PWM LED Ramping Circuit (Slow Bright, Slow Fade)

An PWM LED Ramping Circuit helps change the brightness of an LED smoothly, so it lights up gradually instead of just turning on and off suddenly.

This smooth lighting effect is really useful for many things, like visual signals, decorative lights and soft signaling systems.

The circuit shown here uses two IC 555 timers, an LM393 comparator and a 2N7000 MOSFET to create this gradual change in brightness.

To work best the power supply for this circuit should be between 3 to 12 volts and using 6 volts is especially good for this circuit.

Circuit Working:

Parts List:

In the above circuit diagram IC1 555 timer works as a square wave generator.

It creates a clock pulse at its output, which is found at Pin 3.

The frequency of this pulse depends on the values of resistor R1 and capacitor C1.

IC2 555 timer device acts as a ramp signal generator.

Here, capacitor C2 charges and discharges causing the voltage to change slowly over time.

The rate of this voltage change is controlled by resistor R2 and capacitor C2 together.

IC3 LM393 comparator component compares the ramp signal to a set reference voltage.

It produces a pulse width modulated PWM signal based on the voltage level of the ramp signal.

Q1 2N7000 MOSFET works like a switch that controls how bright the LED is.

The PWM signal from the comparator adjusts the MOSFETs conduction which makes the LEDs brightness gradually slow bright, slow fade.

Formulas with Calculations:

Formulas with Calculations are mentioned below for PWM LED Ramping Circuit (Slow Bright, Slow Fade):

IC1 555 Square Wave Oscillator Frequency:

f = 1.44 / (R1 × C1)

where,

• R1 is 47k
• C1 is 0.001µF

Calculation:

R1 = 47kΩ (47000Ω)

C1 = 0.001µF (0.000001F)

f = 1.44 / (47000 × 0.000001)

f = 30.6 kHz

IC2 555 Ramp Generator Time Constant:

t = R2 × C2

where,

• R2 is 1M
• C2 is 1µF

Calculation:

R2 = 1MΩ (1000000Ω)

C2 = 1µF (0.000001F)

t = 1000000 × 0.000001

t = 1 second

MOSFET Current Calculation:

I_LED = (V_supply – V_MOSFET) / R4

where,

• V_supply is 6V
• V_MOSFET voltage drop is 0.2V
• R4 is 180Ω

Calculation:

I_LED = (6 – 0.2) / 180

I_LED = 32mA

How to Build:

To build a PWM LED Ramping Circuit (Slow Bright, Slow Fade) following steps should be followed to design your own circuit for connections:

• Gather all the components as mentioned in the above circuit diagram
• Connect pin 1 of IC1 to GND of the circuit
• Connect pin 2 of IC1 to pin 6 of IC1
• Connect pin 3 of IC1 to pin 6 of IC1 through R1 resistor.
• Connect pin 4 and pin 8 of IC1 to positive supply of the circuit.
• Connect capacitor C1 from pin 2 of IC1 and GND
• Connect pin 1 of IC2 to GND of the circuit
• Connect pin 2 of IC2 to pin 6 of IC1
• Connect pin 3 of IC2 to pin 6 of IC2 through R2 resistor.
• Connect pin 4 and pin 8 of IC2 to positive supply of the circuit.
• Connect capacitor C2 from pin 2 of IC2 and GND
• Connect pin 1 of IC3 to GATE pin of MOSFET Q1
• Connect pin 2 of IC3 to pin 6 of IC1
• Connect pin 3 of IC3 to pin 6 of IC2
• Connect pin 4 of IC3 to GND of the circuit.
• Connect pin 8 of IC3 to positive supply of circuit
• Connect resistor R3 from pin 8 of IC3 to pin 1 of IC3
• Connect the DRAIN pin of MOSFET Q1 to cathode of LED1 and anode of LED1 connect to one one of resistor R4 and other end of R4 connect to positive supply.
• Connect the SOURCE pin of MOSFET Q1 to GND of the circuit.

Conclusion:

This PWM LED Ramping Circuit gradually makes an LED slow bright, slow fade giving it a nice fading look.

It uses 555 timers, a comparator and a MOSFET to keep everything running smoothly and reliably.

You can change the fading speed by tweaking the resistor and capacitor values which makes it useful for various projects.

References:

PWM Led Fade out slowly help required!