LDR, Transistor, and Electromagnet

[Fahmi88]

I want to make an electrical circuit like the picture i uploaded. This circuit will make the electromagnet on if LDR is illuminated and vice versa. I use 5v red laser to illuminate the LDR and i have 2 kind of LDR (in the picture, i use only one of them).

The question is:
1) what kind of transistor should i use and is it pnp or npn?
2) which LDR should i use?
3) if i replace potentiometer with resistor, what is the value of it?

you can modify my circuit but you cannot change the value of the battery, ldr, and electromagnet because i have only these. Also you can search the transistor from this site : http://alltransistors.com/.

 

[Martaine2005]

Hello,
The transistor is an NPN.
The LDR and resistor depend on when you want it to switch on or off.

Martin

 

[AnalogKid]

Do you want the magnet to switch on full force when the light reaches a certain intensity, or come on gradually as the light increases and decrease gradually as the light goes out?

ak

 

[Fahmi88]

Hello,
The transistor is an NPN.
The LDR and resistor depend on when you want it to switch on or off.

Martin

Hi martin,
I want the resistor to switch on when the LDR is iluminated and off when the LDR is not illuminated. Do you know what transistor should i use. There ara many kinds of transistor and i'm confused to choose the right one.
Thank

 

[Fahmi88]

Do you want the magnet to switch on full force when the light reaches a certain intensity, or come on gradually as the light increases and decrease gradually as the light goes out?

ak

Hi ak,
I want the magnet to switch on full force. Do you have any suggestion for my project
Thank

 

[AnalogKid]

When interacting in a question-answer forum, it is *critical* that you do not keep operational details secret. Now that you say you want full-on/full-off action (no intermediate magnet strength), you have not said how quickly the light is increasing or decreasing. Why do you think this is not important enough to mention?

I doubt that the circuit you posted will do what you want, but as long as you hold back project details you will not get satisfactory answers.

ak

 

[Fahmi88]

When interacting in a question-answer forum, it is *critical* that you do not keep operational details secret. Now that you say you want full-on/full-off action (no intermediate magnet strength), you have not said how quickly the light is increasing or decreasing. Why do you think this is not important enough to mention?

I doubt that the circuit you posted will do what you want, but as long as you hold back project details you will not get satisfactory answers.

ak

Ok, actually i want to make flying magnet. I uploaded the second picture. The idea is when the magnet is attracted to the electromagnet and passes the laser beam, the resistance of the LDR will increase and will make the resistor and electromagnet off. After that because there is no attraction, the magnet will fall and the LDR is illuminated again and the magnet is attracted again. And so on. This will make the magnet seem to fly. I don't have electrical background so i want to make as simple as possible.

 

[AnalogKid]

I suspected that this was the application. This problem actually is a bit more complex than you think. First, if you want the floating magnet to be stationary in the air rather than bouncing up and down, you *do* want a magnet controller that makes a variable magnetic strength rather than simply full on/full off.

Second, gravity. While it looks like it should be easy for the system to stabilize with the magnet partially breaking the light beam and the electromagnet exerting just enough force to hold it there, there is this thing called gravity working on the magnet, and gravity is a square-law force. As the magnet moves up and down, it does not move linearly - it accelerates and decelerates exponentially due to gravity. Also, as it gets closer to the electromagnet the force increases exponentially. Stable magnetic levitation is a classic control system problem, and is a great example of the difference between common sense and science.

ak

 

[Arouse1973]

I suspected that this was the application. This problem actually is a bit more complex than you think. First, if you want the floating magnet to be stationary in the air rather than bouncing up and down, you *do* want a magnet controller that makes a variable magnetic strength rather than simply full on/full off.

Second, gravity. While it looks like it should be easy for the system to stabilize with the magnet partially breaking the light beam and the electromagnet exerting just enough force to hold it there, there is this thing called gravity working on the magnet, and gravity is a square-law force. As the magnet moves up and down, it does not move linearly - it accelerates and decelerates exponentially due to gravity. Also, as it gets closer to the electromagnet the force increases exponentially. Stable magnetic levitation is a classic control system problem, and is a great example of the difference between common sense and science.

ak

Interesting comments ak.
I thought gravitation was an inverse square law not gravity?
Adam

 

[AnalogKid]

I thought gravitation was an inverse square law not gravity?

Don't quite understand your question. Gravity and gravitation are the same thing in the general context of his project. Also, while technically it is an inverse square law relationship, I was using "square law" as a descriptor of a more complex level of mathematical relationship, like "second-order". My point to the OP is that it is something significantly different than a simple linear or inverse relationship.

ak

 

[Arouse1973]

Don't quite understand your question. Gravity and gravitation are the same thing in the general context of his project. Also, while technically it is an inverse square law relationship, I was using "square law" as a descriptor of a more complex level of mathematical relationship, like "second-order". My point to the OP is that it is something significantly different than a simple linear or inverse relationship.

ak

OK, thanks
Adam

 

[Fahmi88]

Now, i get confused with your explanation. So do you think my project is impossible?
I'm always impressed with magnetic levitation demonstration i see on youtube so i want to make demonstration like that with my simple idea. Do you have tutorial or link maybe?

 

[Martaine2005]

Do you have tutorial or link maybe?

Martin