boylesg
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Your attachment does not work.
Without seeing the schematic I am guessing that the 47k resistor powering the electret mic is connected to 6VDC. Then with a mic current of 0.5mA the voltage across the 47k resistor is 47k x 0.5mA= 23.5V which means the mic has no voltage and will do nothing.
The value of the resistor is selected to produce 2VDC to 5VDC across the mic. With a 5V supply the mic will have 2.5V across it when the resistor value is (5V - 2.5V)/0.5mA= 5k ohms.
With the circuit powered, measure the voltage across the mic. It should be 2VDC to 5VDC.
Maybe the schematic or parts list is difficult to read and the 47k resistor is actually 4.7k?
...hard to tune in to a particular frequency because your hand in close proximity to the circuit or a screw driver touching the trim cap changes the transmission frequency slightly.
And you have to turn the trim cap by minute amounts - very difficult.
I purchased a kit of plastic tools for adjusting trimmers of various types. They do not cause frequency shifts. Hex wrenches for inductors. Screwdrivers for pots and capacitors.
Very little. It's the capacitance of the tool and your body when connected to the tuned circuit that shifts the frequency. A plastic tool is best but there are some things you can do to make it more stable:I don't suppose coating metal ones with a thin film of plastic would make much difference?
Very little. It's the capacitance of the tool and your body when connected to the tuned circuit that shifts the frequency. A plastic tool is best but there are some things you can do to make it more stable:
1. put L1, the 10pF capacitor and the trimmer inside a screened can (the whole oscillator would be better), ground it but obviously leave a hole to make adjustments.
2. make sure the screw slot of the trimmer is connected to the +9V rail. It works equally both ways around but the adjuster is usually connected to one of the pins and touching the transistor end will shift the frequency more than the supply end.
I suspect your "10K 16" is a model or batch code with the "16" probably meaning it was manufactured in 2016. My guess is it isn't an electret though. They cost pennies so I advise you buy a new electret and use that.
Brian.
When I said your attachment does not work I did not say your circuit does not work. Instead I do not see the schematic, instead I see this:
Doesn't that FM transmitter produce muffled high audio frequencies? It is because it is missing pre-emphasis (high audio frequencies boost) that all FM radio stations have and all FM radios have the opposite de-emphasis that cuts the boosted highs down to normal and cuts hiss.
My FM transmitter has pre-emphasis calculated for the different amounts in the world so it sounds perfect. My circuit uses R5 and C4 to do it. C2 prevents the audio preamp from amplifying the radio frequency.
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