A few questions about a 555 AM transmitter

Discussion in 'The Projects Forum' started by EarlAnderson, Nov 29, 2011.

  1. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
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    4
    Hi, I am designing an AM transmitter based on the 555 timer, and i just have a few questions to ask.

    1. My transmitter's carrier signal is currently a square wave. is this okay or does it need to be a sine wave instead, and if it does, what is the best way to do so.

    2. How can i get the maximum efficiency and range if i power my transmitter off of a 9V battery.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    A sine wave will put more power into the right frequency, but is a bit trickier to implement. A square wave spreads power over a broader frequency range but is simple to implement with your 555.

    To get better help here, you should detail what your goals are; range, power supply, size, cost, target frequency, etc.
     
  3. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
    4
    my goal is to get the maximum range possible off of a 9V battery supply.
     
  4. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
    4
    Okay here is what I have come up with (schematic attached below). This setup oscillates a square wave carrier signal of 1200 kHz, but whenever I tune my radio to that frequency, i don't get any reception, even with the antenna right next to the transmitter.
     
    Last edited: Nov 30, 2011
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The schematic is not visible. It also seems likely that your circuit is not producing the desired frequency, assuming that you have a working radio appropriately tuned. Even a few volts of signal into a short wire ought to produce some response with the radio so nearby.

    Edit: Of course, if the signal is not modulated, the radio's response to it may not be obvious, maybe just a quieting of the normal noise.
     
  6. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
    4
    okay try checking again. the schematic should be there now
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I think that the frequency you are trying to obtain is a bit high for a 555 -. see page 7 of this document:

    http://www.doctronics.co.uk/pdf_files/555an.pdf

    Try increasing the value of R1 to aim for a frequency less than 1MHz: this may give a better chance of getting the IC to oscillate.

    In any case, you need a capacitor across the power supply input to this IC: try 100nF, placed as close to the IC as possible - see page 5 of linked document.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Make sure you're actually sending an audio signal or as noted you might not be able to tell when you're transmitting. Does your receiver use digital tuning? Be sure to scan a wide range. The frequency will drift and is not precisely predictable.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Why not build a simple transistorized AM transmitter? It will work much better overall, a 555 is simply not designed to do this very well.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I've actually made an AM transmitter out of a 555 for a giggle. It is not a quality transmitter, it is more of an experimental learning tool. If you get the right one you can set a 555 to run in the lower range of the AM band.

    It is not surprising you don't receive a signal from your unit as you are not modulating the frequency. Inserting audio at pin 5 just changes the duty cycle of the oscillation, not the amplitude.

    To modulate the amplitude you need to change the voltage supplying the 555 with the audio. I believe it was Indestructibles that had an article on doing that with a transformer in series with the 9V. Radio Shack still sells an 8 ohm: 1000 ohm transformer you can use. Connect the audio to the 8 ohm side and put the 1000 ohm side between pins 4 & 8 and the 9V.

    If you use an antenna at all put it on pin 2 not pin 3, the harmonics there are lower (it's a triangle and not a square wave, closer to the sine wave you really want). Also swap R1 and R2 to get a more even high and low time.

    With a foot or two of wire as an antenna I could hear my signal from across the room. I got it on just about every station frequency I tried too. I was fine with that as I was just demonstrating a concept and not trying to send any real data beyond my signal generator output.
     
  11. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
    4
    you are right. I have built a 2 transistor square wave oscillator, that broadcasts in the lower AM band. It does a much better job than that lousy old 555 timer. Although i have built a much better oscillator. However, i am not an "expert" at electronics (I'm only 13 years old), so i need some help coming up with a modulation circuit. I would like the modulation circuit to be transistorized as well, just to fit in with everything else ;)
     
  12. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
    4
    you are right. I have built a 2 transistor square wave oscillator, that broadcasts in the lower AM band. It does a much better job than that lousy old 555 timer. Although i have built a much better oscillator. However, i am not an "expert" at electronics (I'm only 13 years old), so i need some help coming up with a modulation circuit. I would like the modulation circuit to be transistorized as well, just to fit in with everything else ;)


    I have attached a schematic of the oscillator circuit below. It is just a multivibrator circuit originally used for flashing LEDs. I just changed some of the values a bit to get it to oscillate at the desired frequency, and i put a long piece of wire on the collector of one of the transistors to act as an antenna.
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Heh, you are going digital for what is a analog requirement. When I was in high school I build every type of oscillator in an attempt to understand them.

    I was thinking more in line of a Colpitts or Armstrong sine wave oscillator, the kinds that use a LC tank to set the carrier frequency. They are extremely simple to build, I use a terminal strip and a block of wood, one transistor.

    By going digital you are splattering the band, wasting a lot of your energy and aggrivating the neighbors (and possibly the FCC). When I have time I'll see if I can point you to better plans than what I've seen.

    The reason you have had poor results with the 555 is you are using a very narrow duty cycle, much less than 10%. This means you have generated very little RF energy.
     
  14. DickCappels

    Moderator

    Aug 21, 2008
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    This one is my personal favorite; it is an elegant way to get AM, using a Gilbert Cell as the modulator. I would also like to point out the loading coil, which few low power AM transmitters on the web have. All in all, a very nice little design. By the way, the crystal can be replaced with an adjustable coil in series with a large capacitor (0.1 uf should be fine) to make a tunable version.

    http://www.techlib.com/electronics/amxmit.htm
    [​IMG]
     
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