Is there such thing as an inverse LDR ?

[isildur100]

Hi,

I'm currently experimenting with a compressor circuit using a CLM6000 photo coupler. As with any LDR I know, the resistance decreases when the led intensity increases. But is there a device that does the inverse of that? The resistance should increase as the light intensity increases.

If not, how can I arrange a circuit that would do that?

John

[mth5044]

What if you inverted the way the LED lite up? For example, when you introduce a sound, instead of the LED increasing in light (decreasing the resistance), have the LED decrease in light, increasing resistance. Is that what you are talking about?

[isildur100]

Yes exactly, but I don't know how to do that...

When the guitar volume increases, I need to make the LED to decrease in light. How would I proceed?

Thanks

[GibsonGM]

Opamp, use the inverting input, the output will be the opposite of input (high in = low out)....a transistor will do this, too, depending on if you take the output from the collector or emitter.

Now, to do this solely based on VOLUME, you'd probably have to amplify the guitar signal, rectify it, and feed it to an opamp as a DC level rather than AC.   So, 1 stage to amplify enough to get thru a diode (make it variable over 10x range, maybe...), then a diode/cap to smooth a little if necessary, input to another stage which outputs to the LED.   Easy circuit with a dual opamp like a 4558 would work. 

At least, that's how I see it!  Someone may have a more direct method, I am going for a 'nice smooth transition' in light.

[Slade]

You should look at the Neutron (Mu-tron III workalike) schematic.

[isildur100]

Ok thanks guys! Will look into your suggestions!

John

[JDoyle]

Invert the control signal.

[R.G.]

I'm really fond of differential amplifiers. If you set up a differential amplifier, the emitter bias sets the maximum current for either of the differential transistors. With an LED in one collector, you can steer 100% of the current into or out of the LED by moving one base above or below the other base by +/-25mV, which is easy enough with resistor dividers from any control signal. Which way the control signal goes is immaterial. If it's going the wrong way, put the LED in the other collector.

This also lets you use two LEDs in opposite-tandem. One goes up, the other goes down, and it's linear (as linear as the light output of an LED is, anyway) over almost the entire range.

Emitter bias is magnitude of peak current, and independent of difference, which is purely done with the diffamp bases.

[isildur100]

Thanks RG,

I think this is exactly what I need. If I understand,

The guitar signal would go to the base of one of the two transistors and a current (or voltage) would be applied to the base of the other transistor. When no sound comes from the guitar, the second transistor lights up the LED. When the guitar signal exceeds by at least 25ma the amount of current on the base of the second transistor, the LED will not be lit or start fading.

I guess that would do the trick in a very simple manner! Now, I only need to tweak the component values to make it work.... 

John

[Top Top]

I have heard a suggestion that if you have an LFO that feeds a PNP transistor to drive an LED, that you can use an NPN in it's place and it will then act inversely.

I don't know if that will help in your case, and I don't know if anything else would need to be changed, but it is something someone suggested to me for sweeping two LDRs to control an effect that uses all three legs of a pot.