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I have finally started my journey of learning something new about our everyday radios and their functionality by making an AM radio transmitter using the 555 timer.

I am trying to understand how LC circuits work so I am currently not very expert in LC oscillator circuits, so I gave it a try with a 555 timer.

Here is the circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

  • R1 = 680 ohm
  • R2 = 470 ohm
  • C1 = 1nf

The frequency of the circuit is somewhere between 850 and 1000 kHz.

I have tried this circuit with a 12 volt 7 ampere lead acid battery which yielded nice results.

The problem is, the transmitted audio can only be heard within 100cm or almost 1 meter radius from the transmitter. The audio quality is crystal clear if it is kept within 60cm. If I exceed this radius, the audio signal gets noisy and weaker. After the absolute 100 cm or 1 meter, the transmitter signal gets out of range.

If we puta thick obstacle between the transmitter and the radio, 60cm crystal clear radius reduces to almost half, 30 cm.

I tried to boost the signal like this:

schematic

simulate this circuit

This solution did not seem to work.

I need the range of this transmitter to be at least 5 - 10 meters. (range of 10-20 meters would be the best.) Currently it is working fine within 1 meter of radius without any obstacle.

How can I achieve the desired range with a good strength and what should the booster circuit look like?

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    \$\begingroup\$ Cool stuff. Reminds me of "back in the day" when I used an AM radio to listen to my subroutines on the 8080A processor. So +1 for that reminder of the olden days. Best wishes! \$\endgroup\$
    – jonk
    Aug 24, 2021 at 6:43
  • \$\begingroup\$ "so i tried to boost signal like this" - how is this supposed to work? \$\endgroup\$ Aug 24, 2021 at 7:03
  • \$\begingroup\$ @Bruce Abbott I already mentioned that I am new in radio and stuff so, why don't youhelp me out with schematic instead of re questioning me ? \$\endgroup\$ Aug 24, 2021 at 7:23
  • \$\begingroup\$ We're questioning you because we all doubt that an NE555 can be used to generate a "proper" AM signal. Why? Because the output of the 555 is "digital" meaning it outputs VSS or VDD, it switches between those. For "proper" AM you will need an output that can also output "in between" values. The 555 can't do that. If you search for "AM modulator circuit" (using Google for example) do you find any circuit using the 555? \$\endgroup\$ Aug 24, 2021 at 7:30
  • \$\begingroup\$ @BruceAbbott seems clear to me the intention is to amplify the current going to the antenna. The obvious mistake is there's nothing to let the current back off the antenna when the wave goes negative! \$\endgroup\$
    – user253751
    Aug 24, 2021 at 8:48

1 Answer 1

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Your circuit has a couple of problems:

  1. you're running the 555 off a 5V 7805 regulator. The 7805 has no capacitors - that's bad juju. Read the datasheet for the 7805 regarding correct usage.

  2. The 555 will happily run on 12V, so why the 5V regulator?

  3. With the output transistor, apart from being drawn upside down, has no base resistor to limit the current. The 555 is running on 5V, so when the 555 output is high (5V), your emitter is at around 4V. What would be more sensible is to connect the transistor emitter to 0V and add a resistor to the base, say around 1k.

  4. Transmitting square waves is not very good. You could make it Class C by adding a LC tuned circuit between 12V and the collector. You'll need a tuning capacitor and a suitable inductor. You could wind this yourself. There's many inductor calculators on the web to assist.

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  • \$\begingroup\$ Well, i am learning more about LC circuits so I don't know how to use them. It will take a little time for me to understand LC circuit. I know that lc circuit is better choice for radio but, as I don't know how to, i gave it a first try using 555 as oscillator \$\endgroup\$ Aug 24, 2021 at 9:37
  • \$\begingroup\$ what I was suggesting was a parallel tuned circuit. After a bit of thought, even easier would be a 'series tuned circuit' and get rid of the transistor. This is fundamental stuff, so there's plenty written on the interwebs to help you. \$\endgroup\$
    – Kartman
    Aug 24, 2021 at 9:43

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