Measuring LDR values

[electricstorm]

I have only been working with LDR's for a short while and had a question regarding measuring light and dark values.

I have measured some LDR's for an effect I want to clone and came up with 6.5K to room light and at least over 20M dark (my meter stops at 20M so I just get "L" on the digital meter screen for over range). The same LDR gives about 39K when exposed to and LED and 1.5M dark (if the LED is driven by an LFO at a moderate rate).

My question is, what is or would be the correct way to measure an LDR for accurate resistance readings? Should there be a waiting period between light and dark before measuring the "Dark" resistance on the LDR? Without a light meter, how much light should I use for the "ON" resistance measurements?

The reason I ask is because I have ordered different LDR's from several suppliers that cover a wide range of values, but when I measure them at room lighting and very dimly lit room for dark, I get almost the same readings from all of them. For example, I have an LDR that says Light resistance 10K - 20K and Dark resistance 10M. When I measure them at normal room lighting and then no lighting at all they measure 2.1K Light and over 20M Dark. Can you see my confusion!!

Any help would be appreciated.

Jim

[LaceSensor]

have you finished that flanger yet?

[electricstorm]

It is finally finished. I am working on the build docs now. I just worked out the calibration procedure last night. I was trying to refine the LDR's but what I have in there seems to be working pretty well.

I had Haberdasher make a revised main board that would allow you to solder the components on the PCB that were originally soldered on top of the components in front of Q9. The biggest problem was calibrating it in a darkened room (or at least a very dim room). I purchased a ? Flanger from Craig's list that was rather cheap (had to replace a stompswitch and the rotary switch and it came back to life) so I have been comparing the clone to the original. There was one surprise though. The depth and rate pots are log and not linear taper on the original. I have linear taper there, so the settings are a little different on the clone.

There are a few errors in the schematic I have posted compared with the original. I will correct these and repost the schematics.

Are you able to make your own PCB's? If so, send me your email addy.

[culturejam]

Different LDRs have different characteristics when it comes to the "recovery period" of going from either light > dark or dark > light. And they are usually different. If you can get a data sheet, you can check the average response/recovery time for each state change. I've not had much luck finding data sheets for LDRs, but you can just about always find them for optocouplers.

I think for our purposes, shorter times are better, since we usually want the LED/LFO to dictate the rise/fall. The longer the response time, the more it jacks with the LFO reaction linearity. This is why sometimes it seems like a given modulation effect seems to "stay" longer in either the upper or lower swing of the wave form (ie - lopsided "swoosh").

[LaceSensor]

It is finally finished. I am working on the build docs now. I just worked out the calibration procedure last night. I was trying to refine the LDR's but what I have in there seems to be working pretty well.

I had Haberdasher make a revised main board that would allow you to solder the components on the PCB that were originally soldered on top of the components in front of Q9. The biggest problem was calibrating it in a darkened room (or at least a very dim room). I purchased a ? Flanger from Craig's list that was rather cheap (had to replace a stompswitch and the rotary switch and it came back to life) so I have been comparing the clone to the original. There was one surprise though. The depth and rate pots are log and not linear taper on the original. I have linear taper there, so the settings are a little different on the clone.

There are a few errors in the schematic I have posted compared with the original. I will correct these and repost the schematics.

Are you able to make your own PCB's? If so, send me your email addy.

Cool news, will be fun to see the build docs. I will likely build one myself.
As for etching a pcb I might be lazy this time and get someone to make me one, not sure yet.

will pm email again

Ian

[electricstorm]

Thanks CJ.

How does the "Light History" play into this? Or does it matter for what we do? I read on one of the supplier websites that LDRs have a "Light History", that is, they remember the level of light or darkness they are stored at. Didn't quite catch on to what that means for using them in circuits.

[gtr2]

Probably just means the response and recovery time of the LDR.

[midwayfair]

Probably just means the response and recovery time of the LDR.

Yes, that's what light history is.

Either Mark Hammer or RG noted that our eyes also have a light history: see something bright, and you get an afterimage when you close your eyes.

In practical terms, I've never been able to measure the light history. Most are going to be under 100ms max.

Oh -- and you could work out dark resistances above 20M algebraicly. Put a 10M in parallel, (10M + x)/4 = y; y = the reading on your multimeter, solve for x.

[electricstorm]

Cool Jon! Thanks. Didn't think about trying to solve it algebraicly.

My question had to do with the Lovetone flanger and Wobulator LDRs. I have one on hand at the moment and had checked them with the meter, but not sure if I was measuring them correctly or giving enough  time before checking the dark resistance. The ones on the main board measured 10M (dark) after a few seconds of turning off the light and then my DMM gave me an 'OL" for out of range. So I know they are over 20M dark. The LDR on the delay board went to 20M and "OL" within a few seconds. The Wobulator LDRs seem to be about 10M, but I need to spend more time with it.

Should I wait a certain amount of time before measuring for dark resistance? I've seen some datasheets say 10 seconds and others don't say how long. Just wanted to try to get as close to the original LDRs as possible. The ones I am using seem to be working fine though.

[midwayfair]

Should I wait a certain amount of time before measuring for dark resistance? I've seen some datasheets say 10 seconds and others don't say how long. Just wanted to try to get as close to the original LDRs as possible. The ones I am using seem to be working fine though.

well, yeah, the Vactec datasheet has their testing as "10sec (or longer)" or something like that. 10s should be more than enough time. I can't imagine anything having a light history that long; it would be completely impractical for any use.