[SOLVED] Simple AM transmitter?

[boylesg]

Can anyone point me in the direction of a simple AM transmitter that I can use to get a signal on a standard AM/FM radio?

I tried this one: **broken link removed**

And this one looks more or less identical: **broken link removed**

If my calculations are approximately correct swapping out the 1nF timing cap (C1) for a 200pF one should give me the full sweep of frequencies the dial of my radio.

I set the radio on a frequency where there is no one transmitting and then adjusted the pot to try and get the audio I was passing in to the transmitter circuit - the national ABC radio station 621.

But I don't seem to be able to get the signal on my radio regardless of the setting of the pot.

Guidance please?

 

[betwixt]

Forget that project - it is absolute nonsense. The IC is incapable of running at the right frequency, the construction will never work reliably and the 'pop can' antenna suggests the writer is doing this as a joke.

Start with a design that uses an LC tuned circuit that resonates on the frequency you want, not a digital timer operating outside it's specification.

Brian.

 

[boylesg]

Forget that project - it is absolute nonsense. The IC is incapable of running at the right frequency, the construction will never work reliably and the 'pop can' antenna suggests the writer is doing this as a joke.

Start with a design that uses an LC tuned circuit that resonates on the frequency you want, not a digital timer operating outside it's specification.

Brian.

I can find abundant examples of FM transmitters but I am struggling to find any credible examples of AM transmitters.

Can you point me in the right direction.

I understand the general idea of a tank circuit but I have no clue where to begin in order to achieve amplitude modulation.

The dial on my radio says x10kHz and the range is 530 1600 to 1600, so that's about 5.3MHz to 1.6GHz.

Yeah and the CMOS 555 I am using only goes up to 2MHz or so, so I miscalculated some where along the line.

 

[srizbf]

..
The dial on my radio says x10kHz and the range is 530 1600 to 1600, so that's about 5.3MHz to 1.6GHz.
..

You might have misread the scale on dial. The MW Band dial should be x khz and not x10 khz . The MW AM Band is from 530 Khz to 1600Khz.

555 timer based am transmitter relies on harmonics for the MWband.

you can try the circuit in :

http://www.techlib.com/electronics/amxmit.htm

(may be the phono oscillator in that page).

 

[boylesg]

You might have misread the scale on dial. The MW Band dial should be x khz and not x10 khz . The MW AM Band is from 530 Khz to 1600Khz.

555 timer based am transmitter relies on harmonics for the MWband.

you can try the circuit in :

http://www.techlib.com/electronics/amxmit.htm

(may be the phono oscillator in that page).

The AM scale on my radio is 53 - 160 x10kHz......530kHz to 1600kHz/1.6GHz

Does not mean the radio manufacturers f#cked up and printed an incorrect scale on the dial?


Does this schematic looked better?

Every time I searched for AM transmitter I seemed to get FM transmitters and that bogus 555 based AM transmitter.

The difference seemed be instead searching for am transistor transmitter - then the above schematic came up near the top of the search list.

Could it be the author of the 555 based circuit did bot intend the signal to be picked up on a conventional AM/FM radio? But in a HAM radio instead?

- - - Updated - - -

http://www.techlib.com/electronics/amxmit.htm
Might just give this one a go for the hell of it. Is 2n4401 substituteable by BC337....looks as though it is from the datasheets.

The only reason why I am pursuing this is because I have an ar$ehole of a neighbor who has picked physical fights with myself and his other neighbor, along with general bullying and intimidating behavior over fences and such like.

He has a security camera pointed down the length of our backyard and the police will not make him move it.

Just for a bit of amusement I want to transmit and an annoying piercing tone over his radio that he has tuned into AM 1224 (SBS) and going 24/7.

Had a bit of amusement with an FM bug and listening into all his daily activities and conversations in his backyard and garage

Terrific opportunity to try some of these things out and learn a bit along the way!

With FM transmitters I can fairly easily conceive of how input to the transistor base can alter the oscillation in the tank circuit on the collector.

But how are these AM transmitters different exactly? What is it that determines whether the frequency or the amplitude is modulated?

