VK3YE’s ‘non-invasive’ LED-LDR AGC add-on

When I saw the AGC circuit by Peter Parker VK3YE, I realised it solved a problem common to many simple homebrew receivers.  You cannot automatically control the gain of a receiver with fixed gain stages. Most simple receivers do not have IF stages, let alone variable gain IFs. And the LM386 has no gain control. So it’s easy enough to sample the audio at the volume potentiometer, amplify it, rectify it for a source of AGC. But then you need a voltage controlled gain stage ( a Voltage Controlled Amplifier  or VCA).  If your receiver doesn’t have one, AGC won’t work, and you with have to either add a VCA in the receiver somewhere or replace one of the fixed gain stages with a VCA.  Until now.

VK3YE’s clever circuit uses a conventional audio AGC circuit which drives a LED that is optically coupled to a Light Dependent Resistor  (LDR). As the signal rises the LED emits more light and the LDR’s resistance drops. The LDR is connected in parallel with the volume control.  In effect, an automatic volume control. As Drew VK3XU once wrote, ‘neat, not gaudy’. A meter connected in series with the LED via a current limiting resistor becomes an effective S-meter.

Circuit function depends on using an LDR with an appropriate resistance range.  My junk box LDR exhibited 40k to 400k ohms.  Too high, it made no difference when paralleled with the volume potentiometer.  Jaycar offered an alternative with resistance of a few k to 40 k ohms. Spot on.

Thanks to Peter VK3YE for the circuit and videos.

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5 thoughts on “VK3YE’s ‘non-invasive’ LED-LDR AGC add-on

  1. […] debugging and testing required to deploy my Arduino Nano keyer and SWR/power meter, and the LED LDR AGC and S-meter modules into my homebrew 40 meter SSB rig (MST Mk1 from OzQrp).  Before reassembly I sprayed the front and […]


  2. […] VK4PLN admits he has tried just two AGC approaches on BITx rigs: the VK3YE AGC (which is shall we say very illuminating) and the ADAFRUIT TPA2016 I2C […]

    Liked by 1 person

  3. […] Nick VK4PLN already has a collection of boards available (at or near cost) for the µBITx.   He is currently working on a new board design for the VK3YE AGC. […]


  4. indrasung says:

    Hello Sir, how much is the resistance of the LDR found that works good with you ?


    • Paul Taylor says:

      Hi, I’m not sure, this mod was done a while back. I tried two LDRs and one worked better than the other. I think you need a high enough resistance when dark to not reduce the audio signal level. So I suggest several k ohms (dark), and a few 100 ohms when lit. What have you measured? Let me know what you find.


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