RGB Bulb using NE555

RGB bulb means bulb made up of three LEDs – RED, GREEN and BLUE. It generates light of many different colours. As we all know RED, GREEN and BLUE are three primary colours and all other colours can be generated by mixing these 3 colours in different amount. So in RGB bulb the intensity of RED, GREEN and BLUE LEDs are varied – means the amount of RED, GREEN and BLUE colours are varied to generate different colour or shade of different colour.

The intensity of LED is varied using PWM generated by NE555 IC. NE555 chip generates PWM when it is connected in astable mode. So three NE555 ICs are used to vary the intensity of three LEDs – RED, GREEN and BLUE.

Circuit diagram:

As shown in figure there are three astable multivibrator built using three IC555 each one for one LED – RED, GREEN and BLUE. In each astable multivibrator block there is one 1 KΩ resistor, one 10 KΩ pot, 2 diodes and a 0.1 uF capacitor connected for RC timing components.

· Vcc supply is given to pin 8 and pin 1 is connected to ground. This will provide biasing to NE555

· RESET input pin 4 is also connected to 5 V to enable output

· Control input pin 5 is connected to ground through 0.1 uF capacitor

· 1 KΩ resistor is connected between Vcc and discharge input pin 7

· Two diodes are connected back to back with pin 7 and two fixed terminals of 10 KΩ pot

· Third slider terminal of pot is connected to 0.1 uF capacitor and threshold input pin 6

· Trigger input pin 2 is shorted with pin 6 to make astable multivibrator configuration

· LED is connected with output of NE555 in current sinking mode with current limiting resistor of 330 Ω

Circuit operation:

(Check the circuit diagram tab for RGB Bulb using NE555 )

Operation of each astable multivibrator block is same. It generates PWM. The width of output pulse varies as pot is varied. Let us see how the circuit generates PWM

· When output is high, the capacitor C1 (or C3 or C6) charges through 1K, D1 and part of 10K resistor (say R1_A) and

reaches to 2/3 Vcc

· The output goes low and now capacitor discharges through another part of 10K (say R1_B) and D2 to 1/3 Vcc

· So by sliding 10K pot left side R1_A decreases – charging time decreases – ON time decreases – duty cycle decreases. But at the same time R1_B increases – discharging time increases – OFF time increases

· Similar to that if pot is turned right side, R1_A increases – charging time increases – ON time increses – pulse

width increases. But because R1_B is decreased, OFF time is decreased

· Thus by sliding pot on both side pulse width can be varied When this PWM is applied to LED, the average voltage

applied to LED varies. This varies its intensity – brightness. Thus three different pots vary brightness of RED,

GREEN and BLUE LEDs. Actually it varies the amount of RED, GREEN or BLUE colour. By varying any pot

from min to max, the amount of that colour is varied from 0% to 100%. So by setting these three pots to any

different value will give different combination of these three colours for example

· When R8 pot is turned to max value BLUE LED intensity will be almost 0%. At that time as other two pots R1 and

R5 are varied, it generates different colours like RED, GREEN, YELLOW, LIME, ORANGE, and may other shades of

GREEN and RED colours

· Similarly when R5 pot is set to max value to turn off GREEN LED and when R1 and R8 pots are varied it generates RED, BLUE, MAGENTA, VIOLET and various shades of RED and BLUE colours

Thus by varying these three pots many different colours can be generated and that’s why its called RGB bulb.