 

[betwixt]

Could it be the author of the 555 based circuit did bot intend the signal to be picked up on a conventional AM/FM radio? But in a HAM radio instead?

Any self respecting HAM would know to avoid a circuit like that (I hope!).
It worked by producing square waves, the transistor at the output served no purpose whatsoever as it never passes any current. Square wave signals comprise many harmonics and it's one of those they picked up on their radio. Bear in mind that the frequency stability would be EXTREMELY bad (think of drifting 10's of KHz as the battery voltage or temperature changed) and the effect that has on harmonics is multiplied.

The first 'techlib' schematic is good, it properly modulates only the amplitude of the signal but has the drawback of not being tunable, you have to start with a quartz crystal at the frequency you want to transmit on. You might be able to change the crystal for an LC circuit but you would have to screen it from magnetic coupling with the output coils.

Avoid any schematic that modulates the oscillator stage, they always suffer from frequency deviation as well as AM. The schematic in post #5 will work but probably produces more FM than AM output. I would also be wary of any design that has no output tuned circuit, especially one that uses 1KV rated capacitors in a design that runs from a 9V battery and has both ends of the tuning control 'live'. It makes it almost impossible to adjust without the frequency shifting as you touch the tuning control.

Incidentally, the range is 530KHz to 1600KHz (0.53MHz to 1.6MHz) not GHz!

Brian.

 

[boylesg]

Any self respecting HAM would know to avoid a circuit like that (I hope!).
It worked by producing square waves, the transistor at the output served no purpose whatsoever as it never passes any current. Square wave signals comprise many harmonics and it's one of those they picked up on their radio. Bear in mind that the frequency stability would be EXTREMELY bad (think of drifting 10's of KHz as the battery voltage or temperature changed) and the effect that has on harmonics is multiplied.

The first 'techlib' schematic is good, it properly modulates only the amplitude of the signal but has the drawback of not being tunable, you have to start with a quartz crystal at the frequency you want to transmit on. You might be able to change the crystal for an LC circuit but you would have to screen it from magnetic coupling with the output coils.

Avoid any schematic that modulates the oscillator stage, they always suffer from frequency deviation as well as AM. The schematic in post #5 will work but probably produces more FM than AM output. I would also be wary of any design that has no output tuned circuit, especially one that uses 1KV rated capacitors in a design that runs from a 9V battery and has both ends of the tuning control 'live'. It makes it almost impossible to adjust without the frequency shifting as you touch the tuning control.

Incidentally, the range is 530KHz to 1600KHz (0.53MHz to 1.6MHz) not GHz!

Brian.

MHz...of course....so it is in range of a CMOS 555 after all.

With this With this circuit: http://www.techlib.com/electronics/amxmit.htm

The purpose of the first stage with the crystal is simply to create a stable oscillation around a central frequency - correct?

Then this central oscillation input into the tank circuit is adjustable, within a range, by the tuning cap - correct?

The key difference between this AM transmitter and the FM transmitters seems to be that pair of 2n4401s that links the tank circuit, with the crystal circuit and with the audio in.

Can you explain to me how this works without getting into too much details?

I have seen that configuration of transistors before in amplifiers etc but I am damned if I can remember what it is called.....long tailed pair or something like that?

How exactly does AM work? Because there seems to be 2 oscillation components here - that produced by the crystal and that produced by the tank circuit.

With FM transmitters there is only ever a tank circuit.

- - - Updated - - -

Just for a bit of amusement I want to transmit and an annoying piercing tone over his radio that he has tuned into AM 1224 (SBS) and going 24/7.

If I can do it then I will thoroughly enjoy watching the brainless twit stress while he tries to figure out what is going on LOL!

The imbecile has huge trouble sleeping due to noises waking him.....doubly SWEEEET!

Bought some 5mW red laser diodes and shone a bunch of them straight into his security camera to blind it with lens flare, but the cops made me take it down :-(

The cops crapped on about damage to property, but they are equally brainless twits because 5mW (or less) lasers wont damage security camera sensors.

