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FM radio receiver

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I've changed some value of the capacitors, since I couldn't get them at my place.
C2, 24pF-->22pF
C4, 2.7nF-->2.2nF
250uF-->220uF
Will these affect the operation of the circuit?
And what's the puspose of center tapping L1 with C2?

Thanks!
 
bananasiong said:
I've changed some value of the capacitors, since I couldn't get them at my place.
C2, 24pF-->22pF
C4, 2.7nF-->2.2nF
250uF-->220uF
Will these affect the operation of the circuit?
They won't make any difference.

And what's the puspose of center tapping L1 with C2?
It creates a Colpitts oscillator which is the most important part of a super-regenerative radio.
Read the article that came with your schematic or see many articles about super-regenerative radios in Google.
 
I got it here:
**broken link removed**

It says this:
The tuning capacitor, C3, has three leads. Only the outer two leads are used; the middle lead of C3 is not connected. Arrange L1 fairly close to C3, but keep it away from where your hand will be.
L2, the RF choke should not be near a ground. The same is true for L1. Capacitance to ground will disturb the feedback.

How not be near to GND? Since L1 and C2 should be placed near to each other, the other end of C2 is GND. So we can't avoid L1 to be far from GND.

The center tap must be exactly center? Because I made a 5 turns coil, if exact center needed, I have to connect a jumper from the top of the coil.
 
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The project says to use the outer leads on the dual tuning capacitor he used. Yours is completely different.

Use his pcb layout as a guide for the space around the RF choke and L1.

Use a 6-turns coil so the center-tap is on the pcb side.
 
Hi,
I've done this circuit finally. But it doesn't work :(
I'm using 6 turns coil.
I'm using 9 volts Energizer rechargeable battery (7.x volts measured)
I'm using the unused resistor pin as the center tap for the coil.

I've tuned both the trimmer capacitor and the volume control, but they gave the same result, no sound at all.

is it the problem of the bf256 transistor? Is there any transistor can be used beside this?
From Google, I found that the pins representative for bf256 are different.
Some showed the pin: Gate, Source and Drain
but some gave me: Drain, Source and Gate

but I've tried both, gave the same thing.

What should I do? :(
 
The manufacturers of the BF256 got the pins mixed up. It is easy to use an ohm-meter to find the gate. The gate is the anode of a diode to the source and to the drain. It is difficult to measure which pin is the source or the drain.

The voltage at the source should be a couple of volts DC.
Try a different numbered FET.
 

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Yes, I found that too. #1 definately wrong. But I've tried both #2 and #3.
The website said that, if the circuit is not working (no sound exactly), I may need to change R4 to 20K (up to 50K). Besides 2N4416, J310 and MPF102 are recomended. Hopefully I can find either.
 
Measure the DC voltage at the top of R4 and tell us what it is. Then we can see if the FET is working.
 
The voltage across R4 (after changing to 33K) is 0V (I'm using analog voltmeter, the battery is 7 volts measured). And, I could't find 2N4416, J310 and MPF102. They have only replacement for them:
2N4416-->BF256
MPF102-->BF245B
:(
 
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bananasiong said:
The voltage across R4 (after changing to 33K) is 0V.
Then the FET is not doing anything. It should be conducting and creating about 2V actross R4. The pins might still be mixed up.
 
audioguru said:
Then the FET is not doing anything. It should be conducting and creating about 2V actross R4. The pins might still be mixed up.
Oh no, both GSD and DSG gave me the same result. Should I try SGD or DGS? That is troublesome, I have to change their location..

*G=Gate, D=Drain and S=Source
 
Try every combination of pins for the FET until it draws some current and starts working.
 
Hi,
Now I can hear some buzzy sound. I was using enamel coated wire as jumper! That's why they were not functioning and I couldn't hear anything! I've measured the voltage across R4, it is around 3 volts with analog multimeter. With the FET's pin is GSD. I've tuned the trimmer capacitor, but I can hear only buzzy sound.
 
Horray, you got your radio working!:D
Maybe it is picking up the video from a TV station nearby, or interference from an electric light dimmer.

It should pickup strong local FM stations and aircraft radio communications if an airport is nearby.

I don't see an antenna on the schematic. Try connecting 20cm of wire to the drain of the FET as an antenna.
 
Hi,
An antenna is now connected to the drain, almost 30cm long. When I tune the trimmer capacitor, within 10 degree I think, I hear some noise (like when you plugging in the audio jack). For the rest, I can hear only buzzy.
I think I have to change my component placement...
 
Try connecting a 22k resistor across R4 to see if it is better or is worse.
Do you have a strong local FM station near you?
 
audioguru said:
Try connecting a 22k resistor across R4 to see if it is better or is worse.
Without removing R4 (33K)?
Do you have a strong local FM station near you?
No, there is no FM station in my state. Most of the FM stations in Malaysia are located more than 100km from where I'm staying.
 
bananasiong said:
Without removing R4 (33K)?
By touching a 22k resistor across the existing 33k resistor, you instantly change their total value to only 13.2k to see if it helps with the lower resistance.

No, there is no FM station in my state. Most of the FM stations in Malaysia are located more than 100km from where I'm staying.
Then the one transistor FM radio will pickup nothing.
What did you expect it to pickup?
 
stevez said:
I do not think it is pointless to construct such a radio - if the point is to learn. It is not a replacement for a good design that has been well constructed and the performance will be poor at best. ...

Agree totally

Think of it as this is the 1st stage.
That q1 will give about 5 to 7 bB gain

Another Q(2) at the node of r1, r5, r7, c4 will get you another 5 dB
this will get you a radio receiver, not a boom box, but a real RX

You can adjust the audio by ading a 2N2222 at the node of r1, r3, d1, c9 node

master the building blocks, this simple radio is a beautiful design to learn about how the value of components affect your Rx

redserpentone
 
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