Simple Automatic Bedroom Lamp Timer Circuit

Designing a lamp timer for your bedroom that shuts off on its own seems awesome but begin with caution.

Do not touch it when it is plugged in because it draws a lot of electricity from the wall socket.

The circuit itself is not that complicated.

When a button is pressed a series of linked components begin to function together.

What is a Automatic Bedroom Lamp Timer Circuit:

A circuit for an automatic bedroom lamp timer regulates when a bedroom lamp turns on and off on its own.

A circuit of this type is often used to provide the illusion that someone is in a room which raises security by giving the impression that the area is occupied when it may not be.

Circuit Working:

Parts List:

When the push button is pressed the C1100uF 25V capacitor starts to charge.

The BC547 receives bias current from the charged capacitor via the 2.2M resistor even after the push button is pressed.

The bias voltage influences the BC547 to stay switched ON which in turn causes the BC557 to switch ON.

The triac is activated by the gate current that the BC557 collector pin supplies to it.

When the triac is turned on the lightbulb illuminates and keeps lighting.

When the charge in the 100uF capacitor completely disappears the transistors can no longer stay in conducting modes.

After a delay that is caused by the 100uF capacitor the transistors turn off deactivating the triac and turning out the bulb.

The cycle is restarted by pressing the push button again, however the lamp stays out of function.

Formulas:

The formula mentioned below connect the resistor R and capacitor C values to the delay time (t) of a basic RC resistor capacitor circuit.

Only delay off times are covered by this formula.

Delay OFF Time (t): t = R × C

here,

• t represents the delay off time in seconds this is the time it takes for the circuits output to turn OFF after a trigger signal is removed.
• R represents the resistance of the resistor in ohms Ω.
• C represents the capacitance of the capacitor in farads F.

How formula works:

Charging the Capacitor:

The capacitors start charging through the resistor when the circuit receives a trigger signal, usually a voltage change.

Slowly the voltage across the capacitor rises as it charges.

Time Constant (τ):

The Greek symbol tau (τ) which represents the product of R and C is the circuits time constant.

It indicates how long it takes for the voltage across the capacitor to reach roughly 63.2% of the applied voltage.

Delay OFF Time:

As soon as the trigger signal is used out the charged capacitor starts to discharge through the resistor.

The delay off time is directly equal to the time constant (τ) according to the formula:

t = R × C

The time constant (greater resistance or capacitance) increases with the duration of the capacitors discharge resulting in the duration of the delay off period until the circuits output turns off.

Example:

Suppose you have a delay-off timer circuit with a 1μF (microfarad) capacitor and a 10kΩ (10,000 ohms) resistor.

Using the formula:

t = R × C = 10,000Ω × 1μF

Converting microfarads to Farads: 1μF = 0.000 001F

t = 10,000Ω × 0.000 001F = 0.01 seconds

This circuit would therefore have a delay off time of roughly 0.01 seconds (10 milliseconds).

Note:

This formula is an estimate that is based on the assumption that the capacitor discharges totally.

There may be voltage thresholds in actual circuits that determine when the output turns on or off.

Basic RC delay circuits can be used using this formula.

More intricate timer circuits might call for extra parts and computations.

How to Build:

To build a Simple Automatic Bedroom Lamp Timer Circuit follow the below steps:

• Place the components on the PCB ensuring proper connections.
• Connect the positive anode leg of the capacitor to one side of the push button.
• Connect the other side of the push button to the positive rail of the PCB.
• Connect the negative cathode leg of the capacitor to the negative supply of the PCB.
• Connect the 2.2M resistor from the negative rail to the base B of the BC547 transistor.
• Connect the collector C of BC547 to the positive supply.
• Connect the emitter E of BC547 to the negative supply.
• Connect the collector C of BC547 to the base B of the BC557 transistor.
• Connect the emitter of BC557 to the negative supply.
• Connect the collector of BC557 to the gate of the triac.
• Connect one leg of the lamp to the main terminal of the triac.
• Connect the other leg of the lamp to the negative supply.
• Connect the positive and negative terminals of the power supply to the respective rails on the PCB.

Testing:

• Press the push button to start the circuit.
• The lamp should illuminate and remain lit for the amount of time specified by the values of the 2.2M resistor and 100uF capacitor.

Adjustments:

• Experiment with different resistor and capacitor values for different delay times.
• Ensure that all connections are secure and that there are no loose wires.

Safety Precautions:

• Avoid touching the circuit while it is powered and uncovered especially when dealing with mains AC.

Conclusion:

You may build the basic automatic bedroom lamp timer circuit on a PCB by following these steps.

To make sure your circuit is secure and working always be cautious and verify your connections twice.

References:

Datasheet BT136