[How-To] Monitor a device using a status light

[TonyNo]

A cord like this may be easier to split if your device is not grounded...

http://www.radioshack.com/product/index.js...rentPage=search

 

[jwilson56]

And don't forget if you want to increase the sensitivity then just wrap more turns around the CR core (always wrap only one of the wires).

Each turn doubles the sensitivity.

John

 

[FrankMc]

I tried this but it doesn't seem to be sensitive enough to light for my application. I am trying to monitor the mute LED light on an A/V receiver. The LED is not very bright when on. Any circuit ideas on how to make the photocell more sensitive to light?

The other option (I would like to avoid if possible) I have is to open up the A/V and monitor the LED directly. What would I need and how would I do this?

Thanks,
Chris

Hi Chris

Not sure which sensor your using ...I use an LDR which is connected to a common 741 op amp ...One of my dvd players uses a red / green led for on/off status...So i used my multimeter to monitor the LDR resistance when the led was red and also when it was green...there was enough change in resistance to get a reliable on/off status ..all i had to do was change the values of my threshold resistances...
Frank

 

[FrankMc]

Hi,

I'm now using some relays and power supply connected in the back of my receivers and cableboxes to my SECU16 input to know theirs power status. That was very simple to build.

But your led detectors tip just gave me an idea... Could i use a photocell over the outical output red light? I don'T know if it possible to get an empty TOSLINK connector and put a light detector in it. It would make a clean looking probe. Nothing clued in the front of the TV. And I don't use the optical out.

I know some of you are using CR Magnets to know if your TV are on or off. I would prefer not to split the power cord. It's a new HD LCD.

Hi Stef

If your toslink connector red light is only there when the unit is on then would imagine you could use an LDR connected to an analogue input or something like my led probe connected to a digital input......
Frank

 

[chrishallowell]

I have a CdS photocell(LDR?) and a LM393(Op-Amp?).

I have built the Basic CdS Photocell Detector circuit on this page: http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html

IC1 = LM393
R1 = (I have tried the full range of resistors with no luck)

I have this circuit working for "normal" light/dark situations but it doesn't work when monitoring the faint LED light on my A/V receiver.

Any ideas on how to make this more sensitive to light?

Thanks,
Chris

 

[FrankMc]

I have a CdS photocell(LDR?) and a LM393(Op-Amp?).

I have built the Basic CdS Photocell Detector circuit on this page: http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html

IC1 = LM393
R1 = (I have tried the full range of resistors with no luck)

I have this circuit working for "normal" light/dark situations but it doesn't work when monitoring the faint LED light on my A/V receiver.

Any ideas on how to make this more sensitive to light?

Thanks,
Chris

Hi Chris

That circuit is very similiar to the one i use for my probe except i use 100k resistors....

You need to connect the LDR to a digial multimeter and monitor the resistance when the led is on and when its off if you get a change in 50k or more then it should be just a matter of altering the theshold level of the non inverting input......

So if you could do that and report back the resistance value i will try and suggest what values to use...Ensure no ambient light can get to the sensor when taking reading as this will effect your reading....
HTH
Frank

 

[chrishallowell]

Thanks Frank. I will take the measurements and report them back.

Thanks,
Chris

 

[chrishallowell]

ok, I get about 20K Ohms when the LED light is on and a 1(Infinite) when it's off.

Thanks,
Chris

 

[FrankMc]

ok, I get about 20K Ohms when the LED light is on and a 1(Infinite) when it's off.

Thanks,
Chris

Hi Chris

Can you please change the value of R1 to 100k and try that.....You should get the led to change state once you do that....

HTH
Frank

 

[FrankMc]

Chris

Also ensure as per your testing that no ambient light can be picked up by the ldr....

I hot glue my ldr,s into small plastic cap ...I use a 3m double sided sticky patch which i punch a small hole in the centre so the ldr can pick up the light from led....

HTH
Frank

 

[chrishallowell]

Thanks Frank. I'll try this when I get home tonight.

Thanks,
Chris

 

[chrishallowell]

Yes, it works! Thanks so much!

One last hurdle....Connecting this to a joystick input.

How would I connect a relay to this to get a contact open/close?

I have a bunch of 12VDC Automative relays sitting around. Can I use these or what kind do I need?

Thanks,
Chris

 

[FrankMc]

Hi Chris

>Yes, it works! Thanks so much!

Great News!!! ;-)


One last hurdle....Connecting this to a joystick input.

I assume you mean connect the led probe output to your pc joystick input ??? If so

One method would be to use a relay but may require a driver transistor connected to the probe circuit.....Personally i think i would use an opto coupler....

Do you have a link to the joystick connections ... ???

i think the automotive relays are a bit too beefy for this application......

Frank

 

[chrishallowell]

Yes, you are correct. I mean connect the led probe output to the pc joystick input.

"One method would be to use a relay but may require a driver transistor connected to the probe circuit.....Personally i think i would use an opto coupler...."
Would the right half of the relay driver circuit work in the below link work?
http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html
(LM393 out to 2.2K resistor to 2N3906 base. 2N3906 emitter to 12+. 2N3906 collector to relay+. Relay- to 12-. Relay common to )

What kind of relay do I need? Radio shack: Model: 275-248?

"Do you have a link to the joystick connections ... ???"
I just ripped open a joystick and spliced into the buttons. I believe this is just a dry contact but I could be wrong.

Thanks again for all your help!
Chris

 

[FrankMc]

Hi Chris

Yes that relay looks fine so long as your using 12v for your circuit...

Frank