Analog ElectronicsUncategorized

How to design and simulate dark activated light/ street light

In this tutorial, you will learn how to simulate a dark activated light. A dark activated light is a device that turns on light, or say an LED when it is dark. This circuit will give you a basic understanding of how the street light works. Previously, I made a more basic video tutorial on how to design a light/dark activated light switch with just some resistors and a light sensor which is a light dependent resistor (LDR) and an LED. See video below.

previous video tutorial

In this very tutorial, I added an extra component to make the circuit more responsive and efficient. We can make the circuit much better by adding a comparator to it. However, for this circuit design and simulation in proteus, we used only the following components:

  • 1 BC547 transistor
  • 1 Torch LDR
  • 1 12 Volts battery
  • 1 10K Potentiometer
  •  2 X 330Ω Resistors
  • 1 LED
  • 1 10 Resistor
dark activated switch
Dark activated switch

Watch complete simulation video below

Circuit simulation explanation

If you have drawn the circuit as shown above in Proteus, once you click the run button, current will flow from the 9V battery and branch into resistors R4 and R5, the current that flows into either branch depends on the resistance in the branch, the branch or path with lowest resistance will receive much current than the other. The current that branched into the direction of the LED will flow through the LED and get to the collector of the transistor, if the transistor is biased, collector current will flow in the transistor down to the emitter and to the negative terminal of the battery which is the ground. When this happens, the LED will come on. But if the transistor is not biased, collector current will not flow, and the LED will not come on.

Now, note that, what determines if the base of the transistor will be biased or not is what happens at the other path of the circuit where the current branched. When current enters the 10k resistor, it is slowed down the more before it branches into the LDR or the direction of the potentiometer. Here also, the amount of current that crosses either the LDR or the potentiometer is determined by the resistance on the LDR and the resistance on the potentiometer. So, in the simulation, bringing the torch closer to the LDR will increase the intensity of light on the LDR, and that will decrease the resistance on the LDR, when this happens, the path of the LDR will have little or no resistance, then causing almost all the current on this path to flow through the LDR and enter the ground. As this happens, little or no current will flow into the potentiometer and no current will flow into the base of the transistor to bias it. Once this happens, collector current will not flow and the LED will not come on. This means that the presence of light has hindered the LED from coming on, and that is what we want, because the circuit is a dark activated light system, so, it is appropriate for the LED not to come on when light is brought to the LDR, this is a typical example of how a street light works.

On the other hand, if the torch is taken back, the resistance on the LDR will increase up to 10M, this will in turn cause little or no current to flow through the LDR, while almost all the current will now flow through the potentiometer, and at the potentiometer, some of the current will find there way to ground, while the rest will enter the base of the transistor through resistor R3. Once reasonable amount of current enters the base of the transistor, the transistor gets biased and collector current flows, as collector current flows, the LED will come on. From here also, we can see that creating darkness on the LDR causes the LED to come on. Exactly as a street light works. In fact, we can call this design street light simulation in proteus.

The work of the potentiometer is to set the sensitivity of the circuit to light, while, the work of the resistor R3 is to make sure the transistor doesn’t get damaged by excess base current.  

This is how to simulate a street light or dark activated light in proteus. You can make it a “light activated system” by switching the positions of the potentiometer and the LDR.

I will be making more simulation tutorials like this in the future, so, try and subscribe to my channel on YouTube so you will always get to know when I upload new videos and tutorials.

Check out this other simulation tutorials and videos

How to simulate battery voltage level indicator
How to simulate digital scrolling text display
How to simulate AC voltage step-down transformer
How to simulate ultrasonic sensor and Arduino in Proteus
How to simulate Arduino in Proteus

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