ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

previous: Ed Rembold As we all know, the Vintage LED/L... -- 2/15/2000 5:41 PM

Re: opinions of optoisolators

2/16/2000 6:18 PM
Mark Hammer	Re: opinions of optoisolators Photocells have timing characteristics, and most commercial photocells (and LED/LDR combos) will have ratings in terms of the rise and fall time (where rise is response to light onset, and fall is recovery in light-absent condition). The "smoothness" will be a function of what you want to use it for. For example, if an LDR or optoisolator is being used as part of an envelope detector/follower circuit, its "sluggishness" may be a desirable feature with respect to smoothing out the envelope votage. In effect, it can work like a lowpass filter on the envelope. On the other hand, if it is being used in a gain control device, like the Anderton compressor, then it needs to have a fast rise time to handle transients. You can always stretch out the lag/fallback time by using a larger value cap in your envelope circuit, but you can't make it go faster than it can go. If the LDR is being used as part of a phaser, the rate of change required is negligible, unless you want to crank up the sweep LFO into the audio range for ring-modulator type effects. Normally, though, asking an LDR to vary at 10hz is well within the capabilities of most LDR's and shouldn't show any variation across models at that rate. Finally, bear in mind that the "smoothness" is also a function of the light source driving the LDR. Diodes respond much faster than incandescent filaments. No doubt this is a small part of what yields the kind of sweep produced by a Univibe, over and above the LFO and LDR characteristics. To sum up: 1) You can make any light-dependent device behave more "sluggishly" with extra componentry, although one always appreciates the reduction in parts count. 2) You can't make it be more responsive than it can be. 3) Responsiveness is an advantage or disadvantage, depending upon application.

2/16/2000 6:18 PM

Mark Hammer

Re: opinions of optoisolators

Photocells have timing characteristics, and most commercial photocells (and LED/LDR combos) will have ratings in terms of the rise and fall time (where rise is response to light onset, and fall is recovery in light-absent condition).

The "smoothness" will be a function of what you want to use it for. For example, if an LDR or optoisolator is being used as part of an envelope detector/follower circuit, its "sluggishness" may be a desirable feature with respect to smoothing out the envelope votage. In effect, it can work like a lowpass filter on the envelope.

On the other hand, if it is being used in a gain control device, like the Anderton compressor, then it needs to have a fast rise time to handle transients. You can always stretch out the lag/fallback time by using a larger value cap in your envelope circuit, but you can't make it go faster than it can go.

If the LDR is being used as part of a phaser, the rate of change required is negligible, unless you want to crank up the sweep LFO into the audio range for ring-modulator type effects. Normally, though, asking an LDR to vary at 10hz is well within the capabilities of most LDR's and shouldn't show any variation across models at that rate.

Finally, bear in mind that the "smoothness" is also a function of the light source driving the LDR. Diodes respond much faster than incandescent filaments. No doubt this is a small part of what yields the kind of sweep produced by a Univibe, over and above the LFO and LDR characteristics.

To sum up:
1) You can make any light-dependent device behave more "sluggishly" with extra componentry, although one always appreciates the reduction in parts count.
2) You can't make it be more responsive than it can be.
3) Responsiveness is an advantage or disadvantage, depending upon application.

Replies:

moocow What about linearity with respect t... -- 2/16/2000 9:13 PM