how to make a 555 FM radio transmitter?

Question

Dear: Element 14

First of all I would like to say is sorry about my last post. My last post about voltage regulation was too difficult for me and that's what it really came down to. So I have decided to give up on that project and start a new one. First of all I am building a FM radio transmitter with a 555 timer. In order to build a 555 timer fm radio you need a capacitor. But I am not sure what farads of a capacitor I need. More or less I am not sure I what I know is correct. I think according to my math I need a 18000000 Farad capacitor which is really impossible to get to. I want my radio transmitter to broadcast up to 108 MHz through 88 MHz which is your basic broadcast signal. But am I right. You see here is my math, one Ampere times MHz divided by six because I am using six volts. Any questions ask me or look at my diagram. Thanks!!!

From: Noah

ntewinkel · Accepted Answer

Hi Noah, The calculator from Neil's link will calculate up to kHz. The reason it won't do MHz is because as John pointed out in a comment above: you can't do that with a 555 timer. According to the data sheet, the NE555 you are using has a maximum frequency of 100kHz. According to Wikipedia most of the world has FM radio in the range of 87.5 MHz to 108.0 MHz. Japan starts at 76MHz. That's starting at 76,000 kHz or 76,000,000 Hz. So that's about a magnitude too high for the 555 to handle. On the other hand, I did find this article (among many others) on how to make an FM transmitter using an NE555 timer: Using 555 as FM transmitter Circuit Diagram | Electronic Circuits Diagram John and Neil, for my education - am I understanding this correctly? the spec sheet says it can go up to 100kHz (0.1MHz), FM radio here needs at least 87.5 MHz, yet there are plenty of articles and schematics out there claiming to use just a single 555 chip with some passives to make a short range transmitter. Noah, What exactly is your question? Earlier you replied to me that it was not "how do I build an FM transmitter" because you can do that most easily by using that RaspberryPi you said you have. Now it looks like the question is "How do I build an FM transmitter using only an NE555 chip with a few capacitors and resistors", but there are so many articles and schematics available for that, that I doubt that's the question or your Google searches would have given you all you needed. Cheers, -Nico

johnbeetem · Answer

Noah Schreiber wrote:

I suggest you take a look at the 555 data sheets. Here's the list of 555-like timers at Texas Instruments: Timers - Products - TI.com . If you look at the column labeled "Frequency (Max)" you'll see that a standard LM555's maximum frequency is 0.1 MHz. The fastest is the LMC555 CMOS Timer at 3 MHz. (LMC555 is great part -- operates from as little as 1.5V!) But a 555 is nowhere near the 100 MHz you need for FM radio. If you go further and look at the data sheets, the first page of the LM555 states: "Timing from Microseconds through Hours". Nothing at the nanosecond level. The LMC555 states: "3 MHz Astable Frequency Capability". The equation you quote is for normal 555 operation where you're not pushing the limits of the part. It's a simplified equation that ignores propagation delays through the part, which are on the order of 100s of nanoseconds. As you approach the 555's limits, internal delays dominate and the equation is no longer valid.

Robert Peter Oakes · Answer

Noah, you need to read up on the 555 timer chip, just Google 555 timer and you will find copious amounts of data, the chip has been around for well over 40 years here is a good place to start http://en.wikipedia.org/wiki/555_timer_IC you will be hard pushed to get it running reliably at 88 to 108 Mhz, it may be possible but you would need to be very skilled at building RF Circuits, you would be better off starting with perhaps an AM radio then work up to it, but there is a lot of self study needed either way Good luck Peter

how to make a 555 FM radio transmitter?

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