Slow Start, Fade Lamp Dimmer Circuit

This article teaches how to construct a light bulb dimmer switch that smoothly fades IN and OUT.

The light will start to get brighter when turned ON and fade away until shutting out completely when turned OFF.

This is a lovely look for living rooms or bedrooms.

Circuit Working:

Parts List:

This circuit for an automatic light dimmer which allow a lighting system to slowly turn ON and OFF.

The process of slowly charging capacitor C1 begins when switch S1 is turned ON.

The LED flashes when transistor T1 starts conducting when the voltage across C1 hits 0.6V.

T1 conducts more current when the capacitor voltage is increased higher which makes the LED light brighter.

The Triac regularly transmits quickly when the LED is lit because the LDR has less resistance.

This method makes it possible for the lighting system to be slowly illuminated.

On the other hand because the capacitor voltage at the base of T1 transistor discharges slowly the LED does not immediately go out when switch S1 is switched off and released.

To create a fading out effect in the illumination the LED gradually dims until it finally shuts out.

It is necessary to set potentiometer P2 such that D1s anode voltage is around 0.7V.

With this setup the capacitor voltage stays at or near 0.5V while the lights are out in standby mode.

Formula:

The RC time delay formula is used to calculate the time constant in the resistor capacitor (RC) circuit shown above.

The time constant 𝜏τ tells how long it will take for the voltage across a capacitor to reach about 63.2% of its maximum value when it is charged or discharged.

Formula for RC Time Constant:

The RC time constant 𝜏 is easily calculated using the following formula:

τ = R * C

where:

• τ is the time constant in seconds.
• R is the resistance in the circuit in ohms.
• C is the capacitance in the circuit in farads.

An RC circuits charging or discharging time delay T is roughly calculated by multiplying the time constant (τ) by a number that represents the percentage of the final voltage.

In most cases it is given for 63.2%, or 1−1/e where e is the natural logarithms base.

From 0% to 63.2% is the final voltage for charging:

T_charge​ = 0.693 * τ

For discharging (from 100% to 36.8% of final voltage):

T_discharge = 0.693 * τ

In an RC circuit the capacitor voltage usually decreases or rises rapidly giving the natural logarithm of 2 (ln(2)) which in this particular case is 0.693.

How to Build:

To build a Slow Start, Fade Lamp Dimmer Circuit following steps are important to follow for connections process:

Connect Transistor T1:

• Connect the emitter of T1 to the ground.
• The collector of T1 should be connected to one terminal of the LED.

Connect Triac:

• Connect the anode of the triac to the collector of T1.
• And the cathode of the triac connect to the positive side of the power source.

Connect LED:

• Connect the other terminal of the LED to the ground.

Connect LDR:

• Connect one end of the LDR to the base of T1.
• And the other end of the LDR connect to the cathode of D1.
• The anode of D1 connect to the junction of the SCR anode and T1 collector.

Connect Capacitor C1:

• Connect one terminal of C1 to the base of T1.
• Connect the other terminal of C1 to the anode of D1.

Connect Resistor R1:

• Connect one end of R1 to the base of T1.
• Connect the other end of R1 to the anode of D1.

Connect Potentiometer P2:

• Connect one end of P2 to the junction of T1 base and C1 and R1.
• Connect the other end of P2 to the ground.

Connect Switch S1:

• Connect one end of S1 to the junction of T1 base and C1, R1 and P2.
• Connect the other end of S1 to the positive side of the power source.

Modifications:

• Potentiometer P2 should be adjusted to set D1s anode voltage to around 0.7V.
• The resistor, capacitor and transistor values should be selected according to the particular needs and the qualities of the components.

Power:

When the switch S1 is closed and the circuit is connected to a proper power source the LED should turn on slowly as explained in the article.

The LED shall dim slowly before shutting off when switch S1 is released.

Take note:

To avoid any problems remember to verify the connections and component values twice.

If unfamiliar with electronic circuits think about visiting a specialist or asking someone with electronics talents for help.

Conclusion:

In the Slow Start Fade Lamp Dimmer Circuit various parts and designs are used according to the expected usage and the required level of control.

As with any electrical circuit it is important to follow to a complete circuit diagram and confirm that the components used are right for what they are supposed to use for.

Taking the proper safety precautions is also necessary while working with electrical circuits.

References:

Fade light 220VAC