I am building a small FM transmitter because we have a neighbor who likes to get plastered and blair the radio. I know it is the radio and what station, and I was hoping to override it with something more preferable, or just silence (nice thing about FM is that you can override it with silence) I found this http://www.pcs-electronics.com/schematics/SimpleFmTransmitter.pdf and it is looking nice and simple and I like it. But I'm not sure if it will have enough power to override the signal. Can I just use a suitable npn transistor with the collector to a higher voltage supply (9v probably)? Also the P1 and P2 in the schematic are variable resistors right? and P2 is only 200 ohm correct (the E is messing me up) Though I could be a total asshat and play a loud 60Hz N wave and really mess with him. Or maybe a nice loud sweep going from 1K-5K and then hold it at 5K for a while. That would hurt your ears. EDIT: I forgot the most important part, the antenna. I need a small antenna becaue I may want to make this portable, I was thinking about just having a piece of wire (or maybe a telescoping antenna from an old radio, go figure )adjusted to the desired quarter wavelenth, will that work? I know I have seen many more complicated antenna designs online.
hmm about how many walls will it have to go through would you suppose? Its a great idea, i hope it turns out for you!
warning: I'm only going to say something useless here, so if you want answers, don't read this post. mmmmm, sounds wonderfully illegal. I was actually considering doing the very same thing to a neighbour of mine. Fortunately he only did it one night, but it drove me so batty that making an FM transmitter became very attractive. I figured I'd broadcast my own voice saying, 'hey, buddy, turn it down' or something... so my heart goes out to you.
It will only have to go through one wall, concrete though. I know it is illegal but the FCC has to catch you. and I would think that this would be low enough power that is could be classified (pawned off as) as just a homebuilt adaptor for putting your CDs on a radio. I looked around for antennas and for in the house I'm considering a dipole antenna, since it can be bi-directional or omnidirrectional. But it needs a balance signal, can I just throw another NOT gate in after the last one and get the -ve from that? or would I have to use 3 gates? 1 gives the -ve, and the other 2 take the signal and invert it twice giving the +ve or will that introduce a propagation delay and make the signal out of phase? Maybe use an AND gate with one input tied high rather than to NOT gates. But I still wan't to use it on the school bus, and a dipole would just be too big, can I use my first idea that way it will be compact. (the local rap station may become a trance/chillout station, or maybe classical. Put everyone to sleep. ) EDIT: Using a homemade wi-fi antenna is technically illegal, since it isn't FCC certified. But most people don't seem concerned about that.
ConK, one thing I think you're confused about is how to invert the signal. You're talking about using logic gates, which will work just about as well as using a sewing machine to solder SMT components. An FM signal is AC, while logic gates work with DC signals. You'd get a nice oscillating output at the exact frequency of the FM carrier wave, but that would do jack as far as transmission goes. I think what you're looking for would be a dual op-amp setup, one buffering, one inverting, both with a gain of 1. That effectively splits the signal into two signals 180° out of phase from each other, and there's no need to worry about propagation delay as both are running through effectively the same circuit, so the delays will be equal.
Whypick ,did you look at the transmitter schematic? Its a 74HC04 (hex inverter) 5 resistors, 2 pots, and 6 caps. The signal is being output by a logic device, so I would think that I could use logic to invert it.
Hmm, I see. I suppose that a buffer hanging off of IC1/E should be suitable, provided it has the same propagation delay as the 74HC04.
Ok and op amp is starting to sound like a good idea, since I can vary the input voltage one it. I just started poking thouhg mouser and came across the TSH93IN high speed op-amp datasheet The bandwith goes up to 150 mhz, so I'm set for FM radio, it can take up to 14V supply, so I'm set there. The one thing that confuses me is that is has an "input ofset voltage" of 4mV. What is input offset voltage? The differential input voltage is +-5V so that means the highest differnece between the inputs for each amp is 5V, right? And, finaly I run the output from the transmitter into the inverting input on one amp(amp "a"), and the non-inverting on another(amp "b"). what do I do with the other input? Do I tie the non-inverting on amp A to ground, and the invertiing on amp B to +5v (the logic Vcc) ? Or do I use the other input with a voltage divider as a gain control?
Hmm, from what I find it seems that the feedback is for controlling gain, Something I'm not too worried about, I can just vary the voltage. If you just feed it a balanced signal it would seem like it would apply maximum gain. If that is the case I wan't to take the output signal from the section that modulates the RF(the above schematic), feed it into a 74VHC04 inverter (lower propagation delay than the 74HC* stuff) and a 74VHC125 buffer. (same propagation delay as the inverter), take the signals from those and feed it into the op-amp to get a balanced signal. From that I'll run dual coax cables to a dipole antenna. Or should I use a single cable with the -ve on the shield? If I'm reading the datasheet correctly, the op-amp seems to like a 600ohm load? How much impedance would a dipole antenna provide? I would probably take the antenna leads, insulate the ends with teflon tape, and clamp them into an Al or PVC block. (most likely PVC since I have some already) I don't have the stuff to measure impedance, just a plain ol DMM.
just a side note on legality; All small band consumer electronics, as per FCC regulations, "Must accept any broadcast interference, even those that may cause unwanted operation" FCC rules, part 68 though it also says that it cannot CAUSE interference, but you arn't moding a device to do that, you are creating a device for that reason. And if you have your HAM licence, you are a radio operator/technican and are fully complying within the rights
Nope no ham license, I'm just asshatting around on the radio because the neighbors are a pain. I would like to do some sutf with a HAM radio, but right now funds are limited.
