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Old 2nd Jul 2019, 8:41 pm   #1
mictester
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Default Another "Pantry" Rig for the IC-allergic

I cheated slightly - there's a three-terminal regulator chip in there, but that could be replaced by a series-pass transistor and a zener diode - at the cost of some stability - but the aim was largely achieved. The 100n ceramic caps either side of the regulator need to be as close to its pins as possible to prevent RF getting in there.

The circuit's nothing particularly special and is made up of a Colpitts oscillator (for stability), a buffer stage (with a bit of gain) to prevent the variable loading of the final "pulling" the oscillator frequency about, and a Class C final, which is operated at half the supply rail voltage under conditions of no modulation. The only slightly critical component is the NPO 220p capacitor in the oscillator. I had an ordinary orange-topped ceramic there at first, and the drift was horrible.

The modulator provides the nominal half rail voltage, and waggles the supply to the final up and down with the applied audio. It also provides a small measure of audio filtering.

It's worth making sure that the emitter of the modulator transistor never quite reaches zero, so that the final is never quite completely cut off - this reduces the (already small) harmonic output considerably.

I used 3N3904 transistors throughout only because I've got a few thousand of them (thanks Ebay!). My brother made one using BC546 transistors throughout, and it worked just as well as mine. Use whatever devices you have - within reason! Most of the component values aren't too critical - if you haven't got the exact values, "close enough" will probably work.

It's worth pre-processing the audio that's modulating the rig - compression and limiting will help this little device be heard above the noise floor! My prototype - thrown together in the traditional 2oz tobacco tin - worked well over nearly a kilometre when given a good earth and a 5m "bit of wire" dangling from a tree, powered from a "12V" gel-cell (actually about 14.4V fully charged) on a well chosen "quiet" frequency.

Have fun!
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Old 2nd Jul 2019, 8:45 pm   #2
MrBungle
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Default Re: Another "Pantry" Rig for the IC-allergic

Very nice. I've added it to my list of projects to build. Thanks for taking the time to write it up!
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Old 4th Jul 2019, 12:00 am   #3
JohnBG8JMB
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Default Re: Another "Pantry" Rig for the IC-allergic

Thanks...nice and straightforward. A question:
How much input for full modulation?
73
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Old 4th Jul 2019, 2:54 pm   #4
PsychMan
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Default Re: Another "Pantry" Rig for the IC-allergic

Are small axial leaded inductors ok for the coils?
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Old 4th Jul 2019, 4:11 pm   #5
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Default Re: Another "Pantry" Rig for the IC-allergic

Do you actually need the 7805 in there? Presumably it's there to improve frequency stability and minimise FM.
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Old 4th Jul 2019, 4:47 pm   #6
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Default Re: Another "Pantry" Rig for the IC-allergic

I'm going to build this over the weekend.

Quote:
Originally Posted by PsychMan View Post
Are small axial leaded inductors ok for the coils?
I wouldn't use one for the 47u inductor. They are terrible things for long term stability. I'd probably use a spectrum communications 45u0L IFT.

Quote:
Originally Posted by paulsherwin View Post
Do you actually need the 7805 in there? Presumably it's there to improve frequency stability and minimise FM.
You would need it for long term frequency stability on a colpitts.

You can side step some of these concerns by using a 1MHz CMOS oscillator module instead of a VFO but you can't tune them away from 1MHz.
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Old 4th Jul 2019, 11:37 pm   #7
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Default Re: Another "Pantry" Rig for the IC-allergic

Would those oscillator modules produce a nice sine wave or are they designed to interface with logic? I have a couple of crystals in the spares box around about the right frequency. I may try these as and when I get round to building the circuit. It's definitely on the list of projects to try. (along with all the others). I would imagine the inductors could be home wound if you have an inductance meter.
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Old 5th Jul 2019, 1:09 am   #8
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Default Re: Another "Pantry" Rig for the IC-allergic

A little further information:

The original prototype was built "Manhattan"-style on a piece of copper-clad PCB material. I used about 4" X 2", and put the completed board into an aluminium box 6" X 4" X 1" that I had lying around for years. The audio socket(s) were phono type, and the RF out was through a BNC. I used a zener diode on the oscillator supply at first - 560Ω from the 12V supply to the top of a 5V6 zener worked quite well, but the temperature coefficient of the zener diode combined with the thermal drift of the oscillator to make things worse! The 78L05 was a quick fix.

