Recently I acquired a 5M length of RGB LED strip using SMD5050 RGB LEDs. It has built in current limit resistors designed for operation from a 12 volt supply. Having thought this would directly attach to the Picprojects MOSFET RGB LED driver project I went ahead and bought one only to discover when it arrived that it wasn’t going to be that easy.
Despite the description and markings on the supplied strip indicating it had a common anode connection it is in fact common cathode. The terminal marked ‘+’ in the photo below is a common ground connection – go figure?
What is required is a high-side driver so the three LED anodes can be controlled by the PWM output from the PIC microcontroller while the common wire connects to ground.
With the requirement defined I decided to put together a quick project to work with the LED strip. The controller on this page is an adaption of the RGB Mood Light 101 project, firmware is the same and can be downloaded from that project page.
You should be aware that not all LED strips use a common ground, I have 1 metre strip that is wired with a common ‘+’ or high-side and works pefectly using the Power MOSFET RGB LED driver kit 106 See my notes on LED strips here
This project is NOT available as a kit, or PCB nor can I supply the LED strip.
The circuit is essentialy the same as the RGB101 Mood Light project and uses exactly the same fimware. Where it differs is in the LED output drive stage. Instead of the BC548 transistor (Q1-Q3) driving the LEDs directly they are used to switch a second set of transistors (Q4-Q6). These are STX790A medium power PNP transistors switching the 12 volt or high-side of the power supply.
The current rating of each colour in the strip is around 1.5 amps which needs a medium power transistor to control it. I’ve avoided using a P-channel MOSFET as they are both expensive and less easy to obtain.
The transistor used for the final output is an STX790A rated at a maximum collector current of 3 amps, with a minimum current gain of 100. The LED strips I used require about 1 to 1.5 amps per colour. Base current for Q4-Q6 is derived from the collector current of the BC548 transistors (Q1-Q3) via R1 – R3. Resistors R1-R3 provide around 20mA of base current to the STX790A. I’ve used 560R 0.25 watt carbon film resistors here, they are operating right on their power dissipation limit for a 0.25 watt resistor. Since the transistors are driven with a PWM signal average power dissipation is lower so not an issue.
If you decide to use an alternative transistor type for Q4-Q6 and need to increase the base current you’ll need to use a 1/2 watt resistor or go for a metal film 0.4 watt or 0.6 watt which are the same physical size as a 0.25 watt carbon film.
For Q1-Q3 any small signal NPN transistor will work. BC546, BC547 or BC549 are also suitable and have the same pinout as the BC548.
If you need more than 2 amps per LED channel you will need to do some redesign of the final transistor output section since the circuit is not designed to handle more than 2 amps on each channel.
The rest of the circuit is straight forward.
The 12 volt input to the board is fed through D1 to a 78L05 5-volt regulator (IC2). D1 provides reverse polarity protection to the regulator though it should be noted this does not protect the LEDs and final driver transistors since due to the high current requirements of the LED strip it is not practical to use a diode here.
Capacitor C1 provides decoupling of the 5 volt supply. Capacitor C2 provides filtering on the input side of the regulator. C1 should be as close to the PICs Vdd/Vss power input (pins 1/8) as practical. The 78L05 and C2 should also be reasonably close to each other and the PIC. R7 provides a pull-up for the PICs MCLR reset input.
S1 is the mode control switch. JP3 just provides a pair of 0.1″ spaced pads for connecting a remote switch if the board is built into a housing.
JP2 is the LED output connector. Take note of the connections on this. The ground, red, green and blue connections have been placed to match the LED strip I was using. You should verify the connections to the specific LED strip you use to ensure they are the same. (see notes on LED strips here)
Also remember the board switches the high or 12 volt side with the ground connection being common to all three LED colours. If you have a common anode strip you will need the Power MOSFET project.
R8/C3/JP1 are not used, do not fit components.
For more detail: RGB LED Strip Controller high-side LED drive for PIC12F629