Presentation
The kind of gadget particularly appreciated by lovers of electronic ...An FM transmitter (FM = Frequency Modulation = Frequency Modulation), also sometimes called "Micro Watch", because it can transmit through a microphone, a conversation from one point to another without a wire connection. But let's call it FM transmitter so here, because of course you know that it is forbidden to spy on someone without telling him (what a strange idea, too, as someone to spy after be informed of your intentions). This assembly is a transmitter, and you'll need a receiver to complete the chain of transmission. Any conventional FM receiver allows to receive the band 88 to 108 MHz (VHF) is perfect. This page has four different patterns:
FM broadcast in stereo or mono?
The transmitter described here provides a broadcast of an audio signal in mono, even those with an entry "Stereo".Diagram of the FM transmitter
If you take the time to search the net you will find lots of patterns of FM transmitters. Myself have made several types. One whose description follows, but other smaller operating with a supply voltage as low as 1.5 or 3 volts. This scheme gives a good fit, but I must still warn you: its scope is very good, but its consumption is not negligible. A 9V battery will last about two hours saline continuously, an alkaline will last up to 5 hours. Its main advantage lies in safe operation, and in its simplicity. In addition, all components are common and very cheap, which should eventually convince the start of the test.
RF side
The oscillator is realized with a transistor (Q2) mounted in an oscillating circuit comprising an inductor (coil L1) and adjustable capacitor (VC1 and VC2). The adjustable capacitor VC1, combined in parallel to the coil L1, is the resonance circuit will set the transmit frequency. There will be two ways to change the transmission frequency, either by varying the value of the adjustable capacitor VC1, either by spacing or a bit of tightening turns of the coil L1. The adjustable capacitor VC2 is used to adjust the reaction rate and ensure reliable starting of the oscillation at each power (it is well known that an amplifier should not oscillate oscillates, and an oscillator should does not start to oscillate forever). Personally, I replaced it several times with a fixed adjustable capacitor of 6.8 pF or 8.2 pF and it still worked. But just to tell you this for your copy do the difficult. So do as you feel. The polarization of the transistor is provided by the emitter resistance of 100 ohms and the resistance of 22 ohms based. It could not be simpler.
Amplitude modulation or frequency?
This type of issuer, oversimplified, produces a residual amplitude modulation (AM) which is not negligible. The amplitude modulation produced will not be seen by FM receiver, but be aware that there are recommendations regarding the value of the maximum amplitude modulation can produce a FM transmitter. We stay here in amateur, it's the only reason for not trying to do better. Anyway, with this type of arrangement, it is hardly possible to improve this.LF side
Frequency modulation is performed by varying the polarization of the transistor mounted oscillator (Q2). This is ensured by Q1, whose collector-emitter resistance will vary depending on the signal applied between its base and collector signal from a microphone-like "crystal". The dc bias at the base of transistor Q1 is provided by the resistor R1 68K.Choice of microphone
A micro crystal? But it's really archaic!why not take a conventional dynamic microphone? Well for two reasons: the first is the simplicity of the transmitter, the use of a dynamic microphone, whose output impedance is much lower, would require an input stage a bit more complicated. The second reason is that the crystal microphones still exist, they are cheap, they are light, and they are perfect for this kind of experimentation. One word of warning, shorten the length of the mic cable to an absolute minimum, because the more the wire is long, and the unintended effects of frequency drifts are felt when you move ...
Realization of the coil L1
The choke (coil) L1 must be made with the wire 8 / 10, the term 8 / 10 corresponding to the wire diameter in mm. So use wire 0.8 mm in diameter. You coil on the body of a large felt pen, to obtain four turns with a diameter of about 8 mm, each turn is spaced from its neighbor from 2 mm to 3 mm. Then from the end to be connected to the two adjustable capacitors VC1 and VC2 (bottom of diagram), you expect a turn and you scratch the varnish that protects the copper wire, so that a weld in this location small wire to act as a cable that will be taken through to the antenna via C2 (taken at 1 / 4). One third of laps later, you do the same in order to weld this place a small wire that will act as intermediary for making the collector of transistor Q2 (taken at 1 / 3). In doing so, you should get something like the following picture:
Prototype
A small example embodiment, plate experimentation pellets.
