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LDR offset + span adjustment


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Hey guys.
Im rather new to this forum, but have been looking around for a while for something that might help me.
I got an assignment from my boss that I should design a new circuit.
The circuit need to be controlled by an LDR and work so that when it gets dark the voltage increase and when it gets dark the voltage decrease.
The one we have now works somewhat alright, it adjusts the voltage output between 0 and whatever max value is chosen.
I have attached a picture of the current schematic.

Now I need to change it a little so that we can adjust and offset and change the span.. And this is where I need help hehe.
At first I made a circuit with opamps setting offset with a summer circuit, but this is no good, cause as it turns out I gotta stick to discrete components so it doesnt become too expensive and too big.

Now.. What im looking for is not necessarily a complete design, but more some directions as to what I should look for and read about. I am an apprentice in an electronics form, so I wanna learn :)

So what im looking for is a way to set offset with - I dunno, a transistor circuit of some kind? And also a way to adjust the span so that we can change how bright it should be before the output reaches maximum.

I apoligize for my bad english grammar, please don't hesitate to ask if I need to clarify something.


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Ok, thanks for, but that is only a quick sketch of the _old_ design.
I need to build a new one where I can adjust offset and span, and for that I am asking if anyone have a few pointers as to what I should read up on.
If possible it need to be designed without the use of opamps.  :-\

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I would think that I would need something like a current or voltage summer in order to create the offset, but I gotta make it without the use of opamps.
For the span im not really sure.
I would really appreciate it if someone could point me in a direction.
Ignore the schematic in the OP, its wrong and outdated :)

I have now tried something that seems to work, atleast somewhat, however, I still have a problem with the use of an opamp. Is there a way to make a DC voltage follower without the use of an opamp? And perhaps even so I can eliminate the problem with my opamps max output being only 3,5V when it's supplied with 5V and same problem with offset in an opamp.


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I'm not sure what you're trying to do.

If all you want to do is make a voltage rise/fall depending on the light intensity all you need is an LDR and a resistor configured as a potential divider. I you need to reduce the output impedance then an op-amp or a transistor will work.

The circuit posted above won't work because the TL082 needs more than 5V to work properly, >6V from memory but check the datasheet. The resistor and LDR potential divider need to be connected to the input of the op-amp not the output.

If you want to switch an appliance on or off depending on the light intensity, you need a comparator. Here's a link to some circuit which do this:

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What I'm trying to do is make the voltage rise and fall within some boundaries.. a MAX voltage and a MIN (offset) voltage. so that I can adjust the span where the voltage should rise and fall, and adjust the offset.
an example; I want to be able to make the voltage rise between 500mV and 3V.
Kinda like the comon 4mA - 20mA applications outthere, only where the sensor is an LDR and the output is voltage.

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  • 4 weeks later...

I remember asking the same thing here years ago ...

Exactly the same issue - you have an output range from A to B and you want to scale it linearly so that A is now A1 and B is B1

A1 and B1 are within reason arbitrarily related to A and B - easy to do in code (with the possible resultant resolution drop), but harder to do with analog electronics

Scaling (multiplying/dividing) is easy
Adding voltage is easy

Some sort of simultaneous equation involved with a circuit - but if I recall subtracting voltage is the hard part

Say you wanted the signal ranging from A to B to equate to A-x to B-x - how is that done ?  probably simple, I just forget

Was about to search for the thread again myself, but they seem to have been purged - may as well tag onto this thread huh  ;)

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