Red Green Blue (RGB) LED's are a marvel to watch in action, and I have got to say, they are worth the effort to play around with. The programming side of things does take a little thought, primarily because you need to control 3 different Pulse Width Modulated (PWM) signals to control the brightness of each LED (Red Green and Blue).
RGB LED's have 4 pins, 3 of them are for a specific colour (either Red Green or Blue), and the forth is a common anode or cathode. I prefer to use the common anode devices, as I like to use the ULN2003 to drive the LED's. This way I can hook up many LED's in parallel to get some very powerful and amazing lighting effects.
The LED's I am using are Common Anode so that I can use the ULN2003 to drive many in parallel. I brought them from www.core-electronics.com.au and I'm very impressed by the vivid colour and extreme brightness of these LED's! Below is the pin-out;
Note the PIC's power supply/oscillator are not shown
The above diagram utilises the ULN2003 to drive much higher current devices then what the PIC can handle so with this design, you could put a lot of RGB LED's in parallel for some really stunning lighting effects.
The RGB LED's from Core Electronics have a forward voltage drop of about 2 Volts for the Red LED, and 3.5 Volts for both the Green and Blue LED's. To ensure that I am running them at full brightness (20mA for each colour), I can calculate the resistance required for each resistor by the following equation;
R = (V - Vf - Vce) / I
Where R = Resistance in ohms, V = Supply Voltage, Vf = LED Forward Bias Voltage, Vce = ULN2003 voltage drop, I = Current
To get the maximum brightness from your LED's, replace the resistors with the following (increase the current if more LED's
are used in parallel);
Red resistor = (5 - 2 - 0.9) / 0.020 = 105 ohm or greater
Green resistor = (5 - 3.5 - 0.9) / 0.020 = 30 ohm or greater
Blue resistor = (5 - 3.5 - 0.9) / 0.020 = 30 ohm or greater
Because the program is interrupt driven, I want to take as little time as possible to service the interrupt. One way is to minimise the code, another is to maximise the clock speed, so I chose a little of both.
The reason why I chose an Interrupt driven routine, is so that once the registers and interrupt is setup, you can do anything you want in the main program, and not worry about the performance of the RGB LED. In the circuit above, I use the 18F4550 with a 20Mhz Crystal Oscillator and PLL enabled (a couple of config settings) to achieve 48Mhz CPU Clock speed, handy.
The program has to incorporate the control of three different PWM signals (one for each colour). I wanted to maintain as much brightness control as possible, and decided to make the Duty Cycle editable from 0 t0 100% by simply editing a register from 0 to 255.
In the example below, I am using the 18F4550 with a 20Mhz external oscillator, and with a few PLL config settings, the CPU will operate at 48Mhz (12 million instructions per second). The Timer 2 interrupt will be serviced every 50uS, that is 20Khz. With this in mind, as each complete PWM signal takes 256 timer cycles, this means that a PWM pulse will be driven 20000 / 256 = 78.125 Hz. Anything above 25Hz will look fluent to the human eye!
This might not be the most efficient way to create three PWM signals, but it allows complete diversity in the fact that you can control the brightness of each LED colour component on the fly while doing anything else! The program also uses the random number user library to create random numbers from 0 to 255.
The 19mS delay between increments/decrements of the colours is to control the time it takes going from one extreme to the other, in this case, 19mS ensures it takes just under 5 seconds for all 256 brightness levels to change.
Device = 18F4550 Clock = 48 Config PLLDIV = 5, CPUDIV = OSC1_PLL2, USBDIV = 2, FOSC = HSPLL_HS Include "RandGen.bas" Dim TMR2IE As PIE1.1, // TMR2 interrupt enable TMR2IF As PIR1.1, // TMR2 overflow flag TMR2ON As T2CON.2, // Enables TMR2 to begin incrementing Signal_Pin As PORTB.0 // Signal output to frequency meter Dim Red_Pin As PORTC.0, Green_Pin As PORTC.1, Blue_Pin As PORTC.2, Red_Duty As Byte, Green_Duty As Byte, Blue_Duty As Byte, Red_DutyVal As Byte, Green_DutyVal As Byte, Blue_DutyVal As Byte, RandomVal As Byte Dim uS As Word, mS As Word Interrupt TMR2_Interrupt() High(Signal_Pin) Save(0) // Back up system variables If TMR2IF = 1 Then // Check if the interrupt was from TMR2 TMR2IF = 0 // Clear the TMR2 interrupt flag uS = uS + 50 If uS >= 1000 Then uS = uS - 1000 Inc(mS) EndIf Inc(Red_DutyVal) Inc(Green_DutyVal) Inc(Blue_DutyVal) If Red_DutyVal > Red_Duty or red_Duty = 0 Then Red_Pin = 0 Else Red_Pin = 1 EndIf If Green_DutyVal > Green_Duty or Green_Duty = 0 Then Green_Pin = 0 Else Green_Pin = 1 EndIf If Blue_DutyVal > Blue_Duty or Blue_Duty = 0 Then Blue_Pin = 0 Else Blue_Pin = 1 EndIf EndIf // Restore // Restore system variables Low(Signal_Pin) End Interrupt Private Sub TMR2_Initialize() TMR2ON = 0 // Disable TMR2 TMR2IE = 0 // Turn off TMR2 interrupts PR2 = 149 // TMR2 Period register PR2 T2CON = %00000001 // T2CON 0:1 = Prescale // 00 = Prescale is 1:1 // 01 = Prescale is 1:4 // 1x = Prescale is 1:16 // 3:6 = Postscale // 0000 = 1:1 postscale // 0001 = 1:2 postscale // 0010 = 1:3 postscale... // 1111 = 1:16 postscale TMR2 = 0 // Reset TMR2 Value TMR2IE = 1 // Enable TMR2 interrupts TMR2ON = 1 // Enable TMR2 to increment Enable(TMR2_Interrupt) End Sub // Start Of Program... Low(Red_Pin) Low(Green_Pin) Low(Blue_Pin) Red_Duty = 0 Green_Duty = 0 Blue_Duty = 0 Red_DutyVal = 0 Green_DutyVal = 0 Blue_DutyVal = 0 uS = 0 mS = 0 RandGen.Initialize(128) // Initialize the Random Number generator TMR2_Initialize // Setup and enable TMR2 While True // Create an infinite loop RandomVal = RandGen.Rand // Grab a random number from 0 to 255 Select RandomVal // Find out what colour to increase/decrease Case 0 To 42 While Red_duty < 255 mS = 0 Repeat Until mS = 19 inc(Red_Duty) Wend Case 43 To 84 While Red_duty > 0 mS = 0 Repeat Until mS = 19 dec(Red_Duty) Wend Case 84 To 127 While Green_duty < 255 mS = 0 Repeat Until mS = 19 inc(Green_Duty) Wend Case 128 To 170 While Green_duty > 0 mS = 0 Repeat Until mS = 19 dec(Green_Duty) Wend Case 170 To 212 While Blue_duty < 255 mS = 0 Repeat Until mS = 19 inc(Blue_Duty) Wend Case 212 To 255 While Blue_duty > 0 mS = 0 Repeat Until mS = 19 dec(Blue_Duty) Wend End Select Wend
Simulation of the program
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