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Quick question about LED/LDR combos

Started by Govmnt_Lacky, February 14, 2013, 01:57:01 PM

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Govmnt_Lacky

February 14, 2013, 01:57:01 PM
If I have an LED/LDR combination... For example an LED lighting a 5K/1M LDR.

Lets say that when I place the LED right up to the LDR, I am getting unwanted noise. To correct this, I move the LED a few millimeters away from the LDR and the noise goes away or is greatly reduced.

Could I "assume" that maybe the full light resistance is too SMALL? Would a logical deduction be that I should maybe go with a 20K/1M with the LED right on top of the LDR and get the same or better results.

These are generalities. I am merely trying to see if there is any truth to my assumption.  8)
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armdnrdy

#1
February 14, 2013, 02:25:08 PM
After "playing" with LDR/LED combos for a delay modulation add on board, I ended up putting a trimmer in line with the LDR so that I could adjust the minimum resistance. With the inconsistent tolerances of LDRs and not having a lux meter handy to dial in each build, I found it the easiest approach.
I just designed a new fuzz circuit! It almost sounds a little different than the last fifty fuzz circuits I designed! ;)

Govmnt_Lacky

#2
February 14, 2013, 02:29:53 PM
Quote from: armdnrdy on February 14, 2013, 02:25:08 PM
After "playing" with LDR/LED combos for a delay modulation add on board, I ended up putting a trimmer in line with the LDR so that I could adjust the minimum resistance. With the inconsistent tolerances of LDRs and not having a lux meter handy to dial in each build, I found it the easiest approach.

Thats a good point Larry. Thanks!

However, wouldn't the inline trimmer also effect the LDR's MAX resistance as well?
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of 'up to and including my life.'

samhay

#3
February 14, 2013, 02:38:40 PM Last Edit: February 14, 2013, 02:41:52 PM by samhay
I would imagine that if the LED you are using is diffused then the further you move the LED away from the LDR, the less light will fall on it.
You can easily measure this if the LED and LDR are not already in situ (are you on a breadboard?) - put some current through the LED and measure the LDR resistance.

Ultra-bright LEDs can cause noise in LED/LDR circuits too, but this is unlikely to be cured in the way you describe.

Edit - didn't see armdnrdy's post. If your LED is mostly off, your LDR resistance will be 1M (ish) and +/- e.g. a 10k trimmer is in the noise. However, if it does cause problems, or you want to adjust the LDRs resistance profile, stick another resistor in parallel with both of them.
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Govmnt_Lacky

#4
February 14, 2013, 02:42:51 PM
Quote from: samhay on February 14, 2013, 02:38:40 PM
I would imagine that if the LED you are using is diffused then the further you move the LED away from the LDR, the less light will fall on it.

The LED is diffused. I am just wondering if what I am assuming is true.

From what you are saying it sounds like it is  :icon_cool:

The less light that falls on it, the HIGHER the resistance at full light (~20K instead of 5K)
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of 'up to and including my life.'

armdnrdy

#5
February 14, 2013, 02:53:42 PM
Quote from: Govmnt_Lacky on February 14, 2013, 02:29:53 PM
[
Thats a good point Larry. Thanks!

However, wouldn't the inline trimmer also effect the LDR's MAX resistance as well?

The highest resistance won't be affected that much. You're adding 5K? 10K? to 1M. I used a 50K trimmer to get rid of my issue at the lowest resistance and didn't hear any problem with the highest resistance area.
I just designed a new fuzz circuit! It almost sounds a little different than the last fifty fuzz circuits I designed! ;)

duck_arse

#6
February 15, 2013, 09:17:37 AM
I rekon the noise must already be there, and the close led/lower resistance is all that makes it audible. the led, run from a clean supply, should not induce any noise, and the ldr itself is just like any other resistor. can you hear more noise with a signal?
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Govmnt_Lacky

#7
February 15, 2013, 09:39:17 AM
Quote from: duck_arse on February 15, 2013, 09:17:37 AM
I rekon the noise must already be there, and the close led/lower resistance is all that makes it audible. the led, run from a clean supply, should not induce any noise, and the ldr itself is just like any other resistor. can you hear more noise with a signal?

The noise is ticking and thumping noise. It is definitely coming from the LED/LDR.  :-\
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of 'up to and including my life.'

~arph

#8
February 15, 2013, 10:21:44 AM
Well, if you move the led closer you basicaly increase the depth of the resistance swing so yeah, that could make thumping audible.. it would help though if you told us what circuit it is.  :-X

Govmnt_Lacky

#9
February 15, 2013, 11:03:55 AM Last Edit: February 15, 2013, 11:14:49 AM by Govmnt_Lacky
Quote from: ~arph on February 15, 2013, 10:21:44 AM
it would help though if you told us what circuit it is.  :-X

Lovetone Flanger?? Specifically, the LED/LDR that controls the oscillator of the BBD clock chip. Essentially, the Manual control.

EDIT: According to the MN3102 datasheet, with a 100pF cap, they list 5K~1M as the resistance value range to produce an oscillator frequency of 6.4 to 520KHz.

Also something to keep in mind is that the original unit has a 120K resistor in parallel with the LDR so any resistance values from the LDR are paralleled with the 120K. I calculated that a 10K/10M LDR would yeild about 9.2-118K resistance.
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of 'up to and including my life.'

pinkjimiphoton

#10
February 15, 2013, 07:49:17 PM
greg, try a bigger current limiting resistor on the led...that's saved my ass a few times!!
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