Battery-less audio amplifier

[JHGibson]

Being a hard-core crystal set builder, I was delighted to read about a battery-less audio amplifier. It used a second crystal set peaked on a local station to charge up a 0.3 FARAD cap. It took most of the day to charge it up to some 1.5 volts , but that was sufficient to run a micropower op amp drawing a microamp for 3 hours.
For details see article 25 on
www.bentongue.com/xtalset/xtalset.html
John.

[Herald1360]

There's a certain elegance about the idea, but in practical terms, unless you want to listen in the dark, a simple solar cell power source would be more effective I reckon. Probably cheaper, too, unless you can use all junk box components for your PSU receiver.

[Coastwatch]

The one I built in the 60's, a PW design I think, had the power supplied from a LW Xtal set with 4 diodes (can't remember which [OA91]) as a bridge rectifier.

Now does anyone remember the WWII spy transmitter that used no power supply. I will start a thread on that one soon.

[kalee20]

Both these approaches have this in common - they use EM energy from another source to provide power. Only Herald1360's approach uses energy from somewhat further up in the frequency spectrum.

Of course, to keep on-topic, you could only use one solar cell, else you'd have a (solar) battery.

Micropower operation is fascinating, however I'd guess that the micropower op-amp could not drive more than a crystal earpiece without significantly increasing its power demand.

[ajs_derby]

A solar cell is less likely to get you into trouble with anyone, as well! (Someone paid for the energy that went up that transmitter mast, and they might well take a dim view of anyone using more than their fair share of it. It would hardly be the silliest thing anyone's ever been prosecuted for.)

At least, until daylight is privatised .....

[G8HQP Dave]

The modern name for this is energy harvesting, and people are looking at it as a way of powering remote sensors etc. Whether the broadcasters, mobile phone firms etc. are happy about this remains to be seen.

The danger I see is that as well as extracting energy which was meant to end up in the front-end of a receiver (so possibly reducing range for legitimate users) it could also radiate interference (rusty bolt effect on a big scale if everyone starts doing it).

[Herald1360]

Originally Posted by ajs_derby

It would hardly be the silliest thing anyone's ever been prosecuted for.....

ISTR "stealing Post Office electricity" was the only charge that would stick in a '70s phone hacking trial.

Back with harvesting, I suppose if you can get 500mV into 50R with a decent antenna, that's 5mW which could be quite useful. Some cleverness with fets could step it up to a useable voltage too.

[G8HQP Dave]

I seem to recall that "dishonest use of electricity" was threatened against people who would signal to the wife that they had just arrived back at the railway station, by doing two rings from the call box.

[KeithsTV]

When I lived in South Birmingham I was about 8 miles from the Droitwich transmitter and with 100ft of wire as an aerial and a decent earth I could get 1v dc, as indicated on my multimeter, from a crystal set tuned to 200kHz.

Keith

[davegsm82]

I've tried to do this before, it never worked particularly well.

In the early 90's (possibly late 80's?) I made a crystal radio from some old cheap chinese AM transistor radio, I put a battery snap (pp3) on the earth and connected this to the radiator. I was astounded to find it didn't need an aerial! The ferrite rod inside was enough. Used to listen to Radio1 on that late into the night, until I got bored or fell asleep and rolled my ear over onto one of those horrible crystal earpieces!

Interestingly, now that I have my own home i've been experimenting more, when I moved in we removed the old gas fire but left the aluminium flue in place, this is insulated from everything by brickwork. Measured with a fluke 175 there is about 500mV DC with respect to earth on it at all times, even after breifly shorting it out. Works quite well as a CB aerial too!

Have a look here, from what I can tell, these crazy russians aren't playing with a live transmitter, they're playing with a disused antenna not far from a transmitter, just look at that spark!...

http://englishrussia.com/index.php/2...-plasma-radio/

Dave.

[JHGibson]

Getting back to the original post, reading Ben Tongue's article 25, the procedure seems that you leave the crystal set tuned to a strong local station when the receiver is "off" and the 0.3 Farad cap will charge up. When the crystal set is "Turned on", the now fully charged 0.3 F cap can run the micropower opamp for some 5 hours and that 20 db of gain really helps. 5 hours of listening time is plenty! No need for additional parts like photo voltaic cells.
John.

[Andrew2]

Back in the 60's there was a company that advertised all manner of odd projects in the back of PE and PW. My mate Pete bought a couple - one was a micro radio powered by a voltage-doubling crystal set and the other was a radio which got its ergs from rotting bread. They worked...just.

[Biggles]

Would it not be better to build two crystal sets (or more) and connect their outputs in series to make a very loud crystal set?
Alan.

[dseymo1]

Would it not be better to build two crystal sets (or more) and connect their outputs in series to make a very loud crystal set?

Trouble is, most crystal sets produce their output WRT earth, so they could only be connected out of phase, producing a null o/p

[murphyv310]

How about synchronous detection using schottky diodes?

[JHGibson]

A micro power RF stage powered from the charged 0.3 Farad cap would be a useful addition to the crystal set. Stronger audio from weak stations and faster charging of the 0.3 F cap when the set is "off". Sounds like a perpetual motion machine!
John.

[G8HQP Dave]

You could connect two crystal sets in series, provided you reversed the diode in one of them. You would need two antennas, and a minor rethink of the arrangements for impedance matching of the output.

If the antennas were really large, so extracting a serious proportion of the available energy, then you might do better to arrange them as a beam instead. Otherwise they might simply share the energy between them.