555 AM Transmitter

This is a very simple easy to make AM transmitter if you don't want to use inductors or capacitors and don't mind not having extreme power. The best part is everythinf you need to build it is at Radio Shack.

Step 1 Building the circuit

Once you have all the parts the circuit is very simple to construct (on a breadboard anyway). The simple seven component schematic is below.

A decoupling capacitor is needed for battery operation =]

Comments

[jazzzzzz]

Simpler TX (range and audio quality suffer a little, looking for a quick fix)

[nqzdepofltr]

What is the equation to find the frequency in this circuit?

[jazzzzzz]

If you wanted the frequency equation because you were having trouble tuning in, the reason for the trouble was the schematic (I fixed the schematic still yet to derive the equation it is harder to regress than I anticipated)

Also remember the decoupling cap for Battery operation.

[jazzzzzz]

I will have to derive one for you (assuming the potentiometer is the only variable)
I will post it as soon as possible

[jazzzzzz]

Frequency = 90000000*Resistance^-2.431
Where Frequency is in kHz
Resistance is in k& (from potentiometer wiper to positive supply)

With R² of 0.993
It will get you close to help tuning but not 100% accurate.

[tsedor56]

I cant wait to build this!
Going to radio shack now!
1 or 2 things though.
are those blue things tuning capacitors or something?
I forget what their called.
and How far will this circuit go?
Hope at least 2 miles like in the specs.
Thanks.

[jazzzzzz]

They are potentiometers Radio Shack doesn't sell trim capacitors
and unfortunately "m" is meters "mi" is miles, I'm working on better range.

[napsterpat]

higher frequencies go farther with a smaller antenna...if you have a low frequency (as you estimated 530khz - 640khz), your antenna would need to be 161 meters (530khz) to 133 meters (640khz) for an optimal antenna to a simple "radio station"

most cell phones operate at around 900 mhz with a 1-3 inch antenna so it is possible to send out radio signals over longer distances without making a long-term investment in a large radio tower

then again i see your concern for a vhf....so you can stick with lower frequencies. just keep in mind that your signal will not travel far if your antenna is not large enough to counter the lower freqency

[aeve1]

RUBBISH!!!!!!

Lower frequency signal have a much longer wavelength, so for every single oscillation (cycle) a greater distance is covered. VHF is line of sight, you must theoretically see it to broadcast to it. The reason GSM signals cover large area is becasue they are relayed from base station, you phone doesn't connect directly to satellites or anything.

Antennas must be matched to the siganl they are attempting to radiate. It is true that lower frequencies require longer antenna, but in this applicatioon (not that it is that suitabel) no antenna would do!

[jazzzzzz]

I agree lower frequencies are better as they can bounce off the ionosphere and curve around the earth (with enough power). Bouncing means longer distance on a curved plane, on a flat one hi low frequencies go the same distance. Transition frequency depends on application, but since this is an AM transmitter I chose the commercial AM band to avoid having to make my own receiver

The antenna does not need to be full half or quarter wave to be matched, it could be 1/8, 1/16, 1/32 etc. They won’t go as far as a full but, they are better than an arbitrary length of wire.

AND IT DOES HAVE AN ANTENNA!!!!! IT IS THE AUDIO IN CORD. Cheap FM scan receivers use the headphone cable as the antenna, so did I.

[tsedor56]

I agree.
I know some one who does ham radio
and He told me when working with FM transmitters and what not
to measure the wavelength or something.
And I defiantly have had luck with smaller antennas!
your right napsterpat.
In my opinion.

[techno guy]

Where's the antenna or transmitter?

[jazzzzzz]

It was discussed earlier, the antenna is the audio in cord. The whole circuit is the transmitter.

[robot797]

THE CIRCUT IS WRONG
it will not work

[jazzzzzz]

Really because it worked fine for me.

Why do you not think it will work?

[jazzzzzz]

I will have to rebuild the circuit and fix the schematic, I don not know what I did. I would try the pin 2 to 6 hookup. Let me know if it works.

If you are talking about the potentiometer one on the right of the breadboard, it hooks to positive on one side and negative on the other, with the wiper (center) on the audio ground.

[napsterpat]

2 goes to 6, and for an astable multivibrator you need a capacitor

[jazzzzzz]

You do not need a capacitor, all it does is control the time constant. At half of the chip’s max frequency it does not need delay, the propagation delay of the flip flops in the 555 have enough delay.

[maclir]

The circuit diagram connects 2 to 5, but unless I'm missing something, the picture of the circuit on the breadboard shows 2 to 6 and 5 unconnected (which matches what I've seen of 555s making square waves).

Also, I'm just learning, so I apologize for the noobie questions, but I don't understand the circuit diagram, how does the resistor in the upper right hand corner connect to the rest of the circuit? (I can't follow the connections on the breadboard to figure this out)

Thanks

[transistorguy]

How many ohms are those potentiometers rated for (the range)?

[jazzzzzz]

Thses are 10k pots

[transistorguy]

Thanks

[jam BD]

Breadboards should really not be used for radio circuits... But at lower frequencies it is alright...

[NoNameme]

Breadboards were originally designed to be used FOR radio circuits.

[jazzzzzz]

yeah its not a vhf circuit so I wasn't to worried

[jazzzzzz]

The audio does plug into the output. The reason is so the audio can push against the carrier wave to modulate it. The other audio goes through a pot to V+ and V-.

[beehard44]

oh ok

[beehard44]

why is the input of the audio on the output of the 555 chip?

[Unit042]

This schematic is not clear. Can you take another look at it? The audio source is plugged into the 555's output. That does not look quite right to me...

[killroy]

Ah the trusty breadboard...

[jazzzzzz]

It doesn't need an antenna the 3.5mm male-male cable to hook it to a source is it (its on pin 3 too =] )

[Kiteman]

Maybe I'm being dim, but I can't see where the antenna is...?

[omnibot]

Looks like there isn't one but if I where to put one in it would be from pin3 on the 555.

[lemonie]

I can't see it either.

L