Simple analogue pantry transmitter ?

[af024]

Seeing these postings, it reminded me that I had a Mykit Series 10-in-1 by gakken many moons ago and that this had a very simple MW AM transmitter as one of its 10 projects.
Would you credit it, but having scrambled up the loft for the Christmas decorations, there was the very kit before my eyes!

Glancing at the ‘as-found’ spring-to-spring wire jumpering, it looks like it was last used as a crystal MW radio, but one of my favourite projects at the time was that of the so-called, ‘HOME BROADCASTING STATION’.

Regrettably you can see that I must have lost some of the wires that came with the kit (I think they were, from memory, red, blue and yellow in increasing length) as you can see a whole mish-mash of wiring on the board. Also, I seem to remember that the one and only transistor bit the dust and a different type was fitted at some point. I can’t find any references to what the original one was other than 2SA Germanium (which covers a multitude of choices of course).

Anyhow, I’ve taken a photo of the published circuit. The variable capacitor doesn’t look like it’s anything special. I don’t know about the aerial coil though (although to me it looks like the kind of thing that was in those cheap transistor MW radios in the 70s).

It simply struck me how simple the circuit was and I also seem to remember that running in on that single 1.5V cell, ok with a huge length of wire for an aerial and a decent earth (radiators!), it actually, and rather frighteningly, got at least ¼ mile of so – no seriously!!
I have no idea how nasty any harmonics might have been, but to a child, it just didn’t matter a jot.

I have attached some photos which may be of interest. If anyone has any ideas as to what the 2SA would have been, then I’d love to hear from you.

Regards,

Andy

[TonyDuell]

I don't own that particular kit, so I can't be sure what the transistor was, but a Google search found this page :

https://www.petervis.com/electronics...ken-mykit.html

which states it's a 2SA52

[af024]

Well what do you know! Thank you Tony.

[Graham G3ZVT]

How about using an MC1374 AM balanced modulator chip, with a free-running oscillator?

http://www.earlytelevision.org/405_modulator.html

Such an arrangement works very well indeed for me at 50MHz, with no significant drift or FM, so with suitable values in the tank circuit it should work superbly at 1MHz.

You would just need to add a PA stage to make it a TX.

[Boater Sam]

It would seem to be a 2SA52 pnp ge transistor of low hfe of 25, 0.06w, 18v collector volts. So just about any ge transistor.
A better shot of the circuit needed I think, or is it me?

[MrBungle]

Interesting kit. I had a Radio Shack kit that had a similar transmitter design in it many moons ago. Also looks a similar sort of arrangement to the 1 transistor FM bugs I made when I was a lot younger.

[af024]

Your right, let me see now...is this any better.

For the avoidance of doubt, the capacitor located between springs 14 and 15 is 100pF.

Regards,

Andy
PS Thank you for the extra info on the transistor. Nothing special then.

[Boater Sam]

That is simple, unbelievable that it works!

[af024]

Sure is!

[Phil G4SPZ]

Prompted by this interesting thread - not that I need any more pantry transmitters, I already have four - I dug out the "Science Fair 60-in-1 Electronic Project Lab" which I bought for my son when he was about eight, in a vain and ultimately unsuccessful attempt to interest him in electronics. Fast forward 25 years, I repaired it a while ago, and my eight-year-old grandson had a couple of days' fun with it until he also tired of it.

Sure enough, it contains a circuit for a 'radio station' which is very similar to Andy's circuit in post #47. Ten minutes fiddling later and I am listening to some 1930s music from it, in what to me is amazing quality. Yes, there's clearly some FM noticeable whilst tuning across the signal, but listening to the output on a decent vintage ITT-KB Golf Preset, you'd never know. I've just tried feeding in some audio from an FM receiver and the results are just as good. The circuit accepts a wide range of audio input levels without distortion becoming noticeable. Power output is definitely in the 'safe' level and the range is feet rather than miles!

The variable capacitor is 265pF and the transistor (according to the parts list) is a 2SC828A silicon NPN epitaxial planar transistor, although the ones in the kit are marked "CR28A". The earphone was originally a crystal type (which had failed) and I found I get best audio quality by substituting a 33k resistor, across which the audio is fed in straight from the headphone socket of the desired portable source. A short length of wire from the top of the ferrite rod coil seems to work best as a radiating element without de-tuning the oscillator too much.

Now all I've got to do is work out the details of the coils on the ferrite rod so I can replicate this little circuit and build it into a tiny box.

Phil

[MrBungle]

Excellent idea!

I had one of those sets in the 1980s. Never crossed my mind to use it for anything like prototyping but it certainly has some convenient parts.

[Boater Sam]

Is there a prize for the most minimal pantry transmitter? There can't be one with much less than these last 2.
Wiring is not up to your usual standard Phil, points lost there. Nice set, not seen one of these before. Is that a motorised volume or tuning control top right?

[raditechman]

Now all I've got to do is work out the details of the coils on the ferrite rod....
Phil

If you do perhaps you could be kind enough submit the coil details as it would be fun to try the little circuit.

John

[Phil G4SPZ]

Originally Posted by Boater Sam

Wiring is not up to your usual standard Phil, points lost there. Nice set, not seen one of these before. Is that a motorised volume or tuning control top right?

Thanks Sam. Sorry about the wiring but clearly, you haven't seen all my projects! The little device you mention is actually a small 3-volt electric motor, which features in some of the experiments. There was also a propeller, but that's long since flown away

Doing a few quick sums shows that, with the capacitor half-meshed and for an output frequency of around 1MHz (300m) the total inductance needs to be around 168uH. I'll try to determine the position of the tapping by measuring the resistances of the windings. The ferrite rod is actually wound with Litz wire, so it does seem to be a reasonably good-quality kit. ISTR it came from Tandy's.

John, of course I'll happily post constructional details when I can.

[Phil G4SPZ]

Okay, here are a few constructional details for anyone wishing to replicate this little circuit.

The only component value not stated on the circuit diagram is the ferrite rod antenna. I wasn't 100% convinced by my estimate of inductance in post #54, so I measured the total inductance of the existing ferrite rod in the kit at 366uH, tapped half-way along. Using this helpful on-line calculator, and entering the details of the piece of ferrite rod I found in the junk box plus the 30SWG enamelled copper wire I had to hand, gave me a total winding of 66 turns, centre-tapped. I had to guess at the relative permeability of the rod, so I just used the default value in the calculator of 600.

The measured inductance of the completed prototype is 276uH. The difference between design and actual is no doubt due to the permeability of the rod being slightly different from the assumed value, and the fact that I used a thin paper sleeve between the rod and the windings. Undaunted, I lashed it into circuit in place of the original and was delighted to find that, with a slight tweak of the tuning capacitor, it works perfectly.

This little circuit draws a mere 0.15mA at 1.5 volts. An alkaline AA cell would therefore last for at least two years, so is it even worth fitting a switch? With a red LED as a power-on indicator, the battery drain rises to just 2.2mA which I suppose warrants an on-off switch!

Phil

[af024]

Not sure whether this makes sense or not, but here are the results of my testing (within the capability of my test kit)-

Variable capacitor ranges from circa 10pF to 270pF
The aerial (following the tappings 3 through to 6):-

3-4: 99 uH, 1.7 ohm
4-5: 112 uH, 1.7 ohm
5-6: 1.5 uH, 0.2 ohm

Weirdly though, if I go between 3-6 I get:-
3-6: 396 uH, 3.8 ohm

Ok, I can probably accept the 3.8 ohm (as it's close to (1.7 +1.7 +0.2) = 3.6 ohm, and the springs are a bit tarnished giving a bit of variation,
but can anyone explain the 396 uH?

Regards,

Andy

[ms660]

Mutual inductance.

Lawrence.

[julie_m]

Inductance is proportional to the number of turns squared; so if you double the number of turns, you will quadruple the inductance.

[ms660]

A rough calc that might or might not be right, coupling coefficient k = 0.9...L1 = 99uH, L2 = 112uH, L3 = 1.5uH, ignoring L3 for the moment....M = k (square root of L1*L2)....M = 94uH.

Lt = L1 + L2 + 2M = 399uH, throwing the 1.5uH back in = 400.5uH.

So 396uH total...coils have a coupling coefficient of just under 0.9

k = Coupling coefficient.
M = Mutual Inductance.
t = Total inductance.

Lawrence.

[julie_m]

That sounds pretty reasonable. You will never quite get perfect coupling on an open-ended core, even with the winding sections on top of one another.