DIY VLF Receiver
This VLF (very low frequency) receiver is used to pick up natural occurring low frequency radio waves created by nature.
Some of the links below have sound files of VLF "signals". Don't be disappointed if you don't hear a
"whistler" (a several second tone decreasing in frequency) right away. Most of the recordings on the links
were received in remote locations after many hours of listening. You should definitely hear
pops and clicks no matter where you are.
You can view the schematic of my circuit in either the PDF format (preferred) or as a .GIF. Note that there is an error in the schematic. The correction is : R6 should go to the other end of C5, where it connects to the + input of the TL082. The links are
General circuit description:
C3 and C7 pass high frequencies to ground that were picked up by the antenna.
The lower frequencies go to pin 3 of U1A
and get amplified slightly (gain=1+(2.2K/1K). The signal is then passed on to U1B to filter out some
of the low frequencies (trying to get rid of some unwanted 60HZ signal). The output of U1B is sent
to a volume control R3 and then to the well known LM386 audio amplifier IC. That is the main circuit
and it is up to you to add a power LED, headphone jack, power switch, and other bells and whistles as
I do not know my RF very well, but looking at Stephen P. McGreevy's design (linked below) I was able to design my own
op-amp version of the VLF receiver. One of the most important features of Steve's design was a FET input for
high input impedance. That is why I chose to use a FET input op-amp (TL082CP). The op-amp is
available at Radio Shack. Since TL082CP package has two op-amps inside, I used the second section
as a high pass filter. Because this unit is a low frequency receiver, it is very good at picking
up 60HZ. That means that you can't be anywhere near a wire carrying 120V 60HZ ( or like).
The receiver is meant to be used far from civilization. You will get beautiful reception that way.
Unfortunately, most of us don't live in a area where we can get miles away from the nearest power wire.
For that reason, the high pass filter was added. It will let you listen at less than ideal locations
without getting too much hummm in your ear. You still will have to be outside in the middle of
yard or parking lot for the reception to be acceptable. The filter created by the second op-amp
and surrounding components creates a 12dB/octave 600Hz high pass filter. It lets you hear
plenty of pops and crackles, but keeps the 60HZ hum down. Since the filter has a fairly shallow roll off (slope)
you are still going to here 300 HZ just fine.
Looking at the picture to the right, you might be able to make out the red volume knob, LED (power on indicator), power on/off
switch, speaker on/off switch, and headphone jack. You can't miss my happy go lucky drilling that
serves as the speaker grill. I used a Radio Shack box (not sure of the part#) that is a bit larger
than other units on the web. I got a great clearance deal on a radio shack speaker that wouldn't fit
in the smaller boxes. The audio amplifier is the well known LM386 (also sold at Radio Shack). BT1
BT2 on the schematic represent two 9V batteries that are set up to provide + and - 9V to power the
op-amps. Eventually I got a small DC-DC converter that takes 9V from a single battery and gives you
an isolated 9V that can be used for the -9V. I probably wouldn't have used the DC-DC converter because they
are expensive, but I received mine for free. Two batteries works just fine.
The only glitch with the unit is when the volume knob is turned up for almost full volume. A oscillation occurs
that can be heard in the speaker. Since I still get plenty of volume and I don't use it more than a few
times a year, I haven't bothered to trace the oscillation down.
A VLF listeners guide by "Mr. VLF... Steve McGreevy
A good explanation of a whistler
Matt Tucker Escondido CA.