Not sure if this is the correct place to ask, but here goes... I've found plans for a simple two transistor AM radio receiver, but I'm not sure how the ferrite loop antenna is connected to the radio. I'm not at all good with schematics, I can understand most of it except for the ferrite antenna bit. I would appreciate any help on this.
Hi, Dave. If you look at the picture, you can see 4 turns of yellow insulated wire around the loopstick. As shown in the schematic, one wire is connected to the base of the first transistor, and the other is connected to the + end of the 47uF cap and the 270K ohm resistor. You've also got two very thin magnet wires (one red, one blue in the illustration) which go to the tuning capacitor.
The idea is, you make the 4 turns with your own wire around the existing loopstick, which you've scrounged from a garage sale transistor radio along with the tuning cap. Be careful not to break the wires coming from the loopstick -- use a tweezers and a lot of care when you pull it.
Hope this has answered your question. The radio shown is fairly simple, and shouldn't be too difficult to build. If you're going with trying to build on a real piece of wood board, use pine instead of oak (softer, easier to work and pound nails), and use brass nails (most nails don't accept solder). If you can find someone with a little electronics experience to help a bit it should be easy.
I'm building it in a plastic project box, for portability reasons. Should a 60-160 pF tuning cap work in this radio? I'm also wondering how to hook up a 1 kohm - 8 ohm audio transformer so I can use my iPod earphones with it. The transformer has three wires on one side, and two on the other from what I can see in the picture.
Chances are pretty good it alreayd has the needed pickup loop.
Likely one winding is quite large, and will resonate with the variable capacitor. And then a small winding of a small number of turns to couple the signal out of that resonant circuit.
As you previously said, it has four wires coming from it. In this case it means it has two windings. The winding you need to use, is the one with the most wire. It is used to resonate with a (maximum) 365 pF variable capacitor. The capacitor you have and want to use will not properly cover the broadcast band (it won't tune into the lower end of the band). The other two wires should be ignored, except do not connect them together.
This looks like a fun project for you. Enjoy experimenting with it and feel free about asking questions.
Yes, you can use the BC547 or the 2N2222A or most any small NPN transistor. Be sure to get the leads right when you connect them to the circuit. In the schematic, the transistor is drawn with an arrow in the emitter lead pointing toward the bottom of the diagram. The base lead is on the other side, in the middle and is connected to the 4 turn loop. The collector lead is at the top, and is connected to three things: the .01 uF capacitor and both the 270k and 2.2k resistors. You can see which lead is which on the BC547 transistor as follows. Go to this site:
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click on "Download BC547 datasheet from Fairchild Semiconductor"
I want to install a fixed capacitor in place of the tuning cap, what value will I need for 1431 kHz? This frequency is home to ABC Radio National, which I enjoy listening to late at night.
If you knew the exact inductance, you could get a precision (Expensive!) capacitor to tune it to 1431 KHz, but it's much easier to just use a variable capacitor and lock the knob once you have it tuned.
You can use a fixed cap, but you have to know the inductance of the coil to figure out what cap to use to resonate. Even then, you'll probably want a trimmer, to "zero in" on the station.
If you cannot fine the crystal ear-phone then you'll need to use the 8 ohm matching transformer. Connect the xformer to an additional BC547 and here's the connection
You can leave the centre tap of the xformer open and just use the 2 end pieces. Play with the restor value to get the best volume. But then it won't be a 2 transistor AM radio any more!
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