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tecco

Posted: 13 April 2017 4:19 PM
 

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Hey Everyone

  Today i thought i would try one last time at making a low power FM transmitter and it worked heres a pic of it:

data-cke-saved-src=/userfiles/forumcontent/907db2aa-ef13-4f69-91fb-ca84eebe768f/image/20170413_154941.jpg

Here is the schematic

data-cke-saved-src=/userfiles/forumcontent/907db2aa-ef13-4f69-91fb-ca84eebe768f/image/kogawa_simplest_transmitter.png

~ Benjamin 

 P.S. Have a good Easter smiley


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joe_ivp

Posted: 13 April 2017 8:31 PM
 

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Good job

If you need a very low value tuning capacitor you can make one with twisted wire. Google for gimmick capacitor

Some radios will tune the transmitter to the approximate frequency by squeezing or stretching the coil, which affects H. Sometimes a piece of foam is put inside the coil and then coated in wax to stop it moving. Then frequency can be fine-tuned by varying the turns of a gimmick capacitor in the oscillator part of the circuit

Joe



tecco

Posted: 14 April 2017 8:58 AM
 

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Hey Joe 

Thank you for bringing up gimmick capacitors i have never heard of them before!! I was thinking i might put some Hotglue inside the coil to stop it from changeing shape / length then the frequncy would change.

Is there anyway of making the circuit put out a little more power? becuase at the moment as soon as i go near the antenna the signal drops out and all i hear is hiss, i did put an inductor across the output of the Amp hoping it would resonate back and forth helps a bit but not enough really i still only get 20Cm of range.

Side note: 

The Raspberry FM radio transmitter i built a couple of months ago has more power than this single transistor transmitter yet the Pi uses clock signals and the transistor one used a LC tank circuit.

~ Benjamin


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joe_ivp

Posted: 14 April 2017 12:45 PM
 

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Hi tecco

A one-transistor design will probably be susceptible to external capacitance, which is what you are, and de-tune. Some devices, like alarms and musical instruments (Theremin and others) make use of that. Andrew recently posted a stable circuit, and I have one here

https://www.flickr.com/photos/97814409@N04/albums/72157658768753134 

Generally more power in = more power out. eg a radio station would be using a hefty PSU and power transistors or valves. V+ could be bumped up to the rating of the 2N3904 (~50V) although you'd need to watch the wattage through it. Also, the 2N3904 isn't a particularly high frequency transistor

https://www.onsemi.com/pub/Collateral/2N3903-D.PDF

The gain drops off as the frequency rises, which you can't do anything about for an actual component

https://en.wikipedia.org/wiki/Gain%E2%80%93bandwidth_product

There are transistors more suited to RF which are specced to 1000MHz or more. You don't need them to run that high but it means they have good gain at the lower frequencies. Just Google RF transistor, plenty of cheap ones

A lot of the design literature is about antennas. The electronics part of the circuit is pretty straightforward but getting the most efficient transfer of energy from circuit to airwaves is a real black art and people are forever tinkering with designs. You could make a career out of it, no problem. The antenna is an output device or load, just like an LED or motor. Unlike those, an aerial is very configurable. You can make it longer, shorter, bent, round, have extra elements, directional, resonant etc etc etc. You can even start getting into quantum effects. Think about the antenna size and efficiency in a cellphone for example

https://en.wikipedia.org/wiki/Antenna_(radio)

For a simple easy transmitter, usually a straight wire (whip) that is some even fraction of the wavelength will do and would be the simplest coupling of electrical energy to radio. If the antenna you're using is the correct length then there's not a lot more you could do (simply anyway - think "a lot of abstract maths") to improve the efficiency

https://en.wikipedia.org/wiki/Whip_antenna

Wavelength is v/f, where v = speed of the wave (usually light speed) in m/s, f = frequency in Hertz

eg for 100MHz (middle of the FM broadcast band) the wavelength is 300,000,000 / 100,000,000 = 3.000m

A wavelength whip antenna would be 3000mm, a 1/2 wave 1500mm, a 1/4 wave 750mm. If the circuit frequency changes then the antenna will be less efficient. Maybe try a stabilised design. Google crystal-locked radio

Joe



Andrew H

Posted: 15 April 2017 11:04 AM
 

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Yep, the single transistor ones the 'Tank Circuit' that sets the frequency is usually a FIXED inductance (turns of wire) and FIXED Capacitance (tiny pf Capacitors. You will see that the antennae on these types of circuits is usually connected to the same / single coil.  This means that any movement of or about the antennae will change the capacitance in the main tuning circuit that is MIGHTY frustrating...  as just a slight bump or moving it a bit can put the frequency allover the FM band or even way outside the tuning range of the FM radio.

A kit set is a good start.

 

Ideas department ?

Can you fingure out what is going on here !!!

This is a VERY 'Smart' FM bug....

What could you use it for ?

(Note the coil inside a 'can' with a tuning slug.  This helps stabilise things beutifully as does placing everything on a breadboard.  This circuit uses the capacitance between the tracks as the tiny pF fixed capacitor !

