Need help with understanding LDR's

[matta]

Hi Guys,

I'm trying my hand at circuit bending an old drum machine (don't ask!) and there is known mod to lower the resistance threshold of a particular resistor to get the circuit to overdrive/distort.

I though it may be cool to do this using an LDR so I can play with light to increase or decrease the distortion color/sound.

I've never used an LDR but understand the basic principal, I just didn't realize the the HUGE variance in resistance they had.

The one I have in full light measures 350R and in total darkness 800K.

The circuit in question requires the value of the resistor to be between 1K5 to 2K2, the lower the resistance the more distortion.

Is there a way to limit the range of the LDR using passive components?

I thought I could parallel a resistor with the LDR and get the MAX resistance down to the 2K2 range, but then it will put the minimum resistance as 0R (I think not even sure this will work).

Any suggestions?

Thanks

Matt

 

[JohnRoberts]

You can put a resistor in series with the LDR so that 350R + series R (say 680R) is more like 1k, then you can put R (say 1.5k) in parallel with LDR so max impedance is in your target range.

This is not necessarily going to give you a good law or nice adjustability but you can manage the range this way.

JR

 

[matta]

Thanks John!

Much appreciated, to be honest it is supposed to sound 'nasty' and I'm not too worried about the law and the more it flips out the better, Hah hah.

If I understand you the series resistor (680R) will bring me up to the min 1K range and then use a 1K5 across the LDR with series resistor to lower the MAX range?

Math is my weak point but if I'm not mistaken the equation for Parallel resistors is:

Rtotal = R1xR2 / R1+R2
Rtotal = 750k x 1k5 / 750k+1k5
Therefore Rtotal = 1K5

1K5 + 680R = 2K2 (rounded up)

Does that look right?

Thanks

Matt

 

[JohnRoberts]

close enough... but I don't know if it will give continuous range of adjustment or just seem like an on off switch between two extremes.

JR

 

[matta]

[quote author="JohnRoberts"]close enough... but I don't know if it will give continuous range of adjustment or just seem like an on off switch between two extremes.

JR[/quote]

I will check it out tomorrow when I'm back at my workbench and report back... I'm hoping for the best...

Cheers

Matt

 

[CJ]

They sell a lot of different cells, you must be foolin with LA2 stock?

Get some samples.

:thumb:

 

[matta]

[quote author="CJ"]They sell a lot of different cells, you must be foolin with LA2 stock?

Get some samples.

:thumb:[/quote]

Hey CJ,

These were all my local store had in stock, seem to be the same values just different size, 5mm, 7mm and I think 11mm... but no different values, at least not from that store.

Cheers

Matt

 

[PRR]

> in full light measures 350R

What is "full light"? The 20FC of the workbench? The 200FC near a 100W lamp? Or the 5,000FC of direct sunlight? Most commercial LDRs saturate long before 5,000FC, and the difference 20FC-200FC won't be 10:1, but it takes quite bright light to really sock the electrochemistry.

> in total darkness 800K.

Wait. In minutes or hours it will go to many Megs.

> value of the resistor to be between 1K5 to 2K2

As you and John say, you put 1K1 in series and 2k2 in shunt with the whole contraption to keep the total in-range.

But as John is saying, out of the total 350R-800++K range, over 11 octaves, only about one octave gives you the full swing you need.

Over most of the middle of the curve, resistance is approximately (and inversely) proportional to Light.

You want a light source with just about a 1 octave range. Maybe an LED fed 5mA to 10mA.

You could have one LED fed 5mA-zero, and a second with a steady 5mA to set the upper limit.

You could put the LDR under a large milky-glass, under a lamp, and put your hand in there. Pick the lamp to get down to the low end of your range. Shadow half the glass with your hand, you get roughly double resistance, which is getting near the top of your range.

All these tricks will need light trimmers. LDRs are moderately stable, but you are asking for a narrow range out of their total range. With temperature, and, and light-history, that "350R" may be 300R next week and 400R next month.