ClassX Video Lessons Youtube Channel The Engineering Mindset LDR and LED Circuit design – Solid State Relay
In this article, we’ll explore a simple yet fascinating circuit that uses a light-dependent resistor (LDR) and a white LED. The LDR is a special component that changes its resistance based on the amount of light it receives. In the dark, it has a high resistance, but in bright light, its resistance drops significantly. The white LED we’re using is designed to operate at 20 milliamps and needs 3 volts from a power supply to reach this current.
When we test the LDR, we notice that in dim light, its resistance is about 40 kilo-ohms. If we cover it with one hand, the resistance increases to around four mega-ohms, and with both hands, it can reach up to nine mega-ohms. However, when the white LED shines directly on the LDR, the resistance drops dramatically to about 66 ohms. If we wrap our fingers around both the LED and the LDR, the resistance is approximately 70 ohms.
For the primary circuit, we need a white LED that has a voltage drop of 3 volts and a current of 0.02 amps. We will control this LED with a switch and power the circuit using a 9-volt battery. To find the right resistor value, we subtract the 3 volts needed by the LED from the 9 volts of the battery, leaving us with a 6-volt drop across the resistor. Using Ohm’s Law, 6 volts divided by 0.02 amps gives us a resistor value of 300 ohms.
To slightly reduce the current and dim the LED, we choose a higher resistor value. By using a 330-ohm resistor and a 22-ohm resistor together, we get a total resistance of 350 ohms. This setup allows approximately 0.017 amps or 17 milliamps to flow through the circuit when the switch is pressed, lighting up the LED.
On the secondary side, we have a red LED that requires a 2-volt drop and a current of 0.02 amps. This LED will light up to show that the circuit is functioning. We place the LDR opposite the white LED, and when exposed to light, the LDR provides about 70 ohms of resistance. To calculate the resistor for the red LED, we subtract 2 volts from 9 volts, resulting in a 7-volt drop. Dividing 7 volts by 0.02 amps gives us 350 ohms. After accounting for the 70 ohms from the LDR, we need a 280-ohm resistor. Instead, we use two 150-ohm resistors, totaling 300 ohms.
Assuming the LDR is at 70 ohms, the total resistance is 370 ohms. Dividing 7 volts by 370 ohms gives us approximately 0.019 amps. When we assemble the components on the secondary circuit board, the red LED lights up because the LDR is detecting ambient light from the room.
To prevent the red LED from turning on due to ambient light, we can use electrical tape. By wrapping a few small pieces around both the LDR and the LED, we block the ambient light, turning the LED off. When we press the button on the primary circuit, the white LED turns on, shining light onto the LDR, which then activates the red LED on the secondary side.
For more exciting projects and to deepen your understanding of electrical and electronics engineering, check out additional resources and videos. This concludes our exploration of the LDR and LED circuit design. Don’t forget to explore more and stay curious!
Gather a light-dependent resistor (LDR) and a multimeter. Test how the resistance of the LDR changes under different lighting conditions. Use a flashlight, cover the LDR with your hand, and observe the resistance values. Record your findings and discuss how this property can be used in real-world applications.
Using a breadboard, connect a white LED with a 330-ohm resistor to a 9-volt battery. Calculate the current flowing through the circuit using Ohm’s Law. Experiment by replacing the resistor with different values and observe how the brightness of the LED changes.
Design a circuit where the LED turns on only when the LDR is exposed to light. Use the concepts of resistance and voltage drop to calculate the necessary resistor values. Test your circuit by shining a light on the LDR and observe the LED’s response.
Set up a secondary circuit with a red LED and an LDR. Use a white LED from the primary circuit to control the red LED. Experiment with blocking ambient light using tape and see how it affects the operation of the red LED. Discuss how this setup mimics a solid state relay.
Combine your knowledge of LDRs and LEDs to create a simple alarm system. Use the LDR to detect changes in light levels and trigger an LED or buzzer. Present your design to the class and explain how it could be used in security systems.
Here’s a sanitized version of the provided YouTube transcript:
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This circuit uses a light-dependent resistor (LDR) and a white LED. The LDR varies its resistance depending on the amount of light it is exposed to. In darkness, it has a very high resistance, while in bright light, it has a very low resistance. This white LED is rated for 20 milliamps. When connected to a DC bench power supply, it requires three volts to achieve that 20 milliamps.