So I have bought a replacement IR LED surround for a security camera and intend to make a IR spot light out of it and blind his camera that way!

- - - Updated - - -

I am looking at this Brian: **broken link removed**

Rather than building my own fixed frequency transmitter.

Looking at the schematic it has a CD4060 frequency divider chip - I understand the concept behind these from Arduino and clock dividers etc.

Could you have a quick look over it and tell me if you think it is worth playing with?

I have this one of **broken link removed**but knowing what I know now (thanks to you guys) I think this is actually an FM transmitter and the seller is confused, or it is another poor attempt at am AM transmitter!

 

[betwixt]

Lots of questions...

Yes, the crystal is to keep the frequency stable. It not only helps to keep you on the frequency you want but is resistant to change by outside influences. That means changes in voltage, temperature and signals applied to it have minimal effect. Frequency modulating a crystal oscillator is therefore quite difficult, at least at the levels needed for radio broadcast.

The Ebay link is actually very good. It combines the stability of a quartz crystal with the ability to select different frequencies by using a PLL (Phase Locked Loop). Basically, it starts by generating a very stable 9KHz reference by dividing down from the quartz crystal frequency and compares it with a divided down frequency from the CD4046 Voltage controlled oscillator. It tries to keep the divided down VCO frequency the same as the divided down quartz frequency and creates a correction voltage that tunes the VCO. The division of the VCO is done inside the CD40103 and you select the divider number with the switches. So for example if you set it to divide by 100, it will tune the VCO so it produces 900KHz (9KHz reference * 100), if you set it to divide by 101 it would retune to 909KHz, always adjusting the output of the divider to be 9KHz.

At first glance I can't see how the CD4053 is used as the modulator, I'll have to pull the data sheet to see how it is connected.
It does also have a filtered output which helps to stop it producing harmonics at the antenna.

The 50mW transmitter is AM. The oscillator frequency is decided by L1 and the capacitors around it. Note that no audio is fed to the oscillator itself. The modulation comes from changing the supply to the amplifier stage Q3 by varying the current through Q5.

Brian.

 

[boylesg]

The 50mW transmitter is AM. The oscillator frequency is decided by L1 and the capacitors around it. Note that no audio is fed to the oscillator itself. The modulation comes from changing the supply to the amplifier stage Q3 by varying the current through Q5.

Brian.

OK so this one warrants a bit more effort before I go and buy the other one.

The sellers suggests that the frequency is not or barely within the AM range.

I tried changed C1 to a 160pF tuning cap and the C15 to a smaller value (the value I got in the kit was not the same as on the schematic)

But I may have got the calculation wrong as I am not comfortably familiar with tank calculations.

What would you change to get the circuit to give you full access to the AM band 530kHz to 1.6MHz?

 

[betwixt]

The calculation formula is:

f = 1/((2 * pi) * sqrt( L * C ))

where
f is in Hz
L is in Henries
C is in Farads

The units are very big for radio use so remember that 'pF' means 10E-12 Farads and 'uH' means 10E-6 Henries.
For example 150pf and 120uH resonate at 1/(6.28 * sqrt(150E-12 * 120E-6)) = 1186872Hz or 1.186872MHz. I approximated 2 * pi as 6.28 to keep it simple.

In the oscillator you have to take into account the total capacitance in the circuit, including the feedback capacitors. If you assume the tuning capacitor goes from rated value at maximum and about 10% at minimum (they never go down to zero) you can work out the anticipated tuning range. In general, if you want a wider tuning range you have to make the variable component larger compared to the fixed ones so it plays a bigger part in the calculation. In the schematic, the tuning capacitance plays a relatively small part, if it is 40pF maximum and lets say 4pF minimum and it has two 100pF capacitors across it, the combined capacitance is 204pf to 240pF. To get full tuning range, I would design it for 1.6Mhz and note the capacitance needed then work out the capacitance needed at 530KHz. The difference is the amount you have to add in the tuning control.

