# Assistance in decoding a circuit diagram

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,194
have no meter. This is my first electronics project.
A cheap multimeter is one of the first investments in this hobby. Without it you'll be lost when it comes to finding faults in your circuits.

#### yolo

Oct 28, 2019
17
Okay I'll buy that capacitor. I cannot go for a kit now because I already bought other components and I cannot afford another kit now. Since this is a school project, I have very less budget to work with. That's why I preferred not to go with a kit as they are costlier than building from scratch.

#### yolo

Oct 28, 2019
17
@Harald Kapp check out the YouTube video link at 1:07 . I guess you will know how to connect the variable capacitor then. I could not figure out by looking at the video.

#### BobK

Jan 5, 2010
7,682
I agree that two of the pins might be connected together. You can determine that using any little circuit, like an led and try connecting one side of the battery through any two connections on the capacitor.

Bob

#### Audioguru

Sep 24, 2016
3,653
That circuit is the simplest, worst quality FM radio that I have ever seen and it is missing most parts that are in a real FM radio.
I think a radio kit will be the best for you to learn how to solder its parts to a circuit board that has a correct layout. It will have the correct parts and will have a good design of a real FM radio circuit that works well.

#### mike wax

Oct 10, 2016
34
if you can't get a meter, you can still get a primitive continuity tester. Get a 12V battery and a wire and short the negative and positive terminals together. It will make a spark when you try to connect the wire from one pole to the other. The spark tells you that the wire is conducting some current. This will also work with a 9V battery but the spark will be really small. Or with a li-ion battery but the spark will be really BIG so you must use a REALLY small wire.
So now if you have a 3-terminal cap, then you use the same method to see if two of the terminals are connected together. They will pass current from one to the other. It won't damage the cap.
But if the 3rd lead is just an anchor lead and is not connected to either side of the cap, then you have a problem.
of course you could also look for the data sheet for the cap in question. if you know the make of the part.

#### yolo

Oct 28, 2019
17
I got this from some website. Can someone figure out what it says? I guess it should say that two collinear pins are short together . Can someone confirm?

#### Audioguru

Sep 24, 2016
3,653
Yes, the two pins at the sides are common (connected together). The pin at the front is the other connection to this variable capacitor.

Oct 28, 2019
17

#### yolo

Oct 28, 2019
17
Finally completed the project guys. Thanks a lot for your help. It works but very very mildly. That'd be enough for my project. Without your assistance I couldn't have done it. It costed 300 rupees which translates to 4 dollars approx so it's really coat effective which was my motto from the beginning. Now I've to figure out how all the parts work together to make this radio work. Any help would be really appreciated. Thanks again guys.

Moderator
Nov 8, 2019
2,920

Oct 28, 2019
17

#### majoco

Nov 10, 2019
50
The reason for the two pins connected together and to the adjuster of your variable capacitor is to ground the adjuster - those two pins should be the ground connections - this prevent the tool you are using to turn the vanes affecting the capacitance.

Also foil wound fixed capacitors often have a line around one end which signifies which lead is connected to the very outside bit of foil so that you can join this end to ground so as to act as a screen around the inside foil.

If you want to make your radio louder (but not more sensitive!) you can increase the audio gain a bit. You'll need a 1.2kohm resistor and a 10uF electrolytic capacitor, join them in series and the join them from pin1 to pin 8 of the LM386. More info here...

Well done in getting your project to work - you can proud of it! I remember the very first transistor radio I built back in the 1950's - a three transistor reflexed broadcast band kitset radio in a smart plastic case - that radio went with me everywhere! Other kids of my age thought I was a genius!

You said "That's why I preferred not to go with a kit as they are costlier than building from scratch." Sometimes this is not true if the kit maker puts together a lot of kits - bulk buying makes things cheaper and you may get a printed circuit board or a cabinet to put it in for a cheaper price than you can but the individual components.

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