When testing the LDR, we find that with dim light, its resistance is around 40 kilo-ohms. When covered with one hand, it measures around four mega-ohms, and with both hands completely covering it, it reaches approximately nine mega-ohms. However, when shining the white LED onto the LDR, the resistance drops to around 66 ohms. If I wrap my fingers around both components, the resistance is about 70 ohms.
In the primary circuit, we need a white LED with a voltage drop of 3 volts and a current of 0.02 amps. We will control this with a switch and use a 9-volt battery to power the circuit. The resistor value is calculated by subtracting the 3 volts for the LED from the 9 volts, giving us a voltage drop of 6 volts across the resistor. The circuit current is 0.02 amps, so 6 volts divided by 0.02 amps equals 300 ohms.
While this circuit will work fine at 20 milliamps, I will use a slightly higher resistor value to reduce the current to the LED, which will also slightly dim its brightness. I will use a 330-ohm resistor and a 22-ohm resistor, which combine to form 350 ohms of resistance. To verify, 6 volts divided by 350 ohms equals approximately 0.017 amps or 17 milliamps.
I place the components into the circuit, and it looks like this. The current will flow through the circuit as shown using conventional current. When I press the switch, the LED illuminates.
On the secondary side, we have a red LED with a voltage drop of 2 volts and a current of 0.02 amps, which will turn on to indicate that the circuit is working. We place the LDR opposite the white LED, providing a resistance of approximately 70 ohms when exposed to light. To find the resistor for the LED, we subtract 2 volts from 9 volts, resulting in 7 volts. Dividing 7 volts by 0.02 amps gives us 350 ohms. Subtracting the 70 ohms for the LDR results in 280 ohms. Instead, I will use two 150-ohm resistors, totaling 300 ohms.
Assuming the LDR is 70 ohms, we have 370 ohms of resistance. Dividing 7 volts by 370 ohms gives us approximately 0.019 amps. When I place the components on the secondary side of the circuit board, it looks like this. Notice that the red LED is on because the LDR is receiving ambient light from the room.
To stop this, we can use some electrical tape. By cutting a few small pieces and wrapping them around both the LDR and the LED, we can block the ambient light, turning the LED off. When I press the button on the primary circuit, the white LED turns on, shining light onto the LDR, which in turn activates the red LED on the secondary side.
Check out one of the videos on screen now to continue learning about electrical and electronics engineering. This concludes the video. Don’t forget to follow us on social media and visit theengineeringmindset.com.
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This version maintains the technical content while removing any informal language and ensuring clarity.
LDR – A Light Dependent Resistor (LDR) is a component that changes its resistance based on the amount of light it is exposed to. – In our physics project, we used an LDR to automatically turn on the street lights when it got dark.
LED – A Light Emitting Diode (LED) is a semiconductor device that emits light when an electric current passes through it. – The engineer used an LED to indicate when the machine was powered on.
Circuit – A circuit is a closed loop that allows electricity to flow from a power source through various components and back to the source. – We learned how to build a simple circuit using a battery, wires, and a light bulb in our science class.
Resistance – Resistance is a measure of how much a component reduces the flow of electric current through it. – The resistance of the wire was too high, causing the circuit to overheat.
Voltage – Voltage is the difference in electric potential energy between two points in a circuit, which causes current to flow. – The teacher explained that the voltage across the battery terminals was 9 volts.
Current – Current is the flow of electric charge through a conductor, measured in amperes (A). – The current flowing through the circuit was measured to be 2 amperes.
Resistor – A resistor is a component used in circuits to limit the flow of electric current. – We added a resistor to the circuit to prevent the LED from burning out.
Ohms – Ohms is the unit of measurement for electrical resistance. – The resistor in the circuit had a resistance of 100 ohms.
Light – Light is a form of energy that is visible to the human eye and is emitted by sources like the sun or LEDs. – The LED emitted a bright light when the circuit was completed.
Power – Power is the rate at which electrical energy is transferred by an electric circuit, measured in watts (W). – The power consumed by the light bulb was calculated to be 60 watts.
