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.

Tagged , , ,

3 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. […]


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

Amateur radio with VK1DA

Amateur Radio, Computing and other activities of Andrew VK1DA/VK2UH

G4YDM Ham-Radio and SWL news

Radio craft, homebrew, QRP/SOTA, AM


Radio craft, homebrew, QRP/SOTA, AM


Amateur Radio Homebrew HF Station

The Daily Squalor

The Church of The Athletes Foot

VK1DA on the net

Amateur radio experiences with VK1DA

Tj Lab



Amateur Radio and unrelated technologies

Paul Gacek


Radio Engineering Projects powered by DK7IH (Peter)

Projects for the radio draftsman. Hardware, software, electronics and test equipment for the radio designer

VK3KR Workshop

Homebrew electronics and a bit about a Tandoor oven

%d bloggers like this: