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Newbie to electronics, trying to build an fm radio

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maltesecorsair

New Member
Hi everyone:)
I am trying to learn a wee bit of electronics stuff at my old age (I'm 42). I am not too good with math but still, I am trying to learn a bit by doing stuff. Anyway, I saw a schematic for an fm radio and am trying to build one if possible. Now, bear in mind I have absolutely no real knowledge in electronics, so please bear with my rather stupid questions. In the pic, the switch is connected to the positive terminal of the battery, which I suppose is because schematics show conventional and not actual current direction, so I just have to connect the battery in the reverse direction right? Can I keep the orientation of the IC and other stuff the same or do I have to change them? Please do advise.
 

Nigel Goodwin

Super Moderator
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The battery and switch are correct as shown - don't alter them. If you reverse the battery (and why would you want to?) the circuit won't work, and you'll kill the LM386.

However, that circuit doesn't seem much of a radio to me, FM or otherwise - if it works at all, it will perform VERY poorly.
 

alec_t

Well-Known Member
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The circuitry around T1 and T2 is unconventional.
Indeed. According to an LTspice simulation it will probably act as a ~250kHz oscillator. I don't see how that will do anything useful with an FM broadcast signal.
 

JimB

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According to an LTspice simulation it will probably act as a ~250kHz oscillator. I don't see how that will do anything useful with an FM broadcast signal.
In which case it will probably act as a super-regenerative detector, which works tolerably well with wideband FM.

JimB
 

maltesecorsair

New Member
@ Nigel Goodwin: Sir, in the pic, the positive terminal of the batter is the one that is attached to the on/off switch. I thought electricity flowed from the negative terminal to the positive terminal, so shouldn't the switch be placed near the negative terminal? Like in this pic, which I got from the book Electronics All in One for Dummies, the positive of the battery is connected to the resistor and the negative to the LED so the current would flow the other way and the LED wouldn't work, would it sir?
Please be patient with me everyone, because I can be rather stupid.
 

audioguru

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Who cares which way the current flows. Simply make the polarities of everything correct. Make the anode of the LED positive like you show and the positive pin of each IC positive. Learn that the collector of an NPN transistor should be positive.
Look here:
 

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KJ6EAD

Active Member
If I were going to build an FM broadcast receiver, I'd explore the possibility of using one of the single chip solutions or at least find a design that incorporates PLL frequency control.

If you're set on building the circuit shown, you have your reasons and I'll help if I can of course.
 

unclejed613

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i think he might be better off with a real super-regen. when i was in grade school i built one that tuned the VHF air band. VHF air comms are in AM, but a super-regen could demodulate FM by tuning off center and doing "slope detection". http://www.ke3ij.com/superrgn.htm
 

maltesecorsair

New Member
First of all, I am really grateful for the replies and for you guys being so patient with my imbecility.
@ audioguru: Sir, I beg you to forgive me but in the second figure that I posted, unless I know from which terminal of the battery the current flows, how am I to make sure that it flows from the anode of the LED to the cathode? I am sorry, but I think I am being unable to communicate with clarity what I mean. Somehow, I thought the current flowed from the terminal marked on the battery with a "-" sign to the one marked "+". So I figured an LED, if put in the circuit should be placed such that it's anode should encounter the current from the - terminal, through the LED and onward to the + terminal. I'm probably a bit mad for thinking like this but I can't help it. I apologize for making you feel frustrated sir, as I think my level of idiocy is something you haven't encountered before.:sorry: I thank you for the diagram, because I didn't know that info, nor the info about the collector of an NPN transistor needing to be positive. I suppose it is the other way around with a PNP one?

@ KJ6EAD : Sir, really the only thing I know about electricity and electronics is that if I am not careful I might get an electric shock. That's my honest statement. I really should have joined up here and then looked for some kind of project to do which would help me understand electronic stuff. But I got here after I actually ordered the components for the fm radio as shown in the first pic in my first post, and I won't have any more money to buy anything until next month, so I am thinking about just trying to do this project when the components get here. I would consider myself honored if you'd help me and advise me sir.

@ unclejed613 : Sir, I have grown older but not at all wiser. I am a weird sort of person with my own incapabilities. What you folk can do with ease, may be nigh impossible for me to do sir.

