Francois Dion - Rocket Science - VLF Receivers

The electronic duct tape

Initially sold as the Archer Mini Amplifier, and currently available from Radio-Shack as the RadioShack Mini Amplifier - Speaker, catalog # 2771008 (here, for a european source) this can really be called the electronic duct tape. I've seen this used in all kinds of experiments, fields and purposes. Of course, it wouldn't have been half as popular if it had cost a lot more. For years, Radio-Shack sold it for $10, then $11 and currently is $12.99. It is made of 1 LM386 type opamp, 7 electrolytic capacitors, 3 ceramic capacitors, 5 resistors, 1 diode, 1 potentiometer, 2 audio jacks, 1 power supply jack, 1 speaker, 1 9v battery connector, on a thru-hole circuit board, all in a plastic enclosure. It would be hard to build one from scratch for less than that, even without counting your time...

Here is the official Radio Shack description:

Perfect for dozens of projects. Has built-in speaker.

Features:
• Great for projects, as a test amplifier or signal tracer
• High-gain IC circuit
• With 1/8" earphone and input jacks, volume control
• Dimensions: 3-3/8" high
• Requires 9V battery or 273-1767 AC adapter (both optional)

I said it's the duct tape of electronics, didn't I? If you look at the range of applications thru a quick search on the Internet, and you understand why:

• Electronic Caller: http://www.nodakoutdoors.com/forums/viewtopic.php?t=11469 (sounds)
• Star wars helmet PA: http://www.metrocast.net/~boucherma/Altmann.htm
• Jove radio group listen: http://radiojove.gsfc.nasa.gov/help/Detailed_set_up_configurations.htm
• For text to speech application for the visually impaired: http://personalpages.tds.net/~ti51/new34.htm
• For treasure hunting with a metal detector: http://www.treasurenet.com/forum/prospecting/archives/19990705/messages/322.shtml
• For audible GPS feedback: http://www.senderogroup.com/mikeproducts.htm
• As a spectrum analyser(!): http://www.autosound2000.com/heresHow/june2003-pt1.htm
• For the receiver of a laser communicator: http://www.scitoys.com/scitoys/scitoys/light/light.html (and here: http://www.engr.sjsu.edu/rkwok/EE164/EE164%20Project%20Report.pdf )
• For portable scanners: http://www.engr.sjsu.edu/rkwok/EE164/EE164%20Project%20Report.pdf
• To amplify a crystal radio: http://www.midnightscience.com/_web/00000d7a.htm
• For ornithology: http://www.birds.cornell.edu/private/MacaulayLibrary/contribute/equipSpeakers.html
• As a headphone amp for an electric guitar: http://my.athenet.net/~daydream/guitoys/axman01.htm
• As an RC Frequency Monitor (with some internal mods): http://www.rc-cam.com/monitor.htm (see also: http://www.rcgroups.com/forums/showthread.php?t=289980)
• As part of a VLF receiver: http://home.pon.net/785/vlfkit.htm
• As an addition (speaker) to another VLF receiver: http://www.auroralchorus.com/bbb4b.htm
• Or even for parts: http://www.williamson-labs.com/480_isd.htm#speaker
• I've personally used this for my Water Radio project, as a speakerphone with a Radio Shack magnetic pickup, as a monitor and as you'll see below, as a VLF receiver.
• Altough not specifically identified, I think Nyle is using one on his Peculiar sound from aluminium experiment
• If you know of other uses not listed here, let me know!

When it comes to very low voltage (1mV sensitivity on the Radio Shack mini amplifier) signals found in chemical reactions, or to convert light or ver low frequency RF to audio, the Radio Shack mini speaker is not the only device available out there. A slightly more sophisticated unit is the one pictured here. It is a Tentec/T Kit, # 1252 Hi-sensitivity Audio Amplifier With Experiments. Altough nicer, its $49.00 price (kit form, unassembled) cannot qualify this as an electronic duct tape! Also, I've not built this kit myself. I have built other Tentec kits such as the 1056 receiver.

