Simple Automatic Bedroom Lamp Timer Circuit

Building a bedroom lamp timer that turns off automatically sounds cool but be careful.

It uses strong electricity from the wall outlet so do not touch it when it is plugged in.

The circuit itself is pretty simple.

Pushing a button starts a chain reaction created by different parts working together.

What is a Automatic Bedroom Lamp Timer Circuit:

An automatic bedroom lamp timer circuit is a device that controls the timing of turning a bedroom lamp on and off automatically.

This type of circuit is commonly used to simulate the presence of someone in a room adding an element of security by giving the appearance that the room is occupied even when it may not be.

Circuit Working:

Parts List:

Upon pressing the push button the 100uF 25V capacitor commences charging.

The charged capacitor furnishes bias current to the BC547 through the 2.2M resistor, even after the user releases the push button.

BC547 influenced by the bias voltage remains switched ON subsequently causing the BC557 to also switch ON.

BC557 collector provides gate current to the triac resulting in its activation.

With the triac switched ON the bulb is illuminated and continues to emit light.

Once the charge in the 100uF capacitor diminishes entirely, the transistors lose their ability to maintain conducting positions.

The transistors switch OFF after a delay determined by the 100uF charge leading to the deactivation of the triac and extinguishing of the lamp.

The lamp remains extinguished until the push button is pressed again restarting the cycle.

Formulas:

Here the below mentioned formulas relates the delay time (t) of a simple RC resistor capacitor circuit to the values of the resistor R and capacitor C.

This formula applies specifically to delay off timers.

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 circuit initiates the capacitors charging through the resistor when it receives a trigger signal, which is often a voltage change.

The voltage across the capacitor progressively rises as it charges.

Time Constant (τ): The time constant of the circuit is the product of R and C, represented by the greek letter tau (τ).

It shows how long it takes for the voltage across the capacitor to equal roughly 63.2% of the voltage that is applied.

Delay OFF Time: The charged capacitor begins to discharge through the resistor as soon as the trigger signal is cut off.

The delay off time is directly proportional to the time constant (τ), according to the formula t = R × C

Put another way, the longer it takes for the capacitor to discharge and, as a result, the longer the delay off time until the circuits output turns off, the bigger the time constant (higher resistance or capacitance).

Example:

Imagine a delay off timer circuit with a resistor of 10kΩ (10,000 Ohms) and a capacitor of 1μF (microfarad).

Using the formula:

t = R × C = 10,000Ω × 1 μF

Converting microfarads to Farads: 1μF = 0.000 001F

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

Therefore, the delay off time for this circuit would be approximately 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.

Construction Details:

• 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 initiate the circuit.
• Observe the lamp lighting up, and it should stay illuminated for a duration determined by the 100uF capacitor and 2.2M resistor values.

Adjustments:

• Experiment with different resistor and capacitor values for varied 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

By following these steps you can construct the simple automatic bedroom lamp timer circuit on a PCB. Always exercise caution and double check your connections to ensure a safe and functional circuit.

References

Datasheet BT136