So really, in the first pic, I just have to connect the positive terminal of the battery to the switch and the negative to the other point, right?
 

maltesecorsair

New Member
Well, I asked my niece about what I was asking and got what I think is a good reply. The "+" on the battery does not indicate charge but that there is more electrons there. So current flows from there to the "-" which is deficient in electrons. I think that makes sense to me, though I don't know if any of you folk would agree?
 

unclejed613

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electrons have a negative charge, and the direction of current flow can be seen with a vacuum tube, because the electrons flow from the cathode to the anode. the same happens in the LED, the electrons flow from the cathode (the bar) to the anode (the arrow). in most electronics, with a few exceptions, the negative terminal of the battery is connected to the ground. on-off switches are usually connected to the positive side of the battery, because if there is an enclosure like a metal box it makes sense to have all of the circuits inside the box at ground potential when the power is off. there is a concept called "conventional current" where you assume "current" flows from positive to negative, even thought it's actually the opposite. "conventional current" is mostly a convenient way to follow what's going on in a circuit even though it is not accurate. it can be confusing to someone new to electronics. if we were still using vacuum tubes it might be easier to understand. even troubleshooting vacuum tube circuits, "conventional current" comes into play, because the terminology calls the positive power the "supply voltage", as if the power comes from the positive supply rails.
 

maltesecorsair

New Member
Oh in an LED the current actually flows from anode to cathode ? hmm. I am learning a lot of things here, which is good. I am a bit confused though, about the term 'ground'. In the first pic, it is a ground connection between the two capacitors as depicted at the top part of the pic, I think? I , pardon my ignorance, thought it just meant I have to connect that to the negative of the battery.
 

audioguru

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ANODE means positive and CATHODE means negative but I found some backwards descriptions in a dictionary.
The polarities in the first schematic are correct and the non-off switch can be in the positive or in the negative side of the battery.

Recently someone posted that horrible "radio" schematic and said it did not work. I agree that it will not work because it is not a real FM radio.

Why do you think about the direction of current flow?? If polarities are correct (positive of the battery to the LED anode and negative of the battery to the LED cathode with a current-limiting resistor in series) then it works fine.

The schematic is missing the most important ground:
 

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Nigel Goodwin

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So really, in the first pic, I just have to connect the positive terminal of the battery to the switch and the negative to the other point, right?
Yes, it makes no difference at all which side of the battery the switch is. On a personal view I would almost always put the switch in the positive lead (as most people do), as the negative is normally chassis/ground - but it makes no difference to the operation of the circuit.

Don't upset yourself with 'electron flow' and 'conventional current flow', it does nothing but confuse you.

Again, on a personal view, I always consider current flows from top of the circuit to the bottom - this works for any circuit, without confusion.
 

maltesecorsair

New Member
@ audioguru : Thank you for the correction to the schematic sir. And ty for clarifying about anodes and cathodes.
@ Nigel Goodwin : Thank you sir, for your reply. And ty for that pointer about current flowing from top of the circuit to the bottom.
I looked up in Wikipedia about ground connections and I gather that it is just a return path for the current into the negative of the battery. I hope I didn't get that wrong. So much to learn hehe.
 

maltesecorsair

New Member
Another silly query coming up :wideyed: . In the first pic, of the schematic of the fm radio, it shows a trimmer capacitor, and just shows two connections to it. But the only trimmer capacitors I can get online or offline have three legs. Could someone kindly tell me how to connect that one to the circuit please? Two of the legs are in one plane, and the middle, third one is behind. Also, I have a couple of trimmers that I do not know the capacitance range of. Is there any way I can find out the capacitance range please? I've just got myself a multimeter which also has a capacitance mode, so could I somehow use that to find the range? Sorry for the silly questions.
 

audioguru

Well-Known Member
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They have two legs connected together which uses only one wire then the third leg uses the other wire. Use the Ohms setting on your multimeter to see which two legs are connected together.
My multimeter measures the capacitance of its internal wiring plus external leads as 200pF and without the leads measures 160pF so it will not measure the maximum capacitance of your low capacitance trimmer.
Many years ago Philips made trimmer capacitors that look like yours and their datasheet shows the color and capacitance of them.
 
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