A Fade On Fade Off LED Circuit enables an LED or a string of LEDs to turn on and off gradually rather than abruptly.
It is also known as a dimming circuit with fade effect.
This produces an effect that is smoother and more aesthetically pleasing.
Circuit Working:
Parts List:
| Category | Description | Quantity | Wattage |
|---|---|---|---|
| Resistors | 100k | 3 | 1/4 watt |
| 1Ω 0.5W (or 8.2Ω depending on LED color) | 1 | 0.5 watt | |
| Capacitors | Electrolytic 100μF 25V | 1 | |
| Semiconductors | Transistor BJT BD679 | 1 | |
| Other Components | ON/OFF Switch | 1 | |
| LEDs | 3W 3.3V | 3 |
A bipolar junction transistor BJT is used in this Fade on Fade off LED circuit to regulate the current flowing to a trio of LEDs.
Three 3.3V 3W LEDs and a 12V power source are intended to be used in this circuit.
Ensure that the LEDs you use have a forward voltage that works with your power source.
This is an explanation how circuit works:
The transistor T1 remains off when the switch is off because there is no current passing through its base.
This indicates that the LEDs are off and not receiving any current.
Current enters the transistors base through the R2 when the switch is switched on.
The transistor is activated by this current, enabling current from the power source to pass through the LEDs and the resistor R1.
The current flow to the LEDs is smoothed out by the capacitor C1.
Additional current flows through the LEDs as a result of the capacitor charging up and providing additional current to the transistors base.
The LEDs gradually fade on as a result.
The capacitor will steady the current flow to the LEDs after it is completely charged.
The capacitor begins to discharge through the R4 when the switch is switched off.
As a result, the transistors base experiences a gradual drop in current flow gradually dimming the LEDs.
The resistance and capacitance settings will dictate the rate at which the LEDs turn on and off.
To get the desired result, try with different values.
Formulas and Calculations:
An LED circuit that fades on and off is seen above.
The combination of the resistor R3 and capacitor C1 produces the fading effect.
The time constant (tau) of the RC circuit, which can be computed using the following formula, controls how long it takes for the LED to fade on or off.
tau = RC
where,
- The time constant in seconds is tau.
- In ohms R represents the resistance.
- C stands for capacitance in farads F.
Since C1 is 100uF and R3 is 100k in the above circuit, the time constant is:
tau = (100,000Ω) * (100 x 10^-6 F) = 10 seconds
This indicates that it will take around ten seconds for the LED to fully fade on or off.
R3 and C1 values can be adjusted to alter the fade time.
Note:
- A longer fade time is produced by a higher value of R3 or C1.
- A shorter fade time is achieved with a lower value of R3 or C1.
- To ensure that the capacitor has adequate time to fully charge or discharge before the LED goes on or off, the values of R3 and C1 should be selected.
Remember:
To get the appropriate fade duration, it is crucial to experiment with different component values even if the formula offers a starting point.
How to Build:
To build a Simple Fade On Fade Off LED Circuit follow the connections process mentioned below:
- Gather the components mentioned in the above diagram
- Connect emitter of transistor T1 to ground.
- Connect collector of transistor T1 to positive supply of 12V through series of 3 LEDs and R4 resistor.
- Connect base of transistor T1 to the junctions of R2 and R3 resistor.
- Connect capacitor C1, resistor R1 and ON/OFF switch in series from ground to positive supply of 12V.
- Connect resistor R3 to ground between base of transistor T1 and resistor R2.
- Connect 3 LEDs 3.3V 3W in series to positive supply from resistor R4.
Safety Measures:
- Make sure you construct this circuit using safe electronics procedures.
- Using a soldering iron and power supply that are rated appropriately is part of this.
- It is important to get advice from a certified electronics expert if you are uncomfortable dealing with electronics.
Conclusion:
In conclude, this basic fade on fade off LED circuit provide a method to achieve a seamless and visually beautiful transition for your LEDs improving the visual appeal of your project.
Transistors and capacitors can be used to create dynamic lighting schemes or to create a fading on and off look that mimics natural lighting.
References:
How to create LED turn on immediately but fade out slowly effect with Capacitor