78L05 fun - Two new project and info pages

Hello Halfgaar,

> Is there still interest in the bug project and you have to turn people away? 

No, there is absolutely no interest anymore. The last batch of 25 I had I gave away as free gizzits to elder children. It seems that the younger generation are not interested in such things. Probably the hassle of getting it, then assembling it. Why bother to learn how to solder when you can buy a pirate RaPi from China?

> Do you have an idea on how to calculate the power of a spark gap transmitter, and subsequently its range? .. I have no idea how to determine VSWR for it ...

This is not an easy one. But let us treat it a little seriously:

POWER
You can simply use a volmeter (diode, resistor and meter) to measure the output voltage, along with an AC current meter in series with the output to measure the antenna current. This will give you a rough idea of the output power.

VSWR
Let us look at a bit of basic antenna information. If you put a "click" into a resonant antenna 1-million times a second, then the +ve pole of that click will:
- travel 1/4-wave down one half of the dipole
- hit the end and bounce back 1/2-wave to the other end
- hit the other end, bounce back and by pass the feed point in time to join the next click.
- for every 1/1 wavelength journey, about 1% of the signal will be radiated with each bounce
- the new "clicks" add to the bouncing power with every new input cycle
- The instantaneous power stored in the antenna can be 50x to 100x the applied power.
- Now substitute "clicks for sinusoidal cycles that are one wavelength long.

This is somewhat over-simplified, but from this story you can see that the wave bouncing back and forth must be in the correct phase, or have a very small frequency shift. If the frequency shifts by just one % then after 25 bounces the new waves and the old waves will have a 90-degrees difference in phase. After 50 mounces they will be 180-degrees out of phase and cancel.

The spark-gap transmitter generates all frequencies under the sun, for example, 1MHz to 10MHz, but an antenna cut to 5MHz will only effectively radiate that portion from 4.985MHz to 5.015MHz = 30KHz bandwidth. This means that the efficient portion radiated is about 3%, the other 97% of your power will be returned (as a near-infinite) VSWR.

You can used very long wires of many wavelengths to improve the efficiency, increasing the number of resonant bits, but you would need a really long wire, certainly several km. You could alternatively use other forms of lower-gain antennas with matching components to get a better match, but you would not really increase the distance much.

Finally, the radiation outside the antenna centre-frequency would be more than sufficient to get you many miles; it is just the 5MHz peak (in out example) that will give you the longest range. But don't worry, everyone will hear you ;-)

> ... counterfeit components. Fact of life these days unfortunately ...

Absolutely. But there are counterfeit copies that actually work. One of the most reliable wrist watches was bought in Thailand. It is a genuine Seiko automatic movement, but has a "Brighthing" dial. It looks like a Breitling, is waterproof and reliable. I have it in the shower, and even when bathing in the sea. I expected a poor quality, but I took a chance. Since then I have bought loads of things from China, especially clocks and watches with in-built hidden video cameras. I expect some failures every time I buy, but on the whole I am satisfied.

But I have never, ever bought something that is about as much use as a kettle made of ice. I expect some form of functionality. But over the years of trading with China I think that I am overall satisfied. I just have to be a bit more careful. But this is just the same with your RJ-45 connectors - some things are good, some things are bad. You just pay your money and take your chances :-)

Very best regards from Harry

Is there still interest in the bug project and you have to turn people away? That would be a bummer. I always thought it was a great project, educational for kids for example. But I can imagine kids these days prefer to get started using a Raspberry Pi. Heck, most of them probably don't have an FM receiver in the house anymore . Come to think of it, the only one I have is in the car...

Do you have an idea on how to calculate the power of a spark gap transmitter, and subsequently its range? I never understood antenna gain, and I have no idea how to determine VSWR for it... 

And the counterfeit components. Fact of life these days unfortunately... I just always buy through authorized dealers. Expensive, but necessary. I do have a suspicious bunch of BC547 and BC557. I think I got them when I did order something from China once. Perhaps I should subject them to a Hfe and current gain test to see if they're real. Or perhaps just chuck 'em.

For some components buying straight out of China is useful. I'm still living off a box of 100 RJ45 plugs bought for less than $5. In retail, these things cost about a euro a piece...

Hello Halfgaar,
Welcome back    As you may have noticed I have been not so active for a while. But the bad luck with the regulators has to be taken as a lesson.
Interestingly I have also had private e-mail about these devices, and one friend wrote that he bought a batch of the same beesties, and they DID regulate. Not quite the correct voltage and they all without exception) permanently failed when more than a few mA was drawn.

I am having a bit of difficulty understanding why they manufacture such crap? To make such a good copy must be a lot of effort that could be better put to making something that actually works.

Ok on the Bug. the V4 was my first and I was always worried about eavesdropping, and microphone sensitivity. That's why it was a bit low.
The V5 was created when I learned that in most of Europe you can record private conversations without other peoples knowledge, providing you yourself take part in the conversation. So, the PCB design for a little hiver power and better RF stability and the V5 was born.

I should really remove the kits from my homepages as I lost the PCB making facilities in the move to Spain. These days other people dictate what I do with my time, but I have just bought the components for a new light-box, so I may be able to put together a few kits.

