Light Dependent Resistor Help

[skyryan]

Hey guys,

I'm stumped on a project I'm currently working on with an LDR and is seeking your expertise or input. Truth be told, my knowledge of electronics is basic at best so bear with me, but I have nevertheless decided to take this challenge on board.

Anyway, the main concept of my system is as basic as it can probably get. I want a system to turn on when the LDR is hit by light and essentially turns off when there is no light. The system is just a simple music in, music out, with music turning off when there is no light and on when there is, and in order to achieve this, I have basically just plugged the LDR in between the player and the speaker hoping that it does what I think it should. I understand that this might probably be conceptually incorrect or whatnot, and I probably should have or could have done it a better way, but my system works in a dimly lit room with the LDR stuck in a dark box. Yay.

However, no matter how much I try to keep the LDR in a dark box and whatnot, whenever I bring the system outdoors, which is where I need it to work, sunlight keeps sticking its head in and won't let the system turn off. I've even stuck the bloody LDR at the end of a 1.5m box with only a small whole on the other end for a laser to point at it but light still gets in whenever there is even a hint of sunlight. I know the sun is damn powerful and all, but is it really THAT powerful that it can reach the LDR through that small hole? Or are LDRs really that sensitive? Are there special circuits or LDRs that will do the job I'm after?

Anyway, I'm now turning to all of you electronic whizzez to see if you have any suggestions and help (pleeeaseee). I know that my idea might not work, which is fine, but maybe, just maybe, somebody have an ingenious solution that will help my situation.

I appreciate all of your feedbacks and help.
Thank you kindly,
Ryan

 

[Reloadron]

Well it won't be quite as simple as you would like it to be but not all that complicated. You can't simply for example place a LDR (Light Dependent Resistor) in series with music to a speaker and get results. In the case of most LDRs when light is striking them their resistance is low and in dark the resistance is high.

What you want is a basic photocell circuit, this can be done usibng a comparator type circuit with your LDR. The was it works is the LDR becomes part of a voltage divider network. As you go from dark to light you will cross a voltage point. That point determines when a circuit (a comparator) will generate an output that could switch for example a relay. That relay would turn the music on or off.

Give this link a read paying attention to the photocell circuits down the page. Something along those lines is what you want with hysteresis. Sounds complicated but with some basic soldering skills and a few components you can get where you want to go.

That or really cheat and buy an over the counter photocell circuit and hack it a little. Even those el cheapo LED lawn lights could likely be hacked.

Ron

 

[skyryan]

Thanks for the quick reply Ron. Much appreciated.

However, I am confused as to how (or why) a comparator circuit is needed conceptually to help remove a majority of the suns effects on the system so that it may only turn on when a laser is pointed at it.

You also said that "In the case of most LDRs when light is striking them their resistance is low and in dark the resistance is high." Isn't that in essence what is needed? or am I completely missing the point? Because, maybe I'm just being naive here, but I thought that this is the property of LDR that is useful to my system? Isn't that the reason why my system is currently turning on and off the music?

A bit of background, I initially used the LDR for a system laser audio transmitter that I followed on the net. However I wanted to change the system so that the music is on the receiver side, rather than on the transmitter's side. That's why I just simply put the LDR in between the music and the speaker, without really thinking about other systems I might have to incorporate. So now that you start talking about comparators, I'm really befuddled Sorry.

Thanks mate for your time and help.
Ryan

 

[Mr RB]

...
However, I am confused as to how (or why) a comparator circuit is needed conceptually to help remove a majority of the suns effects on the system so that it may only turn on when a laser is pointed at it.
...

In layman's terms; The LDR is a resistor that only changes its resistance through a limited range of values. So it acts like a resistive volume pot with a limited range that cannot ever be turned fully on or off. Just slightly louder or quieter.

A comparator *compares* the signal from the LDR with a specific setpoint (that you can set yourself), so when the LDR is >setpoint the output turns full ON, and when the LDR is <setpoint the output will turn fully OFF.

Basically the comparator makes the LDR much more sensitive, and lets it turn something completely ON or OFF instead of just "up and down a bit".