# Large LDR

Discussion in 'General Electronics Chat' started by Bri626, Feb 5, 2016.

1. ### Bri626 Thread Starter New Member

Feb 5, 2016
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Hi, Thanks for taking the time to read my post.

I am trying to find a large LDR (2-3 inch diameter) but have had no luck in finding one. Can anyone help me out?

Thanks,

Bri

2. ### Roderick Young Member

Feb 22, 2015
408
168
Would that be a light dependent resistor? Any chance you can put an array of smaller ones on a PC board? Or use a solar cell, and adapt your circuit?

3. ### Bri626 Thread Starter New Member

Feb 5, 2016
14
1
Hi Roderick, thanks for the reply.

Yes a light dependent resistor. I thought about placing multiple smaller LDR's together but I don't think that will work ideally for my project. I'm making a laser reactive target so I can practice shooting at home and I don't want any area in the "target area" to not be recognized by the laser (the areas in-between the LDRs might be counted as a miss).

I will have to research the solar cell. I'm an electrician however new with electronics (but I am slowly learning...) I don't know if a solar cell would react as quickly as I needed it to.

Thanks again!

4. ### ScottWang Moderator

Aug 23, 2012
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Or you can use a bigger convex lens with the LDR to expanding the sense area.

Roderick Young likes this.

Nov 12, 2008
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6. ### Bri626 Thread Starter New Member

Feb 5, 2016
14
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Ron and Scott, thanks for the suggestions I appreciate it. If I can't find a big enough LDR I will experiment with your ideas.

7. ### ScottWang Moderator

Aug 23, 2012
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How about the distance from the laser to the LDR?
If the distance is long enough then you don't need the big LDR, just adjust the distance of convex lens and LDR and using the laser to do the test, to find out the good distance for convex lens and LDR.

8. ### Bri626 Thread Starter New Member

Feb 5, 2016
14
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The distance from where the laser originates to the LDR will be approximately 4-6 meters. The LDR is going to be mounted on a suspended steel plate and when the laser hits the LDR a solenoid will strike the plate to simulate a hit. If the convex lens was made of plastic and could somehow be mounted directly to the plate that could work.

9. ### tracecom AAC Fanatic!

Apr 16, 2010
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LDRs don't respond very quickly. You may find that the pulse from your light gun may be too short to give you a proper response from an LDR. A phototransistor might be a better choice.

10. ### ScottWang Moderator

Aug 23, 2012
4,933
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If the pulse of laser light is too short then it could be as tracecom mentioned, so here you need to check the pulse width of laser light, and comparing with the response time of LDR.

Laser light → LDR → voltage comparator → monostable circuit → mosfet → solenoid

If the pulse width of laser light is enough to strike the solenoid then the monostable circuit is unnecessary.

11. ### Bri626 Thread Starter New Member

Feb 5, 2016
14
1
Thanks for the reply tracecom. As long as the response time is under .25 seconds I would be happy. I actually got the idea from a video I viewed online and he was using a LDR and the reaction time was very quick, seemed almost instantaneous (using the same laser I would be using). However, I have no clue which LDR he was using or where he purchased it form. If all else fails I my have to go the phototransistor route but I would need to install maybe 20 or 30 of them to get the coverage area I want.

12. ### Bri626 Thread Starter New Member

Feb 5, 2016
14
1
Scott, again, thanks for your advice. Unfortunately, due to being new to electronics, you lost me when you got to the voltage comparator & monostable circuit. I understand laser light, LDR, mosfet transistor, and solenoid but it looks like I need to do more research and learn more.

13. ### tracecom AAC Fanatic!

Apr 16, 2010
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1,396
I guess this is the video.

It does show that an LDR can be made to respond quickly enough, and is a very interesting concept. As an avid shooter, I may try to create something similar. The actual LDR being used is probably not critical. I think I understand what you want, i.e., a metal plate that will ring whenever it is hit anywhere on its surface by the laser. The electronics to make this happen are relatively simple, but I don't have in mind a solution to your issue, but I do think there is one.

14. ### ScottWang Moderator

Aug 23, 2012
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777
LDR → voltage comparator → monostable circuit → mosfet → solenoid

LDR + voltage comparator -- it can be use the voltage comparator as LM339 or LM393 or using op amp as LM358.

monostable circuit -- it can be use as NE555 or CD4047 or others
Mosfet to drive solenoid -- it can be use the mosfet and bjt to drive solenoid, but using mosfet is better.

15. ### Bri626 Thread Starter New Member

Feb 5, 2016
14
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Wow your quick! This is actually the video I was referring to but that one is similar. I like the plinking sound from the plate. I think it better simulates a real life hit. It should be relatively simple to put together but with my lack of electronic knowledge I'm a little stuck.

16. ### hp1729 Well-Known Member

Nov 23, 2015
2,105
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The biggest I have is about 1". I don't think I have seen larger.

17. ### tracecom AAC Fanatic!

Apr 16, 2010
3,879
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The largest I have found is 20mm. GL20539, but I have not found a domestic source.

18. ### tracecom AAC Fanatic!

Apr 16, 2010
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Looks like the guy came up with a cool product. Wonder if he ever found a buyer?

I went back and looked at the vid comments. Apparently, not.

Last edited: Feb 6, 2016
19. ### Bri626 Thread Starter New Member

Feb 5, 2016
14
1

That is bigger than what I have found. I'll take what I can get. Can you share where you found it?

Thanks!

20. ### Bri626 Thread Starter New Member

Feb 5, 2016
14
1
Thanks Scott!