Foxhole Radio tuning help. My first circuit!

[monkeybanana]

Hello everyone, this is my first post here.

I am building my first circuit (outside of class) and am having trouble tuning my radio.

What I have:

So far I just have a coil made from the wires inside a telephone cord wrapped around a toilet-paper-roll tube, an antenna (also a telephone wire), and an IN4002 diode that my friend gave me (I bought a zener diode, whoops).

Luckily with 95 turns on my first try I got Radio Disney! That's AM 1310 here in San Francisco. I never enjoyed listening to crappy pop music so much.

Here is the problem:

I would like to change the resonant frequency in the "tank" with a variable capacitor but when I insert a capacitor in parallel with the coil no signal comes through the speakers or headphones! I've looked at many schematics and pictures of these radios with the same set-up so I don't know what I am doing wrong. I have used capacitor values from .01 uF to the higher pF ranges.

I can try to get a picture of the set-up as soon as I go to my friend's house again where he has all the wires and breadboard I can use.

p.s. Looking at several schematics, some authors have the diode pointing towards the coil while others have it pointing out to the earphones. I am wondering if the cat-whisker or razor-blade and pencil detector worked backwards?

edit: I forgot to insert a diagram!

Thanks,
Yann

 

[Raven Luni]

Maybe this calculator thingy can help:
http://www.thetaeng.com/calc_LC_tank.htm

 

[duke37]

The diode that you have is a power diode with a large internal capacitance and is slow. You would be better off with a 1N914 or 1N4148 which are very common. Even better is a small Schottky diode such as BAT43.

A small capacitor (1000pF) across the speaker would also help.

Crystal sets work with the incoming energy and any loss in the circuitry has a big effect.
There is a compromise to be made between sensitivity and selectivity.

 

[monkeybanana]

Thank you both for the quick replies!

@Raven L. That calculator beats doing it by hand! I'm just not confident with my value for L at the moment but I guess I could get a ballpark value for C.

@duke. Thanks for the tip on diodes. I'm going to order some today. Now as for the cap across the speaker, I thought the speaker itself had a capacitance which coupled with the circuit filters out the high freq. carrier wave thus adding another in parallel with the speaker would increase the filtering? (My goal is to understand all the steps in the circuit.)

Thanks again, after I can get this tuning to work I plan to make an amplifier for this thing.

 

[alfa88]

If you're in the Bay Area you have a plethora of part sources. Radio Shack for limited selection, Fry's better and Jameco Best. For a crystal radio I'd go with a Germanium diode Jameco has1N270 in stock. As far as the capacitance goes. pF is smaller than uF. Next. Your inductor sounds like it has some capacitance in it. Maybe due to the fact that you're using telephone wire and not 'magnet' wire. Magnet wire has an enameled coating which allows for a tighter wrap and reduces capacitance. Using that may allow you to use a capacitor in the tank that will allow some tuning.

 

[duke37]

A germanium diode is good but difficult to find on this side of the planet.

The speaker is not a capacitace, it is an inductance. There will be some capacitance from wiring etc. but a small capacitor across the speaker enables the circuit to supply pulses of current through the diode without too much opposition.

 

[monkeybanana]

@alfa88. Awesome about having places to go. I love Fry's but it's a little on the far side since I don't drive. I will try Jameco on Monday, looks promising.
I was hoping to get this radio to work with my ghetto coil but if it fails I will make one with magnet wire. I should be able to put a capacitor in series with the coil to get the capacitance lower, right? Thanks for the heads up about pF being smaller than uF, I noticed that after posting (I'm still getting comfortable working with small units)

@duke. So does not having a capacitor across the speaker block current due to high impedance from the speaker? This is very helpful!

 

[KMoffett]

...Your inductor sounds like it has some capacitance in it. Maybe due to the fact that you're using telephone wire and not 'magnet' wire. Magnet wire has an enameled coating which allows for a tighter wrap and reduces capacitance. Using that may allow you to use a capacitor in the tank that will allow some tuning.

I thinks that's wrong. Intra-winding capacitance goes down with increased winding spacing. Same as increasing the spacing between capacitor plates decreases capacitance.

http://d4magnetics.com/page7/page7.html

For the "same" wire gauge, tightly spaces enamel wire would have a higher distributed capacitance than tightly spaced, plastic sleeved telephone wire. The inductance would also be higher for the enamel wire.

Ken

 

[KMoffett]

For a real trench radio, you can also make your own diode with a piece of graphite "pencil lead" and a "Blue Blade" razor blade. For the variable capacitor you can use two pieces of foil or sheet metal on opposite sides of an insulation sheet, like paper, plastic, or glass.

Ken

 

[monkeybanana]

Thanks Ken. For now I want to get the circuit working and since I can easily switch and control values for capacitors and diodes I want to make the radio with modern components. Down the line I do want to make the radio out of recycles parts.

Good to know about the wiring.

 

[alfa88]

Me thinks you're right Ken. I didn't think too hard on that one. Perhaps Mr. Banana will need to experiment with the number of windings. This has piqued my curiosity. Now I wanna make one.

 

[monkeybanana]

For anyone reading this in the future, a germanium diode makes a big difference as does a good antenna (I attached my wire with alligators to my blinds rolled up at the top of my window. The variable capacitor now works and I get up to 5 stations! More if I add capacitors.

Also, note that the variable capacitor adds capacitance to the circuit even when set to 0 pF! Mine adds about 180 pF. This makes the receiver jump from 1350 kHz to 810 kHz!!!!

 

[KMoffett]

Variable capacitors don't go to 0pF. They go from some "maximum"value to some "minimum" value, but not to zero.

Congratulations!

Ken

 

[alfa88]

Good stuff, congrats.

 

[monkeybanana]

Variable capacitors don't go to 0pF. They go from some "maximum"value to some "minimum" value, but not to zero.

Congratulations!

Ken

That makes a lot of sense

Thanks everyone for all the help.

 

[duke37]

Excellent job.
Please let us know how you get on, have you tried a Shottky diode, have you fitted a capacitor across the speaker? What differences did these make?
Will you be trying home made diode detector?

Duke

 

[monkeybanana]

Excellent job.
Please let us know how you get on, have you tried a Shottky diode, have you fitted a capacitor across the speaker? What differences did these make?
Will you be trying home made diode detector?

Duke

I have not tried a Shottky diode but I'm curious if it will perform differently. I will get my hands on one and see. As for the home made diode, I am going to "blue" a razor at my friend's house where there is a gas stove (I have an electric) and try to make the detector.

The 1000 pF capacitor does help a bit for some of the stations that have noise. There are a couple of stations that I can't really isolate and that's where the cap comes in handy.
I have a .1 uF capacitor that I got for my power supply project and put it in for fun which muffles all the stations.

Thanks duke!