# Silly me trying to build an FM Radio.



## buccaneer (Mar 3, 2005)

Hello everyone:grin:








I am trying to build an FM radio with the schematic as below. 









I have a couple of questions (noobish silly ones of course!) please. One is, for the volume control, I couldn't get a logarithmic pot, so I am using a linear one but how do I connect the three legs? I mean do I just connect the wire from pin 3 of the IC to the middle leg of the pot? How about the other two legs? 

Another is, I have a few variable capacitors which arent labelled so I do not know their capacitance, they look like the bottom pic:








They have three legs, instead of two so how should I connect one to the circuit and how can I find out their capacitance ? They dont have any numbering on them. I have been told that I cant find out the capacitance with a multimeter though so I wonder how to go forth from here. In the pic, it says 22p VC, but would 3-18p be good enough? 



I hope I will get someone to patiently explain stuff to me as I am not too brainy, but still hoping for some help hehe. :hide:


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## Done_Fishin (Oct 10, 2006)

Hi Buccaneer 


Get out from under that chair and sit on it !! :grin:

With regards to the Vol control .. normally the two outer pins are the resistance and the middle pin is the wiper ... I would suggest that you wire it up with leads so that if you find your Volume increases when you try to turn the volume down .. you can just reverse the outer two wires to make it work right.

With regards the ceramic trimmer .. I believe it's similar .. you should be able to look at it and see what pins are connected to which part of the capacitor .. the wiper will be connected to the part of the plates that moves and the other side(s) ... if the third pin is connected to anything .. ignore it or short it to the wiper .. I have a strange feeling though it's just there for stability of the component when soldered to the board .. It has been a long time since I have handled one so my memory is a bit hazy. 

The value may of may not impede the functionality .. most of these devices have a tolerance and may have several pF difference in the value stated to the actual value .. I would expect that 3-18pF would be the guaranteed working value. Only way to check is to check the value is with a multimeter that has a pF range, but don't forget that your test leads will also add a little capacitance to the reading, as will track layout on the board if you are not careful. Result of not having that 18-22 pF would be that you may not be able to receive some frequencies.

Good luck with the project .. not being silly at all !!!

:thumb::thumb::thumb::thumb:

Edit: .. you may find that the volume control doesn't work the way you expect it to with a linear pot instead of a log pot .. so be prepared to try and find the correct device .. 

Linear devices tend to change the resistance equally across a certain distance whilst log ones tend to cover 1, 10, 100, 1000, over similar distances .. if that is not clear to you consider a ruler either in inches or centimeters .. the linear will go 1-2-3-4-5-6-7-8-9-10 for each unit of length whilst the log one will have, in those same unit lengths, 1-10-100-1000-10000-100000 etc


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## buccaneer (Mar 3, 2005)

Ooo ty sir for your kind reply.:grin:
I am glad you replied and explained about these two things sir. For some reason, I cant find a log pot online or at the electronic parts stores. I went to the part of the city which has some electronics parts stores and nobody has a logarithmic pot. Furthermore, the electronic stores in the city has volume controls with 5 pins. 









I wonder what the pins 4 and 5 are for. Maybe to input power/voltage? I am at a total loss as to how to connect those. However, I do have a pot with just 3 pins, which I bought from an online store. If you have any thoughts about the 5 pin pot, please do share sir. 

Btw my multimeter which is Fluke 101, does have a capacitance option, would that be enough to measure the trimmer sir? Or do I need a multimeter with a capacitance range option?
Thank you again, sir, for your kind input.
Edit: the pots kinda click 'off' when you turn them anti clockwise far enough. So that must be an on/off switch built in?


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## WereBo (Apr 5, 2008)

Hi buccaneer :wave:

The 'pot' you've shown is indeed a 'switched pot', you can connect the 2 terminals (4+5) either way round :wink:


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## buccaneer (Mar 3, 2005)

Hey there Werebo!:grin:
Hmmm so I have to connect the battery to the terminals 4 and 5? You seem to have forgotten that I am an ID10T  and would like to humbly ask if I should directly connect two more wires from the battery to the five terminal pot or should I somehow run wires from the very first diagram to the terminals 4 and 5 so that when the pot is in the 'off position, no power runs through the circuit? I am not sure how I can do that, so umm umm perhaps you could walk me through that with an edit to my first schematic diagram, if possible? I know I am asking rather stupid questions, so accept my humble apologies please. 

Also sir, if you have some time to spare, could you look at the schematic and tell me whether the capacitor c7 (towards bottom, right) needs to be like 47nF 25 Volts? I am asking because I only happen to have 47nF 100 volt rated capacitor, so would that be like really stupid to put in?


Rest assured that I am not trying to intentionally ask stupid questions, I just dont have a clue about electronics and I am trying to learn by doing, which might be a dumb idea? I dunno.


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## Stancestans (Apr 26, 2009)

Lot's of good materials to get you started on your new endeavour.


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## buccaneer (Mar 3, 2005)

Oooo! Thank you sir, for the links. I will download the pdf's and try to assimilate the info therein into me brain. Ty so much :grin:


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## Done_Fishin (Oct 10, 2006)

Sorry for my absence .. I was away & forgot to refresh the page and hadn't seen that the thread had progressed 



buccaneer said:


> Hmmm so I have to connect the battery to the terminals 4 and 5?


