MiniMod pantry transmitter
 

Just a quick mention of the MiniMod, a micro-power pantry transmitter that appeared in the Winter 2011 BVWS Bulletin, that I've just finished building.

This is an elegant little circuit that performs brilliantly, and the audio quality is excellent, so congratulations must go to the designer Ian Liston-Smith. I built mine on a piece of matrix board less than 2" square and housed it in a Maplins plastic case, the most expensive item in the parts list! The specified coils are still available from Spectrum Communications for 90p each. I had most of the passive components in stock or donated by friends, and I only had to buy a 2N3819 plus the coils. All told, I don't think the cost exceeded £10.

Apart from having to trace and rectify one wiring error :-] it worked first time. I added an LED indicator fed via a resistor and five diodes to give me an "on" warning light and battery state indicator, which goes out below about 5 volts. The output is good enough to cover one or two rooms with five feet of wire as an aerial.

A couple of photos are attached.

Phil

Re: MiniMod pantry transmitter
 

Hi Phil.

Many thanks for posting that - it's always good to get feedback, particularly if it's good!

The two identical coils should be available for the foreseeable future. Although in screened miniature IF-type cans, there's nothing special about them and they could be hand wound. (Did I include details in the article for anyone who wanted to wind their own? I don't have a copy available right now.)

I didn't include an 'ON' LED in the design as it would nearly double the current consumption. But a low-current flashing LED would be a good idea.

I really like the label you put on yours. Very professional! White-on-black and printed on what?

(The wiring error... yours or a misprint in my article? I made four spares from the published text/circuit so didn't think I missed anything...)

Thanks and regards,

Ian

Re: MiniMod pantry transmitter
 

Yes, Ian, you did, although a fixed-value home-wound coil would have needed a variable trimmer to give some frequency adjustment capability. The specified coils are ideal and very cheap.

Indeed it does, but mine draws 15mA and an alkaline PP3 should last around 40 hours, which is more than adequate for the occasional use this device is likely to receive at G4SPZ.

Nothing more than Microsoft Word, using white text with dark grey "Shading", and this function is available under "Paragraph". It's actually a very dark grey background, which looks less stark than black. I printed it on ordinary 80gsm paper, cut it out and stuck it to the case with Pritt Stick! No doubt a matt photo paper would be more durable, but if it ever gets tatty I can always print a replacement. The label also hides the slightly oval hole for the on-off switch, which I managed to drill slightly out of place...:-]

My error entirely, Ian. In planning the layout of the circuit on matrix board, and trying to minimise the space required, I inadvertently put the oscillator coil the wrong way up so the tapping was nearest the top, not the earthy end. This led to the FET being turned hard on with no oscillation. I swapped over the connections to the coil and away it went!

Thanks again for a really good design. It compares very favourably with a professionally manufactured AM-03 which I bought some years ago. The Mini-Mod has half the component count of the AM-03 but produces just as good quality and amount of output.

Kind regards,

Phil

Re: MiniMod pantry transmitter
 

Another positive recommendation from me too.

- Joe

Re: MiniMod pantry transmitter
 

Nice to see this Modulated Oscillator getting some positive feedback........

Re: MiniMod pantry transmitter
 

Hello,
I have a couple of questions:

1) I would like to build this transmitter. Would it be possible to obtain a copy of the article?

2) If the answer to 1) is yes, could someone please provide contact information for Spectrum Communications? When I Googled them from here in the USA I got links to all kinds of businesses, but not the correct one.

Thanks in advance,
Colin

Re: MiniMod pantry transmitter
 

If you PM your email address, I'll send it. I can't remember how to attach documents that are not images via the forum email or PM system. :-[

Ian

Re: MiniMod pantry transmitter
 

I wanted a flashing LED on my AM-03 Modulator as I thought it would serve as a good reminder to switch off when not in use.

Had to replace it with a standard one as I could hear the pulsing of the LED on the received audio.

SR

Re: MiniMod pantry transmitter
 

Spectrum are at www.spectrumcomms.co.uk

Re: MiniMod pantry transmitter
 

Ian - PM has been sent.

Phil - Thanks for the info.

Regards,
Colin

Re: MiniMod pantry transmitter
 

OK Colin. You need two coils, type 90u0L and they are a standard stock item - the shipping charges will probably exceed the price of the parts!

Re: MiniMod pantry transmitter
 

Hi I would like a copy if poss for the minimod. Any costs incurred just PM me. My email adress is REMOVED BY MODERATOR. Many thanks electronwilly (G7LER).

Re: MiniMod pantry transmitter
 

It is perhaps worth pointing out that the MiniMod was described fully in Ian's excellent article, which was published in Volume 36 Issue 4, the Winter 2011 edition of the BVWS Bulletin. The Bulletin is distributed free to BVWS members, but back numbers are available direct from the BVWS at reasonable cost.

Re: MiniMod pantry transmitter
 

Phil,
I didn't think to ask if back issues of the bulletin were available, as in my previous experiences with other clubs this service was not provided. BVWS is obviously a cut above the rest! Thank you for pointing it out, I will visit the website.

Regards,
Colin

Re: MiniMod pantry transmitter
 

Phil,
I visited the BVWS website, but couldn't see how to order a back issue of the bulletin. I did see the table of contents for the Winter 2011 issue and it seems to have a lot of interesting articles in addition to the MiniMod, so I'll be happy to purchase it. Would you please be so kind as to point me in the right direction.

Many thanks,
Colin

Re: MiniMod pantry transmitter
 

Well, it took a little longer than anticipated, but my MiniMod is finally up and running. What an excellent design. I have to thank Ian not only for designing this in the first place, but also for sending me a copy of the Bulletin article and the ExpressPCB layout, which helped me tremendously with the construction. Very much appreciated. I attached a couple of photos.

Cheers,
Colin

Re: MiniMod pantry transmitter
 

Nice work, Colin.

I have just put together a collection of parts to make another MiniMod, I just need an LM386. I will be using Veroboard and not going for such miniaturisation this time.

A really excellent design, thanks again Ian.

Re: MiniMod pantry transmitter
 

Very nice indeed and as legal as it can be.

Re: MiniMod pantry transmitter
 

Hi Phil, if you are using veroboard beware of the track capacitance as it may throw you tuning well off. Perforated board is much better.

Ed

Re: MiniMod pantry transmitter
 

Hi Ed. You make a valid point, but I think at these relatively low frequencies you may be worrying unnecessarily.

I have just checked the capacitance between two adjacent tracks on the 4"x3" piece of 0.1" matrix Veroboard on which I propose to build this project, and measured just 2pF. As the tuning capacitors for both oscillator and output stage are 270pF, this will swamp any stray capacitance introduced by the Veroboard, and the range of variable inductance available from the specified coils should be more than sufficient to compensate.

In his original article, Ian did say that construction wasn't critical and Veroboard or other methods could be used, so I'm not anticipating obtaining an output signal on Long Wave or the 49 metre band...! :laugh1: