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.
Watch the full series of electronics: • How to spot a fault in...
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Website: accidentalscience.com