So it is 'Reasonably stable'...

~ Andrew



joe_ivp

Posted: 16 April 2017 3:49 PM
 

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Hi tecco,

the 'gimmick capacitor' effect is often very much unwanted, especially in comms or fast logic circuits

The capacitance of twisted or closely-parallel wires together with any resistance forms a low-pass filter, and that rounds the corners of square waves.

This can seriously affect data detection at the receiver. If the rounding is bad enough this lengthens the time for the voltage of the data bit to reach the receiver's '0' - '1'transition voltage or shorten the time to the '1' - '0' transition point. Protocols that start sampling after an edge or level detection may then produce bit errors. For instance I2C is a protocol which specifies R and C at certain speeds, eg if the pull-up R is too high then corners get rounded too much. As supply or signal voltage is reduced or transmission speed is increased the problem gets worse

Another example is ribbon cables that are too long and sometimes the cause of programmer failures

Joe



tecco

Posted: 18 April 2017 5:05 PM
 

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Hey Andrew & Joe 

 Does the picaxe chip control the output frequency or make tunes?

 Oh i thought gimmick capacitors were good Haha 

 

~Benjamin


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Andrew H

Posted: 20 April 2017 12:27 PM
 

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The picaxe gates on:OFF the base bias power to the FM oscilator.  Look for the 10k Base Bias feed from Pin 4 of the picaxe.

Now............

The fun part....  IF you tuned this to the exact same freq as your usual / fav station / on in the background all the time) etc

AND placed it near by your FM radio so that he power of this feeble FM Tx'r can 'capture' the FM radio channel then you can make some funky things happen....

* If you use the tune or sound command in the picaxe even on another pin just into a resistor load you will hear the tune the picaxe is playing

* Eg sort of an idea.... If your picaxe was outside on the window sill and say it measured the temperature, it could blip the 10's and the 1's  of the temperature into your radio say every hour / OR when the sun comes up and your FM radio clock has just turned on then ecven without opening your eyes you will 'hear' the temperature as a series of long (10's) and short ('1s) bleeps

* OR any sort of a trigger / alarm system (dishwasher / washing machine is flooding / bath over flow (again!!!)

* Letter in letter box.

This litle project will run off that coin cell potentially for a year as the picaxe can sleep when not in use drawing < 1uAmp (whith help of some code I can give you)

OR just put a hald decent battery in it and beef up the RF energy with a 2nd stage and make it say work like a remote letter box alarm / smart door bell that detects people walking up path so you go open the door just as they are about to knock...

OR beef it up or use a more grunty FM bug then use an HES and a suspended bar magnet and detect when a car / vehicle (and what size vehicle ) has just driven up (or stolen down the tanker track) driveway by playing a little tune length of tune / bleeps is approx = to the amount of metal / magnetic deflection.  This works over about 5 to 10m so could be mighty handy...  Measure the average swing of the magnet with the HES on knife edge setting.  Calculate and store the average magnetic flux.  When large metal blob passes by then detect the swing :)

OR

Anything else you could use a uAmp sleeping uFM Bug a Lug transmitter for

~ Andrew

 

(Farmer stealing his own quad bike)

 

 

 

 

 

 

 

 

 



Andrew H

Posted: 20 April 2017 12:29 PM
 

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A lot of FM bugs, cheapo ones esp... 
can transmit sound of just the wiring vibrating to sound waves. 
Try tapping it with a pencil and you will see !!!
Called 'Microphonics' in the trade...
~ A

 



joe_ivp

Posted: 20 April 2017 2:08 PM
 

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Thanks for that explanation Andrew

I had a feeling it was on-off or amplitude-shift keying (OOK, ASK) but didn't know what the s/w was doing

https://en.wikipedia.org/wiki/on-off_keying

""I'm OOK. You're OOK ?"

https://www.maximintegrated.com/en/app-notes/index.mvp/id/4439

Joe



Andrew H

Posted: 21 April 2017 7:56 PM
 

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Kind of budget uAmp system that allows for playing of tunes over the FM bug as any music played by the picaxe even into a resistor load wobbles the supply voltage enough to be heard perrfectly on the FM (wobulation) I.e. modulates the frequency.  But the OOK / switching on and Off would sure make a big noise.

Messy but fun :)

Note the 08M2 by itself can generate a nice ASK audio tune into the MW frequency PWMOut into the MW and lower SW band  as well if you want to make a short range AM transmitter.  Handy as a temperature tranmitter / alarm bug under the bonnet of a car etc.  I set one up so that it transmits suspect radiator temperature as a 10's and 1's bleeps into the car simply by tuning into the AM band on the radio.  It shuts down when car is not in use / cool etc.

Either of these could make a great 'frost alarm' if driving and road / air temps approach zero...
(alert via fav radio channel audio squark etc...)

~ Andrew 



Rorence

Posted: 2 May 2017 7:06 PM
 

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Thanks for your nice post to teach us more in this field. After reading on here, we can learn more in this topic. 



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