Calculating Resistors for a Photocell Dark Switch Circuit

In summary, the person is trying to use a cadmium sulfide photocell to turn off an LED in a circuit when visible light is detected. They understand they need to use a transistor as a switch for the LED and know how to calculate the resistor for the LED. They are having trouble figuring out how to calculate and place resistors for the photocell in parallel to turn on the transistor. They are asking for tips on where to start and formulas to use. Additionally, they mention getting about 4V after the photocell when lights are on and ask for a schematic. The other person suggests experimenting with the actual situation and measuring the resistance at the desired light threshold to determine the necessary current for the transistor's base. They also recommend
  • #1
Jaron Wiley
4
0
Hi I am trying to make an LED 3.5V 0.02A run in a circuit that will turn it off when visible light comes on. I have a cadmium sulfide photocell I took out of a night light. The power input is 12V 250mA. I understand I need to use a transistor as a switch for the LED and I know how to calculate the resistor for the LED. I am having trouble figuring how to calculate and place resistors for the photocell parallel so that the current turns on the transistor to the LED. Any tips on where to start or formulas? I am getting about 4V on after the photocell when lights are on if that helps. Thank you!
 
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  • #2
A schematic would be helpful.
 
  • #3
The photocell's resistance changes an awful lot depending on the incoming light. You light find some clues in its datasheet, but it's better to experiment in the real situation. Ah, and don't hope two photocells to be identical.

Once you've measured the resistance at the desired light threshold, compute how much current flows through it from 12V, check it exceeds what the transistor's base needs (take a Darlington if needed, like BC517, or a MOS like BSS170), add a resistor between base and emitter that gives 0.65V at this curernt.
 

Related to Calculating Resistors for a Photocell Dark Switch Circuit

What is a photocell dark switch circuit?

A photocell dark switch circuit is an electronic circuit that uses a photocell, also known as a light-dependent resistor, to automatically turn on or off a light source depending on the amount of light present in the environment. When it is dark, the resistance of the photocell increases, causing the circuit to activate the light source. When it is light, the resistance decreases, turning off the light source.

How does a photocell dark switch circuit work?

A photocell dark switch circuit works by using the change in resistance of a photocell to control the flow of electricity through the circuit. When there is a high amount of light, the resistance of the photocell decreases, allowing more current to flow and turning off the light source. When there is low light, the resistance increases, reducing the current flow and turning on the light source.

What are the common applications of a photocell dark switch circuit?

Photocell dark switch circuits are commonly used in outdoor lighting, such as streetlights, security lights, and garden lights. They are also used in automatic night lights and sensor-activated switches.

What are the advantages of using a photocell dark switch circuit?

There are several advantages to using a photocell dark switch circuit. It is energy-efficient as it only turns on the light when it is needed, saving electricity. It is also convenient as it eliminates the need for manual switching and can automatically adjust to changing light conditions. Additionally, it can extend the lifespan of the light source by reducing its usage.

Are there any limitations to using a photocell dark switch circuit?

One limitation of a photocell dark switch circuit is that it may not work well in areas with constantly changing light conditions, as the light source may turn on and off frequently. It also requires a power source to operate, so it cannot be used in areas without electricity. Additionally, extreme temperatures and weather conditions may affect the performance of the photocell and the circuit.

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