Questions on LDR's am i thinking right?

Hello there!

I have a project laying around bugging me.
This project is what i think basic. And the function im trying to make is..
I have one LDR used as a refrence.
I also have three other LDR's that i want to use as thresholds for each of the op-amps.
The refrence voltage from the refrence LDR is fed into three op-amps one for each of the other LDR's.
So if one of the LDR's voltages drop below the LDR Ref voltage the op-amp will give
a positive output. The voltage is used to trigger an open collector output.
The power supply in my circuit is 24VDC

A picture on the circuit https://obrazki.elektroda.pl/8671431300_1394657506.jpg.
instead of LDR's in the circuit i used resistors, while testing i just adjust the values of them to see what happens. The Values are just values at the moment since i dont have real component values yet.

Questions about this. Well since the circuit seems to be working i just want someone to look at it and tell me if im in the right direction in my thinking of solving my funtion.

I also want to be able to adjust the sensitivity of the refrence voltage. Since components and conditions are not as in the simulator. So adjustable sensitivity allowing the three LDR's to differ abit from the refrence.

About the open collector outputs. Is it possible to make them voltage insensitive.. meaning if i want to connect a voltage lower than the supply voltage for the circuit, like a relay contact?

Cheers!

To adjust sensitivity you can use variable resistors.

You can use open collector mode, in your case connect a high value resistor from op amp output to the base of BJT else the base region will get damaged. If you have different voltage source, you can use that for connecting load at collector.
In your case since the 24 V is constant and the load resistance is almost constant (relay), better to go for a resistor in series with the relay rather than using a linear regulator to convert 24v to relay voltage. Calculate the resistance and wattage for the series resistor by considering the relay resistance, relay voltage and the total current passing through the series.

Most of my many solar garden lights are turned on and off by LDRs. The actual resistances of the LDRs are very different and change as the LDRs get sunburned.
You will have much better accuracy if you use photo-diodes or photo-transistors instead.

vinodstanur said:

In your case since the 24 V is constant and the load resistance is almost constant (relay), better to go for a resistor in series with the relay rather than using a linear regulator to convert 24v to relay voltage. Calculate the resistance and wattage for the series resistor by considering the relay resistance, relay voltage and the total current passing through the series.

Click to expand...

though not often seen on relay data-sheets there are two currents involved;


1) initial current to latch
2) current needed to maintain the latched state - this is what is usually given

a relay may not latch if the resistor prevents sufficient current to obtain the latch

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Audioguru said:

Most of my many solar garden lights are turned on and off by LDRs. The actual resistances of the LDRs are very different
You will have much better accuracy if you use photo-diodes or photo-transistors instead.

Click to expand...

you could buy 'dozens' of LDRs and match them yourself

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sidewayzz said:

Hello there!
I also want to be able to adjust the sensitivity of the refrence voltage. Since components and conditions are not as in the simulator. So adjustable sensitivity allowing the three LDR's to differ abit from the refrence.

About the open collector outputs. Is it possible to make them voltage insensitive.. meaning if i want to connect a voltage lower than the supply voltage for the circuit, like a relay contact?

Click to expand...

Not sure what you mean by sensitivity.......do you mean the voltage? sorry, but I don't follow

For driving a load with a lower voltage there are a few ways; depending on how much current needed for the load, current sinking of the op-amps. How stable is the voltage for the LDR/op-amp circuit? Is current drain an issue?

vinodstanur said:

To adjust sensitivity you can use variable resistors.
In your case since the 24 V is constant and the load resistance is almost constant (relay), better to go for a resistor in series with the relay rather than using a linear regulator to convert 24v to relay voltage.

Click to expand...

By variable resistors you mean a potentiometer? Sorry for being stupid

Yes, i have been thinking about that. But a transistor is smaller. What about a galvanic separated output?

Audioguru said:

Most of my many solar garden lights are turned on and off by LDRs. The actual resistances of the LDRs are very different and change as the LDRs get sunburned.
You will have much better accuracy if you use photo-diodes or photo-transistors instead.

Click to expand...

I have been trying to find a good way to use photo-transistors. But how do i adjust the sensitivity?
Doesn't the photo-transistors work like a true or false way, either on or off?

kam1787 said:

you could buy 'dozens' of LDRs and match them yourself

Not sure what you mean by sensitivity.......do you mean the voltage? sorry, but I don't follow

For driving a load with a lower voltage there are a few ways; depending on how much current needed for the load, current sinking of the op-amps. How stable is the voltage for the LDR/op-amp circuit? Is current drain an issue?

Click to expand...

*LDR matching
Well yes i guess i could do that. Hopefully i would find some that are pretty close to eachother.
Good tip!
Another thing i have been wondering about is how to choose wich LDR to use. There are a lot of different
types ranges etc to choose from.

*Sensitivity
Well the circuit i have made it really depends on if the components are 100% accurate otherwise it wont work.
In other words it wont work in reality
So i have to have something to adjust the circuit with and make it less sensitive for voltage variations due to component accuracy and surrounding conditions.
For example if LDR ref (Refrence voltage, negative on all op-amp's) and LDR 1 (positive on op-amp 1) has the same amount of light on them. LDR ref could have a greater voltage that LDR 1 never could give out.
Also if LDR 1 has less light on it than LDR ref i want to be able to change the offset voltage to the op-amp.
In my circuit example LDR ref has 100% light and gives a refrence voltage and if LDR 1 has less light and gives 99% it will trigger the output on the op-amp. I want to be able to adjust the allowed offset voltage.

*Current drain
I dont think its a problem. If it is i will have to get a larger rectifier. At the moment the maximum is 24VDC/2A

Thanks!

it is easy to adjust the gain of a photo-transistor for a sensitivity adjustment.

from the sounds of it I really doubt you can achieve what you want, especially using LDRs

maybe if you shared the purpose of your design, we could point you in the right direction

Audioguru said:

it is easy to adjust the gain of a photo-transistor for a sensitivity adjustment.

Click to expand...

A phototransistor yes. After giving this a second thought now with the input i get here. Maybe that would work.
The reason i chose LDR at the beginning was simple, the only thing i knew
After thinking about it for a while now i could use photo-diodes or photo-transistors. Im not really after the value, just the offset between the refrence and the others. Or the amount of offset i choose to accept.

kam1787 said:

from the sounds of it I really doubt you can achieve what you want, especially using LDRs

maybe if you shared the purpose of your design, we could point you in the right direction

Click to expand...

I hear you. I just started thinking of how i could do this in another way.
The purpose of this is to read angles as the sun moves. Refrence is towards the sun and the offsets compared to that will be sent to a PLC slave wich is hooked to a computer wich logs.

When i started this "project" i wanted to make a senor that could give the PLC 4-20mA values.
When i spoke to my guru on PLC he gave me some pointers how to make a simple circuit with just a resistor and a LDR (not exact and would prob overheat the LDR) sinking that to AIN of PLC. In this case i would use several analog inputs to calculate the angles of the sun and it would get me a more exact location as well.

I found it hard to get good examples to work with to make a "proper" sensor that could provide me with 4-20mA so i changed to binary in hope that would be easier for me to work with as a beginner.

So maybe if someone has an idea on a 4-20mA solar sensor. Point me in the direction and i will do my best!
Also considering the other option aswell!

Thanks!

there are motorized sun trackers for solar panels - use the same approach but read the angle of the sensor platform to get the sun's angle

I will have a look at the solar trackers.

I found a site about 4-20mA.
**broken link removed**

Is this good info that could help me accomplish my function?

Cheers!