Ok on the spark-gap. I experimented with a 12VDC to 230VDC inverter, then used trigerable deviced to discharge the capacitor into another transformer. The spark makes a really loud crack, and it can jump a gap of about 3cm. Also about 5 discharges per second. Initially I was thinking about an electric fence for cats, but I think I grossly over-engineered it, but I suspect a 750,000 volts may be considered an illegal weapon!

I wonder if you will go QRV with the spark-gap transmitter? We could perhaps have a sked from Spain - and have blanket coverage of 3.5MHz to 7.4MHz?    It would be easy to find you on the band 

Very best regards and I hope that you can find the board useful. I do so want to promote the board a bit more now that I am allowed to have a bit more time.
Harry - SM0VPO

Hi Harry,

(by lack of a small talk section, this topic seems to be the go-to place)

I haven't been a site user ever since I was kid, really, but after the e-mail update today, I just wanted to drop a line. When I picked up my electronics interest again in 2006, I went the direction of DIY audio and it's been a very educational ride. However, I often think back nostalgically to the bug v5 and other transmitters like it I built (and even to the frustration of it changing frequency when I pointed at it...), and all the reading I did on your site. I actually just this week found a print-out of the bug v5 circuit board on a piece of paper, that I actually used to etch a board. Looking at the projects page, I see the kit is still for sale

Working with audio frequencies, thinking about RF circuits always boggles my mind. Basically any audio stage has no closed-loop gain at MHz frequencies, so I always wondered how one designs amplifiers for GSM/4G and such... I do kind of want to build some RF stuff again. When I do, I know which website to go to first.

On a side-note, I am very proud of the 10 kV spark-gap LC resonant circuit driven by a neon sign transformer I built, though. I never measured the inductance (and therefore its resonance frequency), but it must have been in the range of ADSL signals, because all internet went off-line when I turned it on  The standing wave nodes could be detected on the antenna with a 230V light bulb...

I am sorry to hear of your ill-fortune. My sincerest wishes go out to you to get well soon.

Hi Densil,
> Hi harry - sorry to hear you are sick. just want to say get well soon.
Thank you.

> knowing you i am sure you will find a use for those stuffed regulators
Not much chance now. They are basically non-linear resistors. They are not transistors in disguise, or anything I can identify.

Best regards Harry

Hi harry - sorry to hear you are sick. just want to say get well soon.

knowing you i am sure you will find a use for those stuffed regulators  
Br /D

Hello Steve,
> It's great to have you back, and with such innovation as the 7805 fun, and am looking forward to ...

I am sorry that I have neglected the forum for so long, but I am sure you know what has happened over the past couple of years. I have even been living in a hotel for a while.

I thought I was going to have a lot more fun with regulators. I bought a few when I was in Spain, from a high-street supplier, but then I ordered a couple of thousand from China. Those from Spain are "used" but those from China are all 100% defective. The only use I can find for them is to convert current into heat.

I wonder if there is a diode function between any of the pins? Perhaps I could make balanced modulators, or something. I have probably got a load of BC547s or something that have been re-printed.

> Perseverance is paying off. Impedance matching? Piece of cake.
That's what I found. It's all down to experience. The only thing I have against experience is that the more of it you have, so the older you have to be to get it.



Anyway Steve, it is very nice to hear from you and I hope that you will return to the forum from time to time. I need to get a little more interestin the site. I also need to modify the link on my Raspberry-Pi. For some reason the http://www.sm0vpo.com/board.htm does not seem to work, but http://213.114.137.49/board.htm does.



Very best regards from Harry - SM0VPO

Hello Harry,

It's great to have you back, and with such innovation as the 7805 fun, and am looking forward to your further experiments with these devices.

I've been following your site for years now, and am pleased to report that I am now pretty much fully obsessed with designing and building my own radio and general RF gear. After banging my head against it for so long now it's all starting to make sense. Perseverance is paying off. Impedance matching? Piece of cake.

Thanks for the inspiration.

Cheers,
Steve VK2AAV

Hi again Ruud and others?
Do you want a good laugh?

I bought a few 78L05 chips at 10 pieces for US$1.5 (total 50 pieces, plus postage) and they work fine. Loadsa experimenting and article writing :-)

So I then bought 1000 pieces from China for US$25 (plus postage) and they are all totally useless. The 5V out is actually 4.2V and the whole chip draws about 8mA. With a 50mA load the voltage falls to 3.5V. They also get hot.

I wonder if they are pre-burnt to save time on experiments?

The 10V regulators are exactly the same: 7.4V non-stabilised regulators. US$76 down the drain, but I have learned something ;-)

BR Harry

Hi all,
I have today put two new pages on the homepages at www.sm0vpo.com:

78L05 Voltage regulators in use and misuse - /reference/ /data-technique/
78L05 Push-Pull AF/RF Amplifier - /projects/ /AF-amps/

I must thank Ruud who so kindly agreed to review the  Push-Pull AF/RF Amplifier pages.

Have fun with the info. A bit unusual, but hopefully will provoke some feedback :-)

Very best regards from Harry