methods for scaling a resistance down?

[caress]

i'm wondering if there's a way to scale a larger resistance down to a smaller resistance?
i guess a good example would be if you were using an LDR to control a smaller resistance, say 10k.  an LDR normally puts out a huge resistance from hundreds to millions of ohms so is there a way to scale that down to 10k-ish at dark?

[R.G.]

DC or AC only?

[ParlorCitySound]

I'm not to sure about using the LDR with it, but parallel resistance will drop resistance.
Maybe across the output of the LDR. May screw with the curve of the LDR's output resistance a bit though.

1/ ((1/R1)+(1/R2))

the end result will always be lower than the lowest resistor in the equation.

R1 = 10k
R2 = 330
Total = 319

[caress]

DC or AC only?

DC.  how would that affect the outcome, though?

?

I'm not to sure about using the LDR with it, but parallel resistance will drop resistance.

i'll try that to start.

[R.G.]

DC.  how would that affect the outcome, though?

Too bad. Transformers will directly scale impedances, but they only work on AC. You can't run a transformer on DC.

?

I'm not to sure about using the LDR with it, but parallel resistance will drop resistance.

For DC, best thing to do is use multiple LDRs in parallel. Using the LDR in parallel with a resistor has a limited range. Could be enough, but limited.

Then, depending on what you are doing,  you might be able to use an active device. Scaling a resistor for DC implies that you're trying to control a DC current or DC voltage. Both of those can be done with active devices. Can't tell with no more information than is here.

[caress]

active is totally fine... 
it's to control a DC voltage - a sort of lo-fi control voltage using an LDR.
the LDR would be placed in series with the supply voltage, probably with a small (100r or so) limiting resistor after the LDR.  i'm just thinking of a general idea that could be implemented in different projects.

[R.G.]

OK, that's easy. Use the LDR and a fixed resistor in a voltage divider to get the voltage range you want from the LDR you have, using as high a resistor as you need.  Then run that into the noninverting input of an opamp whose input is specified to be able to go near ground without problems. The LM324 does this, among others, and so do all the "rail to rail input" opamps. You may still have to limit the positive swing of the input voltage with some opamps. The output of the opamp follows the LDR voltage, but with a low impedance, and current-limited to boot. Takes an opamp, but you get to use the LDR you have and not worry about scaling a resistance. You can also put a cap on the junction of the fixed resistor and the LDR and slow down the voltage movement to any degree you like.

[caress]

would this be more or less what i'm looking for?



R1 is there to limit the incoming voltage to the LDR and to set the lowest point but i'm not sure if it's 100% necessary, especially since one will already most likely be in series for supply filtering.
R2 would set the higher limit, thereby effectively setting the LDR to only go to 10k resistance?

[cpm]

how is the LDR used?
if you have an LED, ther you may have a proper control voltage, wouth the need of ldr

[caress]

it would be mounted externally.

[R.G.]

would this be more or less what i'm looking for?

Yep.

This way light make the voltage go up. If you swap position of the LED and 10K, light makes the voltage go down.

R1 is there to limit the incoming voltage to the LDR and to set the lowest point but i'm not sure if it's 100% necessary, especially since one will already most likely be in series for supply filtering.

The amount of limiting R1 does needs looking at. The input voltage to the opamp is always Vin* (R1)/(R2+LDR+R1). As a practical matter, the LDR may vary from 500 ohms to 1M or even 10M. If it goes to 1M, then the voltage is Vin * (10K)/(100R + 1M +10K) or effectively, Vin * 1/100. You have to know the possible range of the LDR to set R1 to keep the voltage up. This is a good thing! Getting enough range is always harder than limiting the range you have. 

R2 would set the higher limit, thereby effectively setting the LDR to only go to 10k resistance?

The LDR does whatever it does, which may be going well under 1K for some LDRs. What R2 does is set the highest percentage of the incoming voltage that the opamp sees. As it sits, the voltage out is a percentage of the incoming supply voltage. We often call that "9V", but it's actually 7V or less up to 9.3 with real batteries, and up to 18V if this falls in the hands of a "doh, I heard somewhere that more volts makes for a better groove..." kind of user. I've often wondered what this kind of guy would do if you told him that for driving on ice, you can get more traction if you drive nails into your car tires so the heads will dig into the ice. Sigh. The statement is true - but there are side effects.

It might be better to make R2 perhaps 2.2K and put a 5V zener to ground from the junction of R1 and the LDR. That makes the voltage to the opamp be 0-zener voltage, and makes the voltage range stable in the face of batteries running down from 9.3V (fresh) to 7V (drained). If you have to have the output voltage swing from ground to nearly battery voltage, you can use a feedback resistor and negative-input resistor to put a gain of maybe 1.5-2 on the opamp and scale your output voltage range.