The Tentec web site mentions:

"A wonderful kit for experimenters of all ages! Build a rugged, versatile, portable utility audio amplifier. Then begin to explore the facinating world of the unique sounds made by all kinds of energy - from lightning whistlers on the other side of the world to the stress noise of a bending paperclip. Listen to radar, to light waves, to your computer. Listen to nearby radio signals on any frequency, any mode. In addition to the kit assembly manual you get a fascinating project booklet based on experiments by USAF Engineer Calvin R. Graf as your guide to adventures with the sounds of energy.".

I tought I'd mention this on Calvin R. Graf as far as these types of experiments: he published the first edition of his book on sound experiments in 1978, ISBN 067221525X, "Listen to radio energy, light, and sound". His followup was published in 1985 as "Exploring Light, Radio and Sound Energy, With Projects", ISBN: 0830617582. I'm not sure if it is either of these that Tentec includes (both are out of print as far as I know).

The $12.99 VLF receiver

Back in september, I posted to the Yahoo VLF group (see also my VLF page for more info) the following:

"Alright, to start with, it doesn't have a notch filter (altough its response is 100Hz to 10KHz...) and all that jazz, but you cant beat the price, and you can mod it fairly easily.

What is it? The good old, reliable, RadioShack mini amplifier speaker: http://www.radioshack.com/images/ProductCatalog/ProductImage/277/277-1008.jpg

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F006%5F011%5F002%5F000&product%5Fid=277%2D1008&hp=search

This works well for all kinds of things, it is more than likely already in your toolbox (if not go and get one now). It has a 1mV input sensitivity which works great with a long wire antenna:

• just a long wire soldered to a minijack
  or
• outdoor antenna kit: 278-758
  with adapter: 278-257
  

and you are good to go.

With a loop, well, a 3ft+, about 40 turns and it is usable. With my square loop with only 18 turns, it's a bit on the quiet side, altough it is very nice with mono headphones, or using this adapter if stereo: http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=274-328

But, it would work better with a little more gain. This is where the fun begins.

First mod:

The opamp is a 386 type. This means that by default it has a gain of 20. For a gain of 200 (max) you put a 10uF capacitor between pin 1 and 8. So how is it hooked up? Pin 7 to ground thru a 1 uF capacitor (bypass) but mostly pin 8 to pin 1 thru a 22uF capacitor (C5) and a resistor R5. Replace R5 by a jumper and you've doubled the gain, that's +3dB right there!

Second mod:

Cover the back inside with foil and tape it over so there are no shorts. The front of the circuit is already partly shielded by the speaker itself and this is fairly stable overall."

60 Hz Blues

I tried at first to add a high pass filter before C3198, but that didn't work out too good. I'll get back to that another time, but I only had an hour or so. Instead, looking at the circuit, pin 5 of the LM386 goes thru a 100uF capacitor, thru the headphone jack, to the 16 ohm speaker. Obviously, this capacitor was put there to help the poor speaker from overloading, effectively creating a 100Hz, 1st order passive Butterworth high pass filter. This is only 6dB per octave attenuation, but that is plenty fine, if we start at 1KHz (particularly with headphones).

First, in choosing the right value to replace the 100uF capacitor, we must take into consideration that the headphones have a higher impedance than the speaker. The speaker is 16 ohm, my headphones are 32 ohms. We must then choose the frequency we want for the headphones and will learn to live with the high pass at twice the frequency (the point at which the frequency response will be 3dB down) on the speaker.

Second, we must choose a commercially available capacitor value. Typical are 2.2, 4.7, 10, 22uF etc.

C = .159 / ( R x f ) * 1000000 (in uF)

A quick calculation shows that for the 32 ohm target for the headphones, and 2000Hz, we get 2.48 uF. There is no such value available so we'd have to go with 2.2uF. 4.7uF would give us 1000Hz which might be acceptable for your application (and 2000Hz cutoff frequency thru the speaker).

So which one is it? It is capacitor C9 on the board, next to the external speaker jack.

This modification works really well for me. Let me know how it works for you at fdion@cimastudios.com.