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This site seems to suggest the use of the 555 timer in astable mode to generate frequency modulation from a source. The typical transmitter uses a tank circuit with an inductor and trimmer capacitor or an inductor and transistor like this one. I would rather use the 555 timer version because of my bad experiences with inductors and the availability of pieces. So my question is first which do you think would have the farthest transmission range and also how would I change the frequency of the 555 timer circuit? Any input appreciated.

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  • \$\begingroup\$ Ask yourself these questions first: what is the maximum frequency that a 555 timer will run at? Is that within the frequency band that you want to receive? \$\endgroup\$ – Dwayne Reid Apr 2 '15 at 22:17
  • \$\begingroup\$ Transmission range has nothing to do with this choice. \$\endgroup\$ – user207421 Apr 2 '15 at 22:17
  • \$\begingroup\$ I would like the entire 87 - 108mhz range. I cannot find the max frequency of the 555 Timer. By the way, I was also wondering what the Diode did in the 555 version. Could someone answer these questions via an answer instead of comment. Thank you for your time. \$\endgroup\$ – NULL Apr 2 '15 at 22:23
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    \$\begingroup\$ Reading the TI version of the datasheet, in the very first line of the description: "The xx555 timer is a popular and easy to use for general purpose timing applications from 10 μs to hours or from < 1mHz to 100 kHz." \$\endgroup\$ – Majenko Apr 2 '15 at 22:38
  • \$\begingroup\$ Do you think the linked 555 timer transmitter could work or what could I do to make it work in the 88 - 108mhz area? Also for the diode I have a 1n4001. Would that work or does it have to be the other one. Thank you for your time. \$\endgroup\$ – NULL Apr 2 '15 at 22:50
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It's probably worth going back to basics to answer this as there seem to be a few things muddle in the question.

A modulation is a means of altering a carrier so that it can carry information. A carrier can itself carry no information as it is periodic. FM is one scheme where the frequency of the carrier is changed slightly with each bit being emitted.

FM broadcast radio is an application of the frequency modulation of an audio signal to impress it upon a sinusoidal carrier, typically at 88-108MHz which is then broadcast as an RF wave.

There are other applications of frequency modulation and the 555 example you give is very much a different one. First of all, a 555 will almost certainly conk out at about 100kHz-1MHz, at very best no more than a few percent of the frequency you need.

Secondly, the values of the accompanying components would be so ridiculously small that they would be dominated by parasitics from your layout, pins etc, and very much not available in the shops.

Thirdly, the signal generated is square, not sinusoidal, so would almost certainly have truly epic harmonics at odd multiples of the carrier frequency such that in the very unlikely event that you got enough power to transmit any distance in the broadcast band, you'd soon be hearing a standard-issue boot at your front door when the relevant regulator arrived for the various shipping accidents and military incidents which your interference had caused. I'm gonna stop at three, but there are loads of other reasons.

Inductors are oddly frustrating to source order, my heart goes out to you on that, but if you want to work in RF you're going to have to get used to them, and also maybe choose a more conventional route in, ideally a legal one such as amateur radio or pre-assembled licensed/license-exempt modules?

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