Another Pantry MW Thing

[mictester]

This is a very low power AM transmitter based on cheap CMOS logic. It uses pulse width modulation to achieve AM, and sounds remarkably good on the air. There is a "peak" indicator that blinks on 95% modulation peaks. It was designed to operate from a wide range of audio sources, so there's an input level pot for convenience. I considered adding basic limiting to the circuit (which is why the level indicator was created), but the range of LDRs available is such that consistent results wouldn't be easily achieved. However, if you're prepared to pay for an NSL32 Vactrol (which will cost as much as the rest of the rig!), you can put the resistive element across the modulation depth pot wiper and earth, and drive the LED from the output of the level measuring comparator, through a 3k9 resistor - in parallel with the peak indicator.

The crystal is available from Farnell / CPC for pennies. The resistor SIL pack can be replaced with vertically mounted 1/8th Watt resistors if necessary...

There's nothing especially novel, but it's cheap and easy to make and gives great results!

[kalee20]

Any chance of the circuit diagram? I'm intrigued!

[ThePillenwerfer]

I know next to nothing about these things but what I'm curious about is why there are four capacitors in series on the RF output. Their total value will be the same as a single 61µF one.

The only reason I can think of for using this arrangement is if that value is critical and it's the only way of getting it due to it not being a preferred value.

I'll watch this thread with interest.

[Boater Sam]

Are they not inductors?

[ms660]

u = uH.

Looks like a pi circuit, inductors bridged to suit then trimmed with a trimmer capacitor.

Lawrence.

Any chance of the circuit diagram? I'm intrigued!

Ditto...

[ThePillenwerfer]

Thanks for the clarification that what I thought were caps are in fact inductors, it makes much more sense now.

— Joe

[Boater Sam]

Type number of chip bottom left, "switch", please?

[TonyDuell]

I assumed 'switch' was what it said it was. An 8 way DIP switch. That is 8 SPST switches between pins 1-16, 2-15...8-9. It appears to control the division ratio of a counter, probably to set the carrier frequency.

[Boater Sam]

Silly me, its too obviously a dil switch. Thank you.

[mictester]

Originally Posted by kalee20

Any chance of the circuit diagram? I'm intrigued!

Here you go - translated from pencil on paper!

[mictester]

The modulator IC does need to be the LM358 (or any other rail-to-rail dual op-amp). The LEDs should be low current / high brightness types. There's one quirk - the 40103 binary down counter takes one clock period to reset when zero is detected, so to divide by 111dec for 999kHz out for example, you actually programme 110dec. The loop filter for the 4046 was determined initially by calculation using the formulae in the Philips CMOS manual, and then adjusted empirically when that failed to work out! The additional 2M2 resistor reduces phase noise on the carrier. The offset in the op-amp bias is deliberate - asymmetrical audio gives symmetrical modulation when done like this!

The audio bandwidth is too much, but you can always add a simple filter before the input. I came up with a simple and cheap audio processor that uses a TL074 quad op-amp and a 2N3819 FET to feed this board. It provides basic low pass audio filtering and compression. However, if you want the real "old time radio" sound, the mod needs to be kept low and uncompressed!

Remember - these logic ICs need to be the "ordinary" types - you can't use the 74LS or 74ALS versions since they're limited to 5V supply, and we need about 12V for a reasonable carrier level. I used HEF4046, HEF40103, CD4060BE and so on. I've also built this on Veroboard and it works just as well.

[ThePillenwerfer]

I've converted the layout diagram into PCB artwork but HAVEN'T TESTED IT so approach it with caution.

[Lloyd 1985]

Might have to have a go at building this! Might be more successful than the valve one I attempted some time ago!

Regards,
Lloyd

[mictester]

Sorry for the copious number of 0Ω resistors. Obviously these can be wire links (offcuts from other component leads are usually favourite). I spent a lot of time trying to minimise the number of links, but always ended up with roughly the same number of them with every board layout I tried!

A friend of mine in Taiwan is using a surface-mount variant of this circuit on a double-sided PCB as a miniaturised "AM Audio Sender" for use in cars. The aerial wire gets wrapped around the coax downlead from the car aerial and there's enough signal to give a strong "extra" station on medium wave!

[mictester]

I've been considering making amendments to the output layout - as presented above, the inductance was chosen by wire links (or solder blobs) to short out inductors that weren't required. I found that I have a small supply of 4-way DIP switches, so I'm going to try a tweak to the layout to use three of the four ways on a switch to select the inductance. This could be a more elegant solution.

There are two "fb" ferrite beads indicated on the diagram on the leads of the level pot. The Veroboard prototype had these because the pot was mounted on longer wires, but they were found to be superfluous - there's not enough RF around to feedback into the audio stages.

There could also be the ultra-cheap version that just uses solder blobs to choose the division ratio and to select the inductance!

Results: With a 2m piece of wire sloping from the curtain rail to my workbench, I got a maximum range of about 50m through the window and down the garden when I'd optimised the output settings using a crude absorption wavemeter. The signal strength is usable throughout my medium-sized 4-bedroom house, but doesn't go too far - you're not going to upset the neighbours.

[Al (astral highway)]

Neat little design, I like it! How did you calculate the antenna loading circuit??that was tricky for me to find an efficient sweet spot with my recent modulator.

It’s possible to save two components... it isn’t necessary to have resistors to combine the two channels of line level input, I’ve tried this with outputs from a smartphone and also a Mac.

Wonderful work and speedy transcription , what did you use to draw the circuit so neatly ??

[kalee20]

That looks a good, workmanlike design. I'm impressed! Thanks very much for putting up the circuit, and also the description!

[mictester]

Originally Posted by astral highway

Wonderful work and speedy transcription , what did you use to draw the circuit so neatly ??

It's a bunch of "cut 'n' pastes" from other drawings, done using "GIMP" on my desktop machine. I have a load of schematic elements saved in a basic sheet. It's just a question of copying, dragging and dropping the various elements to create a circuit diagram. The last step is annotation, but that's quick and easy.
I've been contemplating writing a good schematic editor programme (I have yet to find one that I really get on with), and I've been experimenting with a tablet and stylus, using handwriting recognition routines to identify scribbled component shapes and convert them to standardised shape. This should allow quick and easy schematic sketches....

[Argus25]

An interesting and unique design for a Pantry transmitter.

There have been a few pantry transmitters based on cmos logic designed down here in AU.

I have attached the circuit of one designed by Mike Osborne.

It uses an L/C oscillator based on a 4069 rather than a synthesized carrier frequency.

One thing about it though, that I do like, it uses a pair of differentially driven 2N7000 output fets with the type of driver often seen in switch-mode psu's to prevent the fets being switched on simultaneously.

The output passes to a 750uH output coupling coil of 80 turns of 0.2mm diameter enameled wire wound on a 48mm length of 90mm diameter PVC stormwater pipe (I know that this coil would appeal to Astral Highway). The wire antenna is then tuned by a 10-70pF tuning cap.

It does use resistors to mix the R & L audio channels and an LM386 modulates the power supply to the fets. There is no audio peak limiting, soft clipping or volume leveling and it would have questionable modulator linearity, so the design is not my cup of tea, however I think the output stage and loading coil design is innovative.

Circuit and picture attached, I hope it is readable.