My oscillator coil was initially an axial choke - it oscillated OK, but drifted badly, so I dug out an Amidon FT37-61 ring core, and calculated that I needed 29 turns for 47H. I put mine a bit higher up the band, by winding 26 turns, and the 40p trimmer would allow me to tune from about 1500 - 1650 kHz. 32 turns put me down to roughly 650 - 800 kHz - as the American cousins would say "YMMV"!. I wound the coil with (roughly) 30swg enamelled copper wire that was actually from the secondary of a dud transformer. Once the turns were on, I held them - and the whole coil - in place with candle wax. The oscillator is surprisingly stable considering its simplicity.

It's fully modulated with a few hundred millivolts of audio - however, you can scale the gain of the modulator by changing the feedback resistor. A fairly crude (transistor and FET-based) audio limiter was scaled for the 0.775V nominal output, and this worked well with the little transmitter. I also did a little audio filtering before the modulator, rolling off the top end at 6.5 kHz, but introducing a (sort of) pre-emphasis HF lift from about 1 kHz upwards, which made the overall sound "brighter".

I was considering designing a PCB for this thing, but haven't found the time. Between my brother and myself, this circuit has been replicated about half-a-dozen times now. None of them are exactly this circuit - the component values aren't "set in stone" - I'd just use the first "approximately right" part that I found on my bench!

Improvements:

Is it worth spending much time on something as crude as this? The oscillator stability could be improved by careful selection of components or (better) by using a crystal. The output matching and filtering could be optimised, and a crude ATU could be constructed to load it into the "dangly bit of wire" more efficiently. You could polish this thing endlessly if you wanted to - the circuit shown is just the starting point!

Last edited by mictester; 5th Jul 2019 at 1:13 am. Reason: Typo
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Old 5th Jul 2019, 1:10 am   #9
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Default Re: Another "Pantry" Rig for the IC-allergic

Quote:
Originally Posted by Biggles View Post
Would those oscillator modules produce a nice sine wave or are they designed to interface with logic? I have a couple of crystals in the spares box around about the right frequency. I may try these as and when I get round to building the circuit. It's definitely on the list of projects to try. (along with all the others). I would imagine the inductors could be home wound if you have an inductance meter.
Alan.
Those little oscillator blocks are (generally) designed to clock logic circuits, so mostly give a good squarewave.

Last edited by mictester; 5th Jul 2019 at 1:11 am. Reason: Bad English!
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Old 5th Jul 2019, 9:25 am   #10
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Default Re: Another "Pantry" Rig for the IC-allergic

Thanks for the write up mictester. Much appreciated. It's an excellent little circuit.

On the oscillator blocks, the square wave doesn't really matter that much if it's a very low power output. The fundamental signal is what you are mostly interested in and there's plenty of that. I have actually built a small CW beacon that uses a 74AC14 as an osc and PA and a crystal modulated by a PIC12C509 driving a 2n7000 to disconnect the oscillator. The matching circuit between the 74AC14 output and the antenna is done with a naturally low-pass LC network and then followed by a 5 pole LC filter so what hits the antenna is pretty harmonic free. I was surprised at "how digital" a transmitter can be! It's also a lot more efficient than running class A amplifiers through each stage.
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Old 17th Jul 2019, 11:30 am   #11
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Default Re: Another "Pantry" Rig for the IC-allergic

I do like the look of this circuit. The modulation applied to the collector of the final output transistor whilst the oscillator fed via a buffer stage to the base of the final output transistor. I can see quite clearly that this would work well and will make for a very interesting project in the near future.
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Old 17th Jul 2019, 12:04 pm   #12
David G4EBT
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Default Re: Another "Pantry" Rig for the IC-allergic

