Can I get some help with a single transistor am receiver?

I just built an AM radio circuit from a diagram I found on the net. I have everything hooked up correctly but I cannot get anything to come in. not even static.
The diagram can be seen here [http://www.circuitdiagram.org/images/simple-am-receiver-diagram.gif]

My thoughts are that maybe the signal is too weak for the ear phones I have and that I may need to use an op amp.

The coil is 26 gauge magnet wire wrapped around 80 times on a toilet paper roll. This should be equivalent to around 1mH according to the inductor calculator I used.
 
The trimmer was one I salvaged from a broken radio. It measures 10pF to 90pF. 

Those combined should give me close to the AM frequency band range I need.

Can anyone offer any advice? 

sort by: active | newest | oldest

You should be getting static.

Go to this sight and get the datasheet on the transistor.

http://www.maxim4u.com/

Make
sure you pick a datasheet that shows you the pin out.

Then make sure the transistor is connected right.

The positive voltage travels through the ear phones.

Make sure they work and the connections are good.

That may not mean you will get a signal from a radio station.

Many radio stations have gone digital and that is an analog receiver.

atrumblood (author)  Josehf Murchison3 years ago

I check the datasheet for the bc547 and I do have it hooked up correctly.

atrumblood (author) 3 years ago

A quick update. I just made an AM transmitter using a 555 timer. I had the antenna right next to the antenna on the AM receiver I made and still nothing. So this must mean I have something wrong with my receiver circuit.

Try a different ear phone the one you are using may have to much impedance.

atrumblood (author)  Josehf Murchison3 years ago

thanks I ordered a crystal radio ear piece and a 365 trimmer capacitor. I am hoping this will helo

Did you get the datasheet on it first and make sure it matched your circuit?

atrumblood (author)  Josehf Murchison3 years ago

Well, to be honest I am not exactly sure how to do something like that. I know that I would need to match the impedance of my circuit to the impedance of the ear piece. Impedance is something I only vaguely understand.

I tried calculating the impedance of the circuit but I don't really understand what it means.

Impedance is just a fancy way of saying resistance.

Think of every thing in a circuit as a resistor.

A capacitor is a insulator with DC current and a resistor with a frequency.

A inductor coil is a dead short with DC current and a resistor with a frequency.

A transistors impedance functioning at Q is equal to the collector and emitter resistance.

atrumblood (author)  Josehf Murchison3 years ago

I am having some trouble figuring out how to calculate the impedance of the transistor. Perhaps I am just over complicating this.

Ok if you have a multi meter check the voltage from the collector to the emitter of the transistor, if it is 1/2 the source voltage you are at Q.

If it is just a little above or below 1/2 the source voltage it should still work.

On that circuit 1.3 to 1.7 volts.

lf the voltage across the transistor is outside that you need to change the base resistor, smaller if the voltage across the transistor is below 1.3 volts, and larger if the voltage across the transistor is above 1.7 volts.

When the voltage across the transistor is 1/2 source your transistor is at Q.

Lets say the ear phone is 8 ohm and the emitter is 22 ohm then the transistors impedance is 30 ohm.

To work right in analog circuits transistors must be functioning at or near Q to transmit the full sine wave or the peaks of the signal will be cut off.

atrumblood (author)  Josehf Murchison3 years ago

Ok, well this may explain why I am not getting anything from my radio circuit. I am reading full Vcc across the emitter and collector. In other words the transistor is in cutoff. Thank you for your help Josehf, you have helped me a lot in understanding this little project of mine.

Yep that is also why you don't get static.

Try this,

Connect every thing but the base resistor and the transistor base.

From + to - connect a 5 k ohm pot with the transistor base connected to the adjusting pin.

Connect your meter to the collector and emitter of the transistor and adjust the pot until you get 1/2 source voltage.

If you cannot get 1/2 source voltage this way your transistor is buggered.

atrumblood (author)  Josehf Murchison3 years ago

Interesting. It only took a value of ~650ohm to bring it to about half. Way off what the diagram asks for of 820K!

The transistor doesn't seem to be bad. Maybe I need to try a different one

Na if you got 1/2 source with a 650 ohm then all you needed is a 650 ohm.

The transistor should be good, these things are built to a tolerance and no two work exactly the same add small values to the mix like 8 and 22 ohms and they tolerate less.

As for the schematic it was built in a circuit simulator.

I design circuits in my head and draw in paint then build it on a bread board.

Here is how I draw a circuit see how clear the joints are

full swing.bmp
atrumblood (author)  Josehf Murchison3 years ago

Thanks again. Now that I know a little more about this circuit I can hopefully try different things to get it to work.

Just a few things I want to clear up though.

1. The diode needs to have a very low forward voltage. Correct?

The one I have is ~90mV.

2. Couldn't I connect the ear piece to the emitter of the transistor?

It seems odd to me that I would connect them to V+.

Gold bonded germanium diodes have a very low voltage drop making them a good choice.

Yes you can put the earphone on the emitter of the transistor it is called an emitter follower, most emitter follower circuits have the output in parallel with the emitter resistor.

atrumblood (author)  Josehf Murchison3 years ago

Hmmm. Ok. I will try to calculate the transistors impedance it was the only one I couldn't figure out. As it stands without the transistor the impedance of the circuit is ~900ohm.

atrumblood (author) 3 years ago

P.S I tested the transmitter on my am radio alarm clock. Works though a little distorted.

The trimmer your using isn't to spec with what the diagram ass for. It would be best if you had a 356Pf variable cap like it wants. The audio output will be quite low so an amp would help.

Also i've tried using schematics from the site to make an FM transmitter and a bug detecter that didn't work out either.

atrumblood (author)  mpilchfamily3 years ago

I would use a 356 trimmer but I didn't have one handy. I have four of them ranging from 90pf to 125pf. So I suppose I could wire them all in parallel to get the value I need. However when I fed the values of the inductance and capacitance into the resonant frequency formula I get a range from ~533kHz to ~1,593kHz. The extra capacitance only gives me a lower frequency of around ~230Khz.

You have to have a good, LONG antenna connected to the top of the coil, and a good earth at the bottom. Do you ?

atrumblood (author)  steveastrouk3 years ago

Well I have a 5 foot antenna hooked up to it and for a ground I am using the earth connection to my house. Still nothing.

Try 50 feet or more. This isn't much more than a slightly souped-up crystal set.

atrumblood (author)  steveastrouk3 years ago

Ok thanks. I will give that a try

Thing is the transistor is only amplifying the audio signal - if there isn't any, it can't amplify it!

When I made one of these many years ago, we had a 35 YARD antenna !

atrumblood (author)  steveastrouk3 years ago

I assume I can also make a amplifier for the antenna?

You can, but a high frequency amp is a bit of a tricky beasty compared to the simple audio amp you have now. Once you're at that level, you've left the "clarity" of the crystal set like circuit you have now, and you're making a "proper" radio.

atrumblood (author)  steveastrouk3 years ago

wow, ok then. I guess if I want this radio to work then I should make my next project an AM transmitter. lol

rickharris3 years ago

Although I have tried many times I have never had good results for any type of home made radio receiver. There are too many variables and you really need some quite sophisticated test equipment to see what is going on.