Photoresistor cell

noanalyst

Hi guys & gals,

New to the forum and a complete novice but I want to build or buy a very cheap dime sized photoresistor sensor that reacts to only direct light coming to it. So it will ignore ambient light but only react to light coming directly to it. Is that possible? Which direction should I be heading? I am planning to use this with my kid who loves his led flashlight but I think I can make it more exciting for him.

Thanks in advance

Pliers

One way is to use a second photoresistor to measure the ambient light light threshold.
Or add a tube to the front of the detector, to shield it from ambient light.

Dr_Acula

+1 to the idea of adding a tube. I did some experiments once - it is amazing how much light leaks through various materials like black cardboard, black paint, electrical tape, plastic straws etc. A simple metal tube solves the problem - brass, aluminium, whatever you can get. The longer it is, the less sensitive it will be to ambient light.

noanalyst

Thanks for the responses. Really like the idea of a tube. Should I just experiment with different lengths to get the right size and distance that blocks out the ambient light. I want to block out all ambient light.

Franklin

. I want to block out all ambient light.

I'm not sure that is possible as there will be light from the front of the tube whenever there is light. You can restrict it to a small amount If this is not good enough you can try modulating the source and the receiver like they do with TV remotes.

Bob Lawrence (VE1RLL)

t. I want to block out all ambient light.

Maybe you should build a IR Flashlight and use a IR detector.

For Example: http://learn.adafruit.com/ir-sensor/overview

IR detectors are little microchips with a photocell that are tuned to listen to infrared light. They are almost always used for remote control detection - every TV and DVD player has one of these in the front to listen for the IR signal from the clicker. Inside the remote control is a matching IR LED, which emits IR pulses to tell the TV to turn on, off or change channels. IR light is not visible to the human eye, which means it takes a little more work to test a setup.


[h=1]Infrared LED Flashlight[/h][h=1][/h][h=1][/h] b

http://www.instructables.com/id/Infrared-LED-Flashlight/

Dr_Acula

Should I just experiment with different lengths to get the right size and distance that blocks out the ambient light.

Aluminum tube at the hardware shop comes in a range of diameters and is very cheap. So maybe take a tube 1/4" diameter, and maybe a foot long, and look down it. That will give you a pretty good idea what any photoresistor or LDR is going to see. If you want to take it even further make it black on the inside eg block one end with tape, pour a bit of flat black paint into the tube, tape the other end, tip it up and down a few times, then pour the paint back into the tin and let the tube dry.

And then if you *really* want to look at something in the distance and block out even more ambient light, get a telescope, or a monocular (search ebay, a monocular can be as low as $4). I did some really interesting experiments with these and a cheap laser pointer over several hundred metres. You can really focus the laser right down to a point. Over these distances the laser/torch needs to be mounted firmly - eg on a tripod or a fence post. Fantastic for sending data long distances. But if children are involved, maybe safer to use leds rather than lasers.

Pliers

Here is a circuit for the ambient light threshold design.
Not tested.

noanalyst

Bob Lawrence (VE1RLL) wrote: »

Maybe you should build a IR Flashlight and use a IR detector.

For Example: http://learn.adafruit.com/ir-sensor/overview

IR detectors are little microchips with a photocell that are tuned to listen to infrared light. They are almost always used for remote control detection - every TV and DVD player has one of these in the front to listen for the IR signal from the clicker. Inside the remote control is a matching IR LED, which emits IR pulses to tell the TV to turn on, off or change channels. IR light is not visible to the human eye, which means it takes a little more work to test a setup.


Infrared LED Flashlight

b

http://www.instructables.com/id/Infrared-LED-Flashlight/

Interesting idea. How about costs though. How do these compare to basic photocell/photoresistor?

noanalyst

Cool idea with the telescope. I will try that this weekend. But looks like I may be going with the aluminum tube at this point.

noanalyst

Dr_Acula wrote: »

Aluminum tube at the hardware shop comes in a range of diameters and is very cheap. So maybe take a tube 1/4" diameter, and maybe a foot long, and look down it. That will give you a pretty good idea what any photoresistor or LDR is going to see. If you want to take it even further make it black on the inside eg block one end with tape, pour a bit of flat black paint into the tube, tape the other end, tip it up and down a few times, then pour the paint back into the tin and let the tube dry.

And then if you *really* want to look at something in the distance and block out even more ambient light, get a telescope, or a monocular (search ebay, a monocular can be as low as $4). I did some really interesting experiments with these and a cheap laser pointer over several hundred metres. You can really focus the laser right down to a point. Over these distances the laser/torch needs to be mounted firmly - eg on a tripod or a fence post. Fantastic for sending data long distances. But if children are involved, maybe safer to use leds rather than lasers.

Sorry to keep asking about this but someone just told me there are filters that can block out ambient light. Does you know of one and will it work alongside an aluminum tube?

kwinn

noanalyst wrote: »

Sorry to keep asking about this but someone just told me there are filters that can block out ambient light. Does you know of one and will it work alongside an aluminum tube?

There are filters that will block out specific wavelengths of light, IR, and UV, but nothing that blocks out ambient light if it is the same wavelength you are interested in. All of those filters will work with an aluminum (or any other) tube. What are you trying to accomplish here?

noanalyst

kwinn wrote: »

There are filters that will block out specific wavelengths of light, IR, and UV, but nothing that blocks out ambient light if it is the same wavelength you are interested in. All of those filters will work with an aluminum (or any other) tube. What are you trying to accomplish here?

Basically trying get an led light or even a regular flashlight to trigger off an action. Right now, I want my son to shine a light at a box with a photosensor in it and have the sensor trigger off his electric train. It works well at night but during the day nothing. Was going to buy a metal tube but a friend thought there were filters that blocked out ambient light.

kwinn

This might be possible by placing the sensor in a tube to block out most of the ambient light and adding a filter to the end of the tube to block all but the wavelength of the led you are using. Even then you may need to modulate the led and filter the received electrical signal as well. Easier to use an IR led and receiver module like the ones used in TV remotes. You would need to build the circuitry to modulate the led but the receiver filtering is done in the module.

noanalyst

kwinn wrote: »

This might be possible by placing the sensor in a tube to block out most of the ambient light and adding a filter to the end of the tube to block all but the wavelength of the led you are using. Even then you may need to modulate the led and filter the received electrical signal as well. Easier to use an IR led and receiver module like the ones used in TV remotes. You would need to build the circuitry to modulate the led but the receiver filtering is done in the module.

Yep, just confirmed with my buddy. That is what he meant by a filter. Ok, will try that.