No you would feed (normally) the positive side of the battery to one terminal and from the other you would then feed the positive rail of the circuit .. if you fed both terminals to the switch it would most likely burn the switch contacts and possibly cause the battery to overheat or in a worse case .. EXPLODE!!
Feeding through the switch allows you to control when the circuit will take power or be functional



buccaneer said:


> would like to humbly ask if I should directly connect two more wires from the battery to the five terminal pot or should I somehow run wires from the very first diagram to the terminals 4 and 5 so that when the pot is in the 'off position, no power runs through the circuit? I am not sure how I can do that, so umm umm perhaps you could walk me through that with an edit to my first schematic diagram, if possible?


You would feed the wire exactly as it shows in your diagram .. Battery + connects to one terminal of the switch and the other terminal will connect through to the circuit .. it doesn't matter which way the switch is wired, just make sure that you don't leave any strands of wire or solder atn the terminals that could make contact from one terminal to the other 




buccaneer said:


> Also sir, if you have some time to spare, could you look at the schematic and tell me whether the capacitor c7 (towards bottom, right) needs to be like 47nF 25 Volts? I am asking because I only happen to have 47nF 100 volt rated capacitor, so would that be like really stupid to put in?


In electronic circuits you can usually ALWAYS use a Capacitor Voltage Rating that is bigger than the one recommended or stated in the diagram .. but NEVER use a Voltage Rating that is smaller .. 



buccaneer said:


> Rest assured that I am not trying to intentionally ask stupid questions, I just dont have a clue about electronics and I am trying to learn by doing, which might be a dumb idea? I dunno.


Someone has or used to have a signature on this forum saying there are no dumb questions .. except the questions you didn't ask !!!


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## Done_Fishin (Oct 10, 2006)

https://www.ebay.com/p/22k-Log-Track-16mm-Variable-Resistor/932568829


Just found this on ebay if it's of any help .. you can mount it on your PCB (Printed Circuit Board) or carefully solder wires to the pins .. No switch incorporated though


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## buccaneer (Mar 3, 2005)

Hello Done_Fishin ,:thumb:
I thought you had had enough of my stupid questions and decided to walk off from the thread instead of having to bash your head against a wall after seeing my noob questions lol. 

Yikes. Good job I asked the question about whether I should wire both battery terminals to the pot. I dont want a battery exploding, no sir hehe. Hmm I think I understand your explanation, and I think if I use the switched pot I can do away with the independent on/off switch. 

So let's say I wired the + of the battery to pot leg 4, from leg 5, I have to connect to the circuit in the first image I posted, near the top right of the diagram before the capacitor C9 connects to the positive rail ? Maybe it makes no difference where the power is put in in a circuit, but due to my dearth of knowledge, I have this tendency to route the power exactly as in a diagram so that the components shown as nearer to the power source gets power first. 

Oh thank you again sir, for that pointer about capacitor voltage rating. I was kinda stupidly thinking a 100v capacitor might discharge enough power to kill me if I touched the circuit even if its capacitance was small. 

I am deeply grateful to you sir, and the others on here, who never ever even roll their eyes at my nutty queries but patiently explains things so poor me can understand stuff. 

(You can roll your eyes, I dont mind hehe:smile


I unfortunately do not have a credit card to buy from international sites sir, and ebay.in just recently got bought up by some other company so I have to rely on local sites. I will look for a log pot in amazon.in though.


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## Done_Fishin (Oct 10, 2006)

I remember back to the days when I was studying Electronics and literally knew nothing .. College course was useless since they expected you to already know stuff and not even the course books were of any help .. It was years later that I understood that a transistor had to be put in circuit in order to amplify a signal .. All we ever got was a diagram of a transistor showing that a waveform in to the base was amplified either to the Emitter and /or the collector !!! :banghead::banghead::banghead:

That is why I am here and why I say to you to stop criticising yourself .. ask as much as you want .. if it is in my field of EXPERIENCE I will help .. I know only too well what it is like to be confused by books and teaching folk !!


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## buccaneer (Mar 3, 2005)

I didn't know that a transistor actually amplified a signal.oooo! Might I say that you are not only an electronics guru, you are a great morale booster too sir!:flowers:
I am sorry for the late reply, an uncle of mine passed away on Sunday and I was caught up with the funeral stuff. 

I am currently, slowly trying to lay out the components on a breadboard and I will of course post again when I make progress or run into any difficulties. 

Thank you again, Done_Fishin.


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## Done_Fishin (Oct 10, 2006)

I'm sorry for your loss .. always sad when we lose a family member .. if you were close remember him for the good times that you had together and respect any disagreements that you had for what they were .. just disagreements !! Everybody is entitled to his or her own opinions .. even when they disagree with our own ..


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## buccaneer (Mar 3, 2005)

Oh it is okay sir, he was 90 and was bedridden and starting to get bedsores. It is better to go away than to silently suffer I think. We did have disagreements but nothing major. I thank you for your kind words sir.


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