Help with crystal radio circuit...

[jaki54321]

Ok first of all I am sort of a complete noob... I made a 'crystal' radio circuit from parts i got out of an old