- Joined May 4, 2020
portable FM receiverWhen you said "FM receiver" is an FM Brodacast reeiver that tunes in the area of 88 MHz to 108 MHz?
Except when crystal controlled, I have not had much luck getting FM transmitters to operate at the desired frequency without some adjustment, so this is a normal situation.
What several of us want to know is what construction method you used.
If it is on one of those plug-in breadboards the next thing to do is get a small piece of PC board material and re-built it dead bug style.
View attachment 210753
Somebody once told me that his LED voltage boost circuit works without an inductor. It turns out that he had built the circuit on a plastic plug-in breadboard and it was operating at VHF frequencies using the inductance of the wires used to connect one part of the circuit to another. Such breadboards are often a problem when using fast components, like the 2N3904, which can oscillate at hundreds of MHz. Put your parts a little closer together than shown in the photo above because you will be working with RF around 100 MHz.
Please double-check your components -the pin-outs for the transistors, etc. and when you think everything is right, with a digital voltmeter you can check the base and emitter voltages on Q2 and see if they seem reasonable. If either the base or emitter is very close to one end of the battery or the other, then there is a bad component or a wiring error.
Make sure the capacitors are what the schematic asks for. Sometimes it is difficult decipher the capacitor's markings.
The emitter should be about 0.6 volts less positive with respect to the battery's negative terminal than the voltage on the base base.
The collector should measure the same as the battery voltage (there should be no DC voltage across L1, only sine waves).
If those look right then its time to tweak the resonance to get the thing to oscillate somewhere in the FM band.
The last couple fixed-frequency transmitters I made were tuned by deforming the inductor. But before doing that, place an FM receiver close-by and turn it on. Tune around listening for a place on the dial where the hissing noise go away is either there or greatly attenuated. If it is not a broadcast station, it is your transmitter.
At that point I would tune the receiver up to where you cannot hear any broadcast stations near the highest frequency on the dial and slowly touch the inductor and listen to see if I could hear the transmitter's frequency sweep through the bandpass of the receiver as the distance between my finger and the inductor changes. If the transmitter gets pulled into the top of the FM band by putting my finger close to or in contact with the inductor, that means that you have to either increase the capacitance across the inductor or add more turns to the inductor.
You can slightly increase the capacitor values or add turns to the coil.
If you were unable to hear any effect of your transmitter then your trasmitter is probably oscillating at a frequency below the FM band and tune the receiver to a spot near the low end of the band where you cannot hear any broadcast stations and try stretching out the inductor to raise the transmitter's frequency.
The above might help.
no, I am using a 0.5w 2ohm speaker in place of condenser micAre you using a "condenser" microphone? Is it powered?
The circuitspedia site on my desktop PC showed a BC547 transistor in a TO-92 package next to the circuit. It's pinout is different than the 2N3904 in the same package. Did you use the correct pinout for the 2N3904?
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by Jake Hertz