# 555 timer FM transmitter

So online, I found this 555 timer FM modulator circuit. But To make this a working FM transmitter, is all I have to do is attach an antenna to the output modulated signal? Or do I have to also have a series LC resonant circuit tuned to the carrier frequency?

Circuit:

• If you add a link to the article where you found this we might be able to point out some context that you've missed. Hit the edit button below your question ... – Transistor Apr 26 at 20:35
• I don't think you're going to ever have much luck using a 555 timer as an FM transmitter. – Hearth Apr 26 at 20:38

The original article states

The carrier frequency is set using the 5 kΩ potentiometer behaving as a potential divider with one end at Vcc and the other at ground. The frequency of the free running oscillator is set to approximately 455.50 kHz.

This is not in the FM radio band which extends from about 88 to 108 MHz so you will not be able to pick this up on your radio - unless you somehow manage to generate the right harmonics.

The preceding paragraph says

Here is a frequency modulation (FM) circuit that uses a 555 timer in astable mode to generate a sine wave carrier. GCSE students might find it interesting as they are often playing with the 555 timer IC.

GCSE is a UK second level school standard so I suspect that this might be part of a course studying basic electronics and that a matching receiver would be required.

• "unless you somehow manage to generate the right harmonics" Like more Dirac-pulse shape and use an LC tuned to the 200'th harmonics :-) – Oldfart Apr 27 at 10:14

With the components shown this circuit only produces about 3.5kHz, not the 'approximately' 455.5kHz claimed by the designer. I built it on a breadboard it using a uA555, then removed the 0.002uF capacitor to get the highest possible frequency, which was ~670kHz with the pot wound down to ground.

Here is the waveform at pin 3 captured on my oscilloscope:-

Without an external capacitor the waveform is highly asymmetrical because it is timed by stray capacitances in the wiring and inside the uA555. The rectangular wave has fast rising and falling edges, so it produces hundreds of even and odd harmonics up to the FM broadcast band and beyond. The signal can be tuned in at many places in the band.

I fed a 1kHz audio frequency tone into the 555's control pin, and it was clearly picked by an FM radio placed next to the circuit. I then hooked a 30cm piece of wire up to pin 3 to act as an antenna, and was able to receive a strong signal from the next room.

The optimum length for a 'whip' antenna is ~1/4 of the wavelength, which for the FM broadcast band is 2.78m (at 108MHz) to 3.41m (at 88MHz) so 750mm is commonly used. This would tune out the lower and higher harmonics to some extent, but to reduce interference at other frequencies to an acceptable level you would need to put a narrow-band tuned circuit between the 555 and antenna.

While this circuit does 'work', its poor frequency stability, reliance on parasitic capacitance and internal delays in the 555 (which could vary depending on the manufacturer), weak output and production of many unwanted harmonics makes it no more than a crude demonstration of FM radio transmission. In most countries it is probably illegal due to the massive amount of interference it puts out. There are many simple FM transmitter designs that are much better than this one - don't use it!

• Thank you so much for giving me such an amazing and detailed explanation! I really appreciate it! And yes sir, I will look for another FM transmitter circuit to build instead of this one. – Highvoltagemath Apr 28 at 12:52
• Ok IMHO this is the best and most correct answer. – DarcyThomas Apr 29 at 6:34
• It might be better, if it explained how putting out 8V pulses with a necessary rise time <= 10ns to make it audible in the FM band also scatters harmonics from the fundamental just below 4kHz being deviated by 25% with thousands of harmonics all the way up to 100MHz FM band but of course emissions are now down by < - 100dB won't work , so the fundamental had to be boosted from 4k to near 670kHz still with up to 25% deviation enough to recover a single tone. – Sunnyskyguy EE75 Apr 29 at 16:40
• @SunnyskyguyEE75 yes, at 3.5kHz the higher harmonics are too close together and too weak at 100MHz. With the original circuit my radio did not detect any signal on the FM band. At 670kHz I had to reduce the audio input level to -20dBV to avoid distortion. – Bruce Abbott Apr 29 at 23:45
• Yes TY Bruce. Maybe we should make a challenge for younger EE's to create a 555 "radio" using PWM to modulate a 555 kHz AM carrier for AM radio all with 555's (lol) – Sunnyskyguy EE75 Apr 30 at 0:41

The author is confusing you with free running oscillator set to approximately 455.50 kHz which is the down-converted intermediate freq. , or IF frequency used for an AM radio.

FM radio's use 10.7 MHz for the IF filter.

This 555 timer circuit can generate FM square waves as a DC-biased VCO (or Astable) BUT only from 3.3kHz to 4.4kHz using the CTRL input, 0 to 9Vdc.

It is never useful for AM, FM or any radio design.

555's never have nor will be used in radio designs or any serious design for that matter.

Mr. Peter J. Vis appears to have good Windows/Network/Router skills but weak on Electronic design. I would look elsewhere for better basic electronics books such as on my profile.

The North America FM band is around 100MHz, which is a couple orders of magnitude higher frequency than a 555 can manage. You would also want a sinusoidal output so as not to splatter harmonics all over the spectrum (at odd integer multiples of the base frequency).

The circuit does demonstrate a FM modulator. It is a little wanting though.

If you wanted to use it as a transmitter well:

First of all it does not transmit at a very usable frequency, as in finding a cheap receiver that can be used to validate if it works will be difficult.

It would need a rather unwieldy antenna (for a class room/ lab demo; about 156 meters long for a 1/4 wave diapole).

I could be wrong, but I'm pretty sure it would not be legal to use as a transmitter, as it is right now. Wrong frequency band and lots of extra noise (from the overtones that you get with a square wave).

That being said, if you added a LC filter network (google: Butterworth filter), tuned to an overtone, in the 88-108 range, added an output amplifier and antenna; it might work.

Square waves (like what a 555 IC generates) are composed of a series of odd harmonic sine waves, of the fundamental frequency. So with a good filter and antenna, to remove the junk, you could get a nice-ish FM modulated sign wave out. Which (big sh-maybe) be able to be picked up by a FM radio sitting right next to the '555 transmitter'.

It would be horribly inefficient, but may demonstrate a basic working FM modulator; in a high school lab setting.

• You'd probably need a buffer to sharpen up the edges of the square wave. – Oskar Skog Apr 27 at 12:54
• It sounds like I'd better not build this circuit. It sounds like it will be a lot more work than I want. Thank you for your answer! – Highvoltagemath Apr 27 at 14:17
• @highvoltagemath if you use it as just a modulator, that you use an oscilloscope to see the output then maybe. But as a transmitter, no there are better things out there – DarcyThomas Apr 27 at 20:18
• Damm it. Now I want to try and build it and get it to work. Haha. – DarcyThomas Apr 28 at 7:00

Well, it may be not be feasible in practice but a frequency multiplier may be of help. It can be used to increase the frequency by a factor as output frequency is proportional to a harmonic of input frequency and then use a BPF to eliminate unwanted distortion and harmonics. In your case, doing some math, you need a frequency multiplier of factor around 200. It was generally used in frequency modulation circuits and in communication circuits and was operated on signals in microwave range. The best device in case of frequency multiplier is PLL(phase locked loop). But the disadvantage is that it make use of non-linear devices for which the signal may be prone to non-linear distortion or variation. Alternatively, you can use a frequency mixer to generate two frequency components 'f1+f2' and 'f1-f2' but this method may still be inadequate to reach in the MHz range.

So, as other answerer has noted that 555 is not capable to reach the radio band frequency and is not used in radio design, you should accept that 555 as FM modulator is solely for academic purpose only.

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