Double LDR circuit

[versimilitude]

Firstly, i'd like to apologise because my base electronics isn't great! I have started to mess about with some electronics and am slowly learning.

I've created a simple light activated switch as per the attached drawing. It works great but now i'm getting a little more adventurous! I now want to use two LDRs so that when either of them senses light the single LED will come on.

any help anyone can give to point me in the right direction would be much appreciated.

Thanks!

 

[ericgibbs]

Firstly, i'd like to apologise because my base electronics isn't great! I have started to mess about with some electronics and am slowly learning.

I've created a simple light activated switch as per the attached drawing. It works great but now i'm getting a little more adventurous! I now want to use two LDRs so that when either of them senses light the single LED will come on.

any help anyone can give to point me in the right direction would be much appreciated.

Thanks!

You could put another LDR in parallel with the existing LDR, you may have to adjust the resistor value from the transistor base to 0V.

Another way would be to make an identical circuit [without the LED and resistor] and connect the two transistor collectors together.

Does this help?

 

[versimilitude]

Thanks ericgibbs !!

I've tried adding it in parallel and that seems to work. I will try your other suggestion as well later and see how it goes as well.

thanks again.

 

[audioguru]

If both LDRs in parallel have a bright light then the base current in the transistor might be too high. Limit the base current with a resistor like this:

 

[versimilitude]

Thanks guys, that's helped a lot !

I have another question now though. Due to some technical issues i'd like to make this circuit out of components that are RoHS compliant. I;m having trouble finding a LDR that is compliant so i was wondering would it be possible to swap the LDR for a photodiode (or phototransistor).

I know that photodiodes have a faster response time but that is not an issue. Would i have to make significant changes to my circuit?

thanks again!!!

 

[ericgibbs]

Thanks guys, that's helped a lot !

I have another question now though. Due to some technical issues i'd like to make this circuit out of components that are RoHS compliant. I;m having trouble finding a LDR that is compliant so i was wondering would it be possible to swap the LDR for a photodiode (or phototransistor).

I know that photodiodes have a faster response time but that is not an issue. Would i have to make significant changes to my circuit?

thanks again!!!

Yes, some changes are required.
What is the type/nature of the light you want to detect and what do want
to happen when the light is detected?
Give us some details of your project so that we can advise you.

Eric

 

[versimilitude]

I have a machine at work with two LEDs that indicate when certain parts of the system are on. Unfortunately the way they are setup the LEDs aren't in a very clear postion to see them so i want to use a LDR/photodiode to sense when they are on (or flashing) and use that input to control another LED in a more suitable position.

cheers!

 

[ericgibbs]

I have a machine at work with two LEDs that indicate when certain parts of the system are on. Unfortunately the way they are setup the LEDs aren't in a very clear postion to see them so i want to use a LDR/photodiode to sense when they are on (or flashing) and use that input to control another LED in a more suitable position.

cheers!

hi,
Have you got access to the pin connections for the existing LED's. [the one's you cannot easily see]?

If yes, and you are permitted to add two wires to each LED, it would be possible to modify the equipment and not use Photo detectors.

Eric

 

[versimilitude]

I have thought about that but it's a route i would rather not go down. It would be extremely difficult to get to and i would be rather worried about messing something up!

That's why i was heading down the route of the LDR/photodiode.

 

[ericgibbs]

I have thought about that but it's a route i would rather not go down. It would be extremely difficult to get to and i would be rather worried about messing something up!

That's why i was heading down the route of the LDR/photodiode.

hi,
Attached datasheet it has a number of fundamental phototransistor, transistor circuits which would suit your application.

If you need a hand to finalise your design, let us know.

 

[audioguru]

Watch out for the DARD current in a Sharp photo-diode.
Why don't Japanese companies use an English-speaking person to proof-read their English translations??

 

[versimilitude]

Ok, phototransistors is stretching my electronics background to its limit!

Eric, i had actually saved that doc onto my computer after having found it earlier. It did help a bit but i'm still not 100% sure exactly how to do it. I have attached a simple diagram of how i figure it might be setup for one light source input (didn't want to complicate things even further at this stage with two inputs!)

My understanding is that when light shines onto the phototransistor it will allow a current to pass through the Collector / Emitter junctions and light up the LED. the resistor R1 can be adjusted to change the amount of light required to switch the phototransistor on. is this right?

 

[audioguru]

A photo-transistor needs to have a very bright light on it for it to pass the high current of an LED.
Replace the LDR in your original circuit (it has a regular transistor as an amplifier) with a photo-transistor then it will be more sensitive.

 

[ericgibbs]

A photo-transistor needs to have a very bright light on it for it to pass the high current of an LED.
Replace the LDR in your original circuit (it has a regular transistor as an amplifier) with a photo-transistor then it will be more sensitive.

I agree with agu.

IMO you do not need the base resistor connected on the photo-transistor.

When you choose the photo-transistor, get one that matches the light output spectrum from your existing LED's.
I expect visible light would be a good starting point.

Remember to keep out the ambient light as much as possible to avoid mis-operation.

 

[versimilitude]

Thanks again guys.

From what i gather from your posts, you're saying that the previous circuit that i'd put up wouldn't give me enought current to power the LED. So by using the phototransistor in conjunction with a transistor i will get amplification of the current therefore powering the LED.

So my new circuit should just be the one that is attached.

Will the value of R1 have to change as well? I was using a 10k potentiometer to allow me to adjust it to the right value.

 

[ericgibbs]

Thanks again guys.

From what i gather from your posts, you're saying that the previous circuit that i'd put up wouldn't give me enought current to power the LED. So by using the phototransistor in conjunction with a transistor i will get amplification of the current therefore powering the LED.

So my new circuit should just be the one that is attached.

Will the value of R1 have to change as well? I was using a 10k potentiometer to allow me to adjust it to the right value.

Hi,
To give you some level of control of the activating light intensity I would use the attached circuit.

The 330R is based on the LED having a 2V operating voltage drop.

Eric

 

[versimilitude]

I appreciate that Eric.

I'll now go out over the weekend and see if i can get the parts i need