Starting and settings
After carefully checking the wiring, you can make an adjustment. To do this, move next to a radio commercial (on preference), and turn your radio on. If you hear a little "plop" into the receiver at the moment is a good sign (but if you do not hear anything, it does not mean that it does not work). Place the adjustable capacitors VC1 and VC2 in their mid-position (half of the slats retracted). Prop your FM receiver to the bottom of the FM band (88 MHz) and speak into the microphone or tappotez lightly, just up the FM band. At a given moment, you will fall on the frequency. If you have come to no avail any FM band, repeat the operation after removing all the blades of adjustable capacitor VC1. If unsuccessful new route of the FM band, repeat the operation after entering all the strips of adjustable capacitor VC1. If still no success (you are not lucky there), space just the turns of the coil L1 and start scanning the FM band. If anything ever again, tighten a few turns of the coil L1, and Hop, left for another round! If after all that you do not hear anything, it's not a good sign, it makes you re-check all wiring.And scope?
I will not be like those you sell products to any fact or as a kit and tell you that the scope can be obtained by direct vision, without obstacle, with a super sensitive receiver ... The scope depends, as for any set transmitter / receiver, the transmission power, quality and receiver sensitivity, antennas, and barriers between the two. You may well get a range of 300 meters in open terrain, and do not get a scope of 10 meters in an apartment with armored walls. One thing is certain. Of all the FM transmitters I have built simple, that's the one who "carried the farthest." But I repeat: at the expense of autonomy.Aerial length
I almost forgot! The length of the antenna plays a lot on the performance of the assembly. Do not, however, looking to use an antenna "cut" in the quarter-wave, half wave or full wave (3 meters anyway), it would be disastrous ... Instead, try with a length of several centimeters to several tens of centimeters, you will get results already very nice (recommended length between 5 cm and 20 cm).PCB
A circuit was designed, but no obligation to follow. If you decide to make, choose preferably a glass epoxy circuit, which has VHF losses less than the Bakelite.
Artwork
You can also join me for my first small assemblies: assembly "wheel", ie components directly welded together. Advantage for this type of production: short connections, which is good for an RF assembly. Disadvantage: troubleshooting more difficult, requiring to be more careful from the start.
Various remarks
- A hum can be observed in certain situations. It is almost certain if you use a power supply instead of 9V battery, even if the power supply is well filtered and regulated. Sometimes the change is taking the mains power supply (phase / neutral inversion) solves the problem ... A hum can sometimes be seen even when the OOD is on battery.- Reception on several places on the FM band for the same frequency setting issue: this is related to parasitic behavior of the radio all the more important that the transmitter is near. It can be said for simplicity that the RF signal received is too strong and it creates entertaining "ghost". The only way to know the main transmission frequency is to remove the transmitter and the receiver to see how often the signal is received loud and clear.
- The adjustment range of the emission frequency is very large, which means that the setting is rather sharp (not easy to lock onto a precise frequency). It is a disadvantage of this type of transmitter. When more serious attack of issuers, we are sometimes faced with the opposite problem, namely difficulty of covering the entire band 88-108 MHz with a single set ...
- Heat of the transistor Q2: this is normal, the emitter-collector current is quite high. Can be reduced by increasing the value of the emitter resistor of Q2, to the detriment of course, lower transmission power and thus its scope.
- Frequency stability: quite fussy, it depends on the position of the transmitter in the room, the proximity of a hand, the supply voltage, temperature ...
- Relationship between the value of supply voltage and frequency of issue: yes, there is a clear link, so the use of battery instead of a battery is recommended.
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