[caress]

would this be more or less what i'm looking for?

Yep.

This way light make the voltage go up. If you swap position of the LED and 10K, light makes the voltage go down.

R1 is there to limit the incoming voltage to the LDR and to set the lowest point but i'm not sure if it's 100% necessary, especially since one will already most likely be in series for supply filtering.

The amount of limiting R1 does needs looking at. The input voltage to the opamp is always Vin* (R1)/(R2+LDR+R1). As a practical matter, the LDR may vary from 500 ohms to 1M or even 10M. If it goes to 1M, then the voltage is Vin * (10K)/(100R + 1M +10K) or effectively, Vin * 1/100. You have to know the possible range of the LDR to set R1 to keep the voltage up. This is a good thing! Getting enough range is always harder than limiting the range you have. 

R2 would set the higher limit, thereby effectively setting the LDR to only go to 10k resistance?

The LDR does whatever it does, which may be going well under 1K for some LDRs. What R2 does is set the highest percentage of the incoming voltage that the opamp sees. As it sits, the voltage out is a percentage of the incoming supply voltage. We often call that "9V", but it's actually 7V or less up to 9.3 with real batteries, and up to 18V if this falls in the hands of a "doh, I heard somewhere that more volts makes for a better groove..." kind of user. I've often wondered what this kind of guy would do if you told him that for driving on ice, you can get more traction if you drive nails into your car tires so the heads will dig into the ice. Sigh. The statement is true - but there are side effects.

It might be better to make R2 perhaps 2.2K and put a 5V zener to ground from the junction of R1 and the LDR. That makes the voltage to the opamp be 0-zener voltage, and makes the voltage range stable in the face of batteries running down from 9.3V (fresh) to 7V (drained). If you have to have the output voltage swing from ground to nearly battery voltage, you can use a feedback resistor and negative-input resistor to put a gain of maybe 1.5-2 on the opamp and scale your output voltage range.

whoops in my last message i flipped R1 and R2 around... i meant R2 would be there because of supply filtering.
i added a new pic with the changes you recommended.  the zener would be a 5v type and R2 would probably need to be picked to find the best range (most likely a trimmer replaced by the appropriate resistor)  R3 and R4 provide a gain of around 2, but would it be necessary to raise each of their values to say 100k/220k?

[R.G.]

Almost!

In your last schemo I would:
1. move the cathode/bar of the zener to the junction of R1 (in the last schemo you posted). This keeps the total voltage from the LDR to the opamp to less than the zener voltage. A 5V zener is low enough that any real 9V battery will keep it at that voltage.
2. Leave R2 connected as it is.
3. Move the end of R3 that you show as connected to the LDR to ground instead.
4. Connect the junction of the LDR and R2 to the (+) input of the opamp.

Now, the zener keeps the voltage into the opamp to less than the zener voltage even if the LDR goes to 0 ohms. The LDR going to over 10M makes the input voltage to the opamp approach be never be exactly zero volts. The R3 and R4 make the opamp produce a multiplied version of the LDR/R2 voltage on the (+) input. This multiple is 1+R4/R3. So if R4=0 or R3 = infinity, or both, then the gain is 1+ (something)/infinity) or just one. If R4 and R3 are equal, the gain is still 1+R4/R3, or 1+1 = 2.

As you've shown it, the gain is 1+ 22k/10K = 1+2.2 = 3.2. A 1V input on the + input will result in 3.2V out.

As shown, the output voltage goes up when there is more light on the LDR. If you want to change that, exchange the LDR and R2. Now the LDR can let the voltage on the + input go high when there is no light and pull it low when there is lots of light.

[caress]

ok let's see if this is correct...
you're really working with me here!  thanks RG
i forgot about the +1 when figuring out the opamp gain, so i switched it back to a gain of 2 using 1 pair of 10k resistors.

[caress]

ok so i got this working and it's great so far.  i need to boost my output a bit more than x2 to get my desired output voltage when in light, but that's easy so no problem there.  i'm wondering now how to limit how low the dark output will go.  at the moment my voltage swing is roughly 7.3V - .35v from light to dark, respectively.  too big for me to actually use...  how could i modify this to get a different voltage swing somewhere in the range that i have?  for example:  a .5v swing?  1v? etc.  from 2-3v?  6-7v? etc.

[caress]

bump for RG to come to the rescue   

[PRR]

[caress]

 

ah yes... that makes a ton of sense.  so simple!  thanks paul!