Incidentally, 47uH with 204pF resonates at 1.625MHz and with 240pF it resonates at 1.499MHz so the existing range is quite small. If you want to cover the whole band I would suggest making the inductor 150uH, the parallel capacitor 33pF and the tuning capacitor 470pF, that gives a resonance range of about 1.6MHz to about 580KHz. The exact range will depend upon other factors in the circuit, for example wiring length and capacitance between and inside other components.

Brian.

 

[boylesg]

The calculation formula is:

f = 1/((2 * pi) * sqrt( L * C ))

where
f is in Hz
L is in Henries
C is in Farads

The units are very big for radio use so remember that 'pF' means 10E-12 Farads and 'uH' means 10E-6 Henries.
For example 150pf and 120uH resonate at 1/(6.28 * sqrt(150E-12 * 120E-6)) = 1186872Hz or 1.186872MHz. I approximated 2 * pi as 6.28 to keep it simple.

In the oscillator you have to take into account the total capacitance in the circuit, including the feedback capacitors. If you assume the tuning capacitor goes from rated value at maximum and about 10% at minimum (they never go down to zero) you can work out the anticipated tuning range. In general, if you want a wider tuning range you have to make the variable component larger compared to the fixed ones so it plays a bigger part in the calculation. In the schematic, the tuning capacitance plays a relatively small part, if it is 40pF maximum and lets say 4pF minimum and it has two 100pF capacitors across it, the combined capacitance is 204pf to 240pF. To get full tuning range, I would design it for 1.6Mhz and note the capacitance needed then work out the capacitance needed at 530KHz. The difference is the amount you have to add in the tuning control.

Incidentally, 47uH with 204pF resonates at 1.625MHz and with 240pF it resonates at 1.499MHz so the existing range is quite small. If you want to cover the whole band I would suggest making the inductor 150uH, the parallel capacitor 33pF and the tuning capacitor 470pF, that gives a resonance range of about 1.6MHz to about 580KHz. The exact range will depend upon other factors in the circuit, for example wiring length and capacitance between and inside other components.

Brian.

Thankyou very much Brian, will give your suggested changes a try.

If I can get it working then boy o boy I am going to have some fun at the expense of that jail trash imbecile next door

 

[betwixt]

If you are successful, remember I didn't help you !

Personally, I would go with camera blinding option but using IR LEDs instead of a laser. Don't forget that transmitting a jamming signal is illegal and also that it may interfere over a greater range than just your neighbor.

Brian.

 

[boylesg]

If you are successful, remember I didn't help you !

Personally, I would go with camera blinding option but using IR LEDs instead of a laser. Don't forget that transmitting a jamming signal is illegal and also that it may interfere over a greater range than just your neighbor.

Brian.

The brainless twit would not be able to figure out that I am sending a jamming signal. And even if he complained, the local cops would not have enough intelligence to recognize a small PCB transmitter if it was thrust under their noses! Apart from the fact that it is 50mW and will only have a VERY local effect and the cops would be highly unlikely to bother pursuing such a petty complaint to any significant degree. And assuming that any other near neighbor's listen to the same radio station - which is virtually nil since I have been here for over 10 years and never heard SBS playing once, other that since dip$hit moved in next door 12 months ago!

Another nasty trick I would love to try is to generate an infrasound square wave from my Arduino, amplify it and play it 24/7 through a woofer into his backyard. See of the often quoted 'fear frequency' has any legs!

Occasionally people turf out old speaker cabinets and they nearly always have a low frequency speaker in them.....keeping my eye out.

 

[Audioguru]

If you can find a low frequency speaker that can play infrasound frequencies (most cannot) then do not play a squarewave because it is full of harmonics that the speaker can produce and that people can clearly hear. Only a low distortion sinewave has no harmonics but most low frequency speakers generate distortion that can be heard.

 

[boylesg]

If you can find a low frequency speaker that can play infrasound frequencies (most cannot) then do not play a squarewave because it is full of harmonics that the speaker can produce and that people can clearly hear. Only a low distortion sinewave has no harmonics but most low frequency speakers generate distortion that can be heard.

What if I use Brian's GND/EARTHtrick?