Quote:
Originally Posted by mictester View Post
Those little oscillator blocks are (generally) designed to clock logic circuits, so mostly give a good squarewave.
The recent 'Turntable Strobe' project used a crystal oscillator module, which are great devices and cheap too, with only three connections - ground +ve and output. However, they're half-way to being an 'I.C.' in that they in a 14 pin IC DIL profile! The waveform was more or less square, but with some ringing, albeit of no detriment.

Good to see yet another inventive take on pantry transmitters, which make nice and useful homebrew projects for little outlay. I must resist the urge to design a PCB for it, or it will be yet another diversion from a growing list of half-finished projects. Meanwhile, my Mini Mod continues to give good service.

It would be nice to hear from forum members who build this circuit, to learn of their experiences on how it performs.
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Old 17th Jul 2019, 8:28 pm   #13
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Default Re: Another "Pantry" Rig for the IC-allergic

I'm halfway to completion with mine, and spare time at work permitting, it should sorted shortly. I will report back when it's done. Hopefully there will be a lull in faults or some bad weather with the day job and I can get on with it.
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Old 17th Jul 2019, 9:00 pm   #14
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Default Re: Another "Pantry" Rig for the IC-allergic

Similar. Haven't had a minute to build it yet!
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Old 18th Jul 2019, 7:42 am   #15
David G4EBT
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Default Re: Another "Pantry" Rig for the IC-allergic

Just one point, given that the supply is 12V, presumably the intention is to power it from a 'wall wart'.

It might be a bit more convenient if powered from a 9V PP3, or would it work down to 9V as it stands?

Or would some of the resistor values need to be reduced, (apart from the oscillator, served by the 78L05)?
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Old 18th Jul 2019, 10:40 am   #16
Ian - G4JQT
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Default Re: Another "Pantry" Rig for the IC-allergic

That's a nice circuit mictester. The VFO buffer should give isolation and the regulator IC should also add to frequency stability.

I'd be minded to make the LPF components variable so that optimum output is reached with whatever (short!) antenna is used.

In my experience even linear wall-wart PSUs need plenty of RF decoupling: X-cap across the mains, and 22nF-ish across the secondary, the diodes and output voltage terminals to minimise modulation hum when received on some nearby mains receivers.

The SMPU wall-warts are a nightmare on many levels and best avoided - not that anyone is suggesting them.

Thanks for sharing the design.

Ian
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Old 19th Jul 2019, 11:00 am   #17
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Default Re: Another "Pantry" Rig for the IC-allergic

I'm thinking of an experimental pantry-transmitter using one of the crystal-oscillator modules driving a VMOS FET, with something like a "LT700" transformer in the drain-lead to supply modulation.

Turn the modulation off and you've then got a nice 1MHz frequency-marker too!

One thing that's often puzzled me is why nobody seems to use a ferrite rod as the antenna on a pantry-transmitter. If the receiver you'll be using the TX with has a ferrite-rod I'd think that a similar rod in the TX would give better (magnetic) coupling than the traditional bit-of-dangly-wire.
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Old 19th Jul 2019, 11:11 pm   #18
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Default Re: Another "Pantry" Rig for the IC-allergic

To add that extra bit of authenticity you could modify the circuit to introduce a bit of fading, coupled with a light sensor to ensure this gets worse after dark. I think the ferrite rod antenna idea sounds good. It could be tuned to resonance at the desired frequency too. I may try this when I get mine working.
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Old 6th Sep 2019, 11:58 pm   #19
mictester
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Default Re: Another "Pantry" Rig for the IC-allergic

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One thing that's often puzzled me is why nobody seems to use a ferrite rod as the antenna on a pantry-transmitter.
The problem with the ferrite rod aerial is that they tend to saturate very easily, leading to distortion and spurious outputs.
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