Automatic Gain Control with your own LDR

An automatic gain control (AGC) with a DIY opto-resistor is presented in this video. AGCs basically are voltage controlled amplifiers where the control voltage is the amplitude of the very same amplified signal. While many designs exist for this kind of circuit, here I show one of the simplest solutions: an operational amplifier where the gain control is set by an opto-resistor, also know as light dependent resistor (LDR) or vactrol. Because it could be hard to source this kind of component I also show how to make one yourself with a simple LED a chunk of tube and a photo-resistor, all kept together with a drop of epoxy resin mixed with grey cement.
This circuit has a dynamic gain of 18dB and a ratio of 80% over the target output level. With some further adjustments it can be expanded to a higher ratio but at that point other circuits would better suit. The advantages of this circuit are its simplicity, very low distortion and low noise. The stray capacitance of the photo-resistor could limit the bandwidth but with the VT900 the circuit has proven to work pretty well with very little attenuation up to 500KHz.
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Website: accidentalscience.com

[@fabiovsroque]

In the first version it may be intersting to change the BC337 for an opamp led drive. The leds goes in the feedback loop of one non inverting opamp. With this configuration the leds turns on gradually from zero.

[@PEGuyMadison]

I just wanted to say thank you for your time and effort making these videos, I suspect you are an analog electrical engineer like myself (we are probably the same age). I wake up in the morning having my coffee and look forward to watching your videos, they really are fun and from the heart. I don't have any more to offer than my thanks, so thanks again. Cheers and I look forward to your videos.

[@AccidentalScience]

I'm glad to be appreciated, so my thanks go to you.

[@Electronzap]

That's a fun circuit :)

[@Enigma758]

Is there any reason not to do this with a non-inverting op amp configuration?

[@AccidentalScience]

No, you can put the LDR to shunt the NI input.

[@anandha12]

Awesome video👌. BF245 isn't available. So, please suggest an alternative.

[@pavelpavlov954]

2N5457 should be fine.

[@anandha12]

​@@pavelpavlov954thanks 👍

[@debmalya6144]

Compressor