FM radio receiver

[bananasiong]

I have just made a coil, the copper is as thick as 2mm. I used calculation to find the number of turns, length, area to work with the 33pF variable capacitor at around 100MHz. I found that, the copper of the coil is too thick and cannot fint in to the veroboard, can I file the leads to be thinner so that they can fit in? Will this affect anything?

Thanks

 

[Nigel Goodwin]

I have just made a coil, the copper is as thick as 2mm. I used calculation to find the number of turns, length, area to work with the 33pF variable capacitor at around 100MHz. I found that, the copper of the coil is too thick and cannot fint in to the veroboard, can I file the leads to be thinner so that they can fit in? Will this affect anything?

Not enough to worry about - however, you're wasting you time calculating anything at these frequencies - the stray capacitance and inductance will make your calculations way off.

 

[audioguru]

I used 1mm wire for the coils on my FM transmitter and they fit the holes in my stripboard perfectly. My stripboard is a Veroboard copycat.

 

[bananasiong]

I've found a question when doing the coil. Even there is no much difference for 5 turns and 6 turns at that high frequency, but I just want to make sure that the way of making it is correctly. If it is turned 6 turns for example, the 6th cycle is not complete right?

 

[audioguru]

You can make the coil 5.75 turns then its ends are not in the same line. Adjust the trimmer capacitor to compensate.
Or you can make the coil exactly 6 turns if you bend the ends so they are in the same line.

 

[bananasiong]

Okay, I got it. Thanks!

If I use LM386 audio amplifier, the volume is adjustable and as high as supply voltage?

 

[audioguru]

If I use LM386 audio amplifier, the volume is adjustable and as high as supply voltage?

If you use a volume control you can adjust the volume from zero to 250mW into an 8 ohm speaker with a 6V supply if the input signal level is high enough. With a 9V supply the max output is 563mW.
Your super-regen "radio" might not have enough output level to drive the LM386 to its max output.

 

[Hero999]

Solder on the unused metal on PCB?

No, solder on the tracks you're using. You're better off removing the unused tracks.

Why?

It doesn't matter in your circuit, I was just providing additional information.

I think cut short the size is easier for Veroboard.

Coating the tracks with solder makes a difference because it increases the conductor's crossectional therefore reducing both the resistance and inductance. I agree though, it's true that making the tracks as short as possible will have a greater effect than coating them with a layer of solder.

 

[bananasiong]

Your super-regen "radio" might not have enough output level to drive the LM386 to its max output.

The how much is the maximum can it give if the supply voltage is 9V?

Coating the tracks with solder makes a difference because it increases the conductor's crossectional therefore reducing both the resistance and inductance.

Yes, that's the key word to make me understand, thanks. But coating the tracks with solders makes the board looked dirty if it is not done well.

 

[audioguru]

The how much is the maximum can it give if the supply voltage is 9V?

It is a very simple, cheap and crappy "radio".
Its output level will change with the strength of the signal until it overloads.
It doesn't have an FM detector, you tune it to the side of an FM station and it detects FM on the slope of its tuned circuit with high distortion.

On a real and good FM radio the volume is the same for a weak distant station as it is for a strong local station and it doesn't overload. Its distortion is very low.

 

[bananasiong]

It is a very simple, cheap and crappy "radio".
Its output level will change with the strength of the signal until it overloads.
It doesn't have an FM detector, you tune it to the side of an FM station and it detects FM on the slope of its tuned circuit with high distortion.

On a real and good FM radio the volume is the same for a weak distant station as it is for a strong local station and it doesn't overload. Its distortion is very low.

Can I know the percentage of this circuit being done? Among you, your friends or anyone doing the same thing. I just want to know it.

Thanks.

 

[audioguru]

Can I know the percentage of this circuit being done?

It might work. It won't cost much and won't take long to make. Try it.

 

[bananasiong]

It might work. It won't cost much and won't take long to make. Try it.

Yes! I just started to make the coil and purchase the parts. Thanks!

 

[bananasiong]

Hi,
I've just modified the circuit, but I'm not sure whether correct or not.
1. I use only a 33pF variable capacitor instead of four.
2. I couldn't find 2N4416 here, I replace with BF256.
3. I replace TL431 with LM386 circuit.
4. Can I use rfc other than 22uH?
5. Should I use trimmer screw driver for tuning?

Thanks

 

[audioguru]

Your circuit is nearly perfect.
C8 doesn't need to be a high value. Use 10uF instead.
Add a 100uF or 220uF supply bypass capacitor across the supply to bypass and filter the audio supply, and add a 1000pF or 2000pF ceramic disc capacitor across the supply to bypass the RF.

The value of the choke doesn't matter. 10uH to about 100uH is a high reactance at 100MHz which is what is needed.

The trimmer capacitor has its adjustment screw connected to one of its 2 terminals. Ground that treminal. If a metal screwdriver gets near the tuned circuit then the tuning will change. Use a plastic screwdriver to adjust it.

 

[bananasiong]

Is this correct?

What is the "10" resistor connected between GND and the 0.05uF capacitor? Is 10 ohm?

The trimmer capacitor has its adjustment screw connected to one of its 2 terminals. Ground that treminal. If a metal screwdriver gets near the tuned circuit then the tuning will change. Use a plastic screwdriver to adjust it.

Do you mean, ground either one at the side and use the center one to connect to the other side of the circuit?
The shop owner told me that the trimmer screw driver can be used to tune trimmer cap. It has a very small flat material in front and it is very long, so that my hand won't be near to it when tuning.
But I don't know whether this is suitable to be used or not because the shop owner will say anything to get their stock sold. And I couldn't find a plastic screw driver.

A trimmer capacitor doesn't have blocker right? I mean, it can turn 360 degree?

 

[audioguru]

What is the "10" resistor connected between GND and the 0.05uF capacitor? Is 10 ohm?

Yes, 10 ohms. The 0.05uF cap can be 0.047uF.

Do you mean, ground either one at the side and use the center one to connect to the other side of the circuit?

Just look at it or measure which terminal connects to the adjustment screw and ground that terminal.

The shop owner told me that the trimmer screw driver can be used to tune trimmer cap. It has a very small flat material in front and it is very long, so that my hand won't be near to it when tuning.

If it has a shaft of plastic, not metal, then it is fine.

A trimmer capacitor doesn't have blocker right? I mean, it can turn 360 degree?

Usually they can turn around and around.

 

[bananasiong]

Just look at it or measure which terminal connects to the adjustment screw and ground that terminal.

Only one out of three is connected to the screw?
Can I use Ohmmeter to measure each pin with the screw to know whether which one is connected to it?
Thanks!

 

[audioguru]

Only one out of three is connected to the screw?

It might be one terminal or it might be two terminals.

Can I use Ohmmeter to measure each pin with the screw to know whether which one is connected to it?

Of course, the screw will be a very low resistance (actually a short) to one or to two terminals.

Haven't you finished making the radio yet? It has only a few parts to connect together.

 

[bananasiong]

Thanks!

Haven't you finished making the radio yet? It has only a few parts to connect together.

Hehe, I just started purchasing parts yesterday and I've done the coil. i need to file the leads of the coil so that it can be fixed into the veroboard.
Besides, I've to attend class and work as well, that's why my progress is quite slow. I'll try finish it asap.

Thanks!