SA TeachPoint

Which sir is getting married? 😆

দেশের সকল শিক্ষাপ্রতিষ্ঠানে ৩ দিন অনলাইনে ক্লাস ও ৩ দিন অফলাইনে ক্লাস। সিদ্ধান্ত দিয়েছে শিক্ষা মন্ত্রণালয়। নিউজ লিংক ও আলোচনা কমেন্টে 👇

⚡ The Power of Potential: Understanding the Voltage Divider ⚡

Ever wondered how electronic devices take a large battery voltage and scale it down to exactly what a tiny sensor needs? The secret is the **Voltage Divider Circuit!

This simple arrangement of two resistors is the "Swiss Army Knife" of circuit design. Here’s a breakdown of what’s happening in the image:

🔹 1. The Core Principle
A Voltage Divider splits the input voltage (V_{in}) into a smaller output voltage (V_{out}) based on the ratio of two resistors (R_1and R_2).

The magic formula is:
V_{out} = V_{in} \cdot \frac{R_2}{R_1 + R_2}

* **Equal Resistors:** If R_1 = 10k and R_2 = 10k, the voltage is split exactly in half (1.5V from a 3V source).
* **Unequal Resistors:** If R_2 is larger (e.g., 20k vs 10k), it "grabs" more voltage (2V from a 3V source).

🔹 2. Sensing the World with LDRs
By replacing a standard resistor with a **Light Dependent Resistor (LDR)**, we turn a static circuit into a **sensor**:
* **Light Up:** When light hits the LDR, its resistance drops, changing the output voltage.
* **Darkness Falls:** As it gets dark, the resistance increases, causing the voltage to shift.

🔹 3. Real-World Application: Automatic Street Lights
The diagram on the bottom right shows how this works in real life. When the sun goes down:
1. The LDR resistance changes.
2. The **Voltage Divider** sends a signal to the transistor.
3. The transistor acts as a switch, clicking the **Relay** to turn on the street lamp! 💡

Pro Tip:** Voltage dividers are perfect for signals and sensors, but **never** use them to power heavy loads (like motors or big LEDs) directly, as they waste energy as heat!

Which part of circuit design should we cover next? Let me know in the comments!

Alternative flash

This circuit uses 3 555 timer ICs. They all function as astable multivibrators. The first 555 has an on/off time of 1 second. This IC controls the on/off period of the other two 555 ICs, which are used to drive two lamps via the two relay switches.

The diodes are used to protect the 555 ICs from surges. The relay switches must have an impedance greater than 50 ohms. They must not draw more than 200mA.
The flash sequence is as follows:
The lamp connected to the first relay flashes 4 times for 1 second. Then the lamp connected to the second relay flashes 4 times for 1 second. After this, the cycle is repeated.
The flash frequency can be changed by changing capacitors C3 and C5. A higher value results in a lower flash frequency.

Keep in mind that the values ​​of C3 and C5 must be equal to or lower than C1.
The value of C1 determines the change-over rate (standard 1 sec). A higher value results in a lower change-over rate.

4-in-1 burglar alarm
On this circuit, the alarm goes off under the following conditions:
1. When light falls on LDR1.
2. If light falling on LDR2 is blocked.
3. If a door switch is open or a wire is broken.
4. If a doorknob is touched.
The light-sensitive resistor LDR1 must be placed in the dark, close to a door, lock, etc. If an intruder's flashlight shines on the LDR, the alarm will sound. If an intruder passes through a hallway and their shadow falls on LDR2, the alarm will sound again. The sensitivity of LDR2 can be adjusted using potentiometer VR2. A long but very thin wire can be connected between points A and B or C and D. If someone cuts this wire, the alarm will sound. Instead of a wire, switches can also be placed between points A and B or C and D. If the switches or wires are not used, points A and B and C and D must be connected in series. A wire can be connected between touch point P and, for example, a doorknob or another material that conducts electricity. If someone subsequently touches the object connected in this way, the alarm will sound. The sensitivity can be adjusted using potentiometer VR3. If you adjust potentiometer VR1, VR3 must also be readjusted. If an alarm has a voltage higher than 6 volts or draws more than 150mA, it is advisable to use the shown relay.

Power failure alarm

Most power outage alarms require a separate power supply to operate. However, the alarm circuit shown here does not require an additional power supply. It uses an electrolytic capacitor to store sufficient energy to keep the alarm circuit running for a reasonable time if the power fails. This circuit can be used as an alarm for power adapters between 5V and 15 Volts.
To calibrate the circuit, the power supply must be connected first. Next, potentiometer VR1 must be turned until the buzzer switches from on to off.
As soon as the power fails, the buzzer will now start buzzing.

Burglar alarm

In this circuit, a 555 timer IC is used as an alarm system to protect your luggage or to prevent theft. The alarm goes off if a thin wire is broken.
The circuit is simple. It uses a 555 IC connected as an astable multivibrator to generate a 1kHz tone to deter a thief. The wire that triggers the alarm can be made of very thin copper wire, such as winding wire.
The circuit operates on a wide voltage range between 5V and 15V.

Rain alarm

This circuit triggers an alarm if the sensor gets wet, for example from rain. A 555 astable multivibrator is used here, which emits a tone of approximately 1 kHz as soon as water is detected by the sensor.
The sensor must be placed at an angle of 30 to 45 degrees relative to the ground. This ensures that the rainwater is drained away and prevents the alarm from continuing once it has stopped raining. The metal used to make the sensor must be aluminum and not copper. This is because copper oxidizes upon prolonged exposure to the elements, requiring frequent cleaning.
The aluminum foil can be attached to a wooden or plastic surface with glue, for example. The X and Y contacts of the sensor can be formed using small alligator clips.