with getting everything set up properly?

Parts:

PSU: Corsair CX

RasPi: Rev 1, model B

Lights: 12v, pulling around 700ma each

Wanted outcome:

PWM control on one LED strip

Normal (on, off) on the other

A living RasPi

Questions:

Could you help me set this up in a way that does not any part?

Do I need a capacitor in front of the RasPi to ensure no harm is done under startup? (Will 4700 microfarads do?)

Should I use a fuse, or is it unnecessary?

Am I asking too many questions?

Robert Peter Oakes · Accepted Answer

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)

Robert Peter Oakes · Answer

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-problems for 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.