How it works:
From left to right, the first part is the microphone and some resistors to get it working. Next we have a capacitor and the first transistor, this amplifies the sound from the microphone so that it can be loud enough to work with. The last part, there is a transistor, a coil and some capacitors. This part generates the radio waves and combines them with the sound from the mic to transmit it thru the antenna.
The coil is made with about 9 turns of wire, use a pencil to get the right diameter for the coil. The capacitor with the arrow is called a trimmer capacitor, it has a small screw to adjust the value, we'll use it to tune a certain frequency or station to transmit on.
(click to enlarge)
11 comments:
Good explanation of a simple FM transmitter. I'm looking into making/installing one on my own car. I was also considering using this for noise reduction: photocoupler
Let me know what you think.
broken link, says item can't be found.
photocouplers, in my experience, don't add much noise reduction in analog circuits (like this one), although it could help to reduce loading of the amplifiers, is this how you plan to use it?
R6- should it be 100k, or is it supposed to be just 100?
R6 I'm pretty sure that it's 100, since we want all the current flow we can get to amplify it (mic gives very low voltage, hence low current), 100k would be a virtual open circuit and you wouldn't get any sound.
You could swap the mic for an audio jack without much change to the circuit, just connect it where the mic would go and get rid of R1.
If your equipment is sensitive or want extra isolation you could try something like optocouplers but that's not really for beginners! look it up if you are feeling curious.
What should be the length of the antenna for any particular transmission frequency? How should we set the antenna length?
For this particular circuit, the length of the antenna is the least of your worries since there's not much stability or power in the circuit to make fine tuning the antenna something worthwhile.
A quarter wavelength antenna (in this case, about 3ft long) gives you the "best" antenna for this particular transmission frequency (from 85 to 90 Mhz), but any length of bare wire should be enough to get a good transmission.
3ft is a long wire, and as i said before, for this particular circuit is not even worth the small gains.
whats the range ? can i use any ordinary radio to hear the sound ?
Can't say for sure about range, depends on many things including antenna, the transistors and other random atmospheric stuff.
safe figure i'd say from 30 to 50 ft, but you have to try for yourself to get the real figure.
any ordinary FM radio can get the signal, the tuning may be a bit off so experiment to find the correct frequency ( seek probably wont detect it, use knob/dial )
I'm curious to experiment this but I have not much knowledge in Eelectroics except that I know some basics. How do I start with? What are the required things (any references/material) and how do I go about doing the experiment? What are the things that I should buy to create the above equipment to experiement (can't understand much from the above given circuit)?
this should be a fun project for my son how would a 3 inch telescope antenna work and if the 9 turn of (copper wire)do i space them 6-8 mm apart what would this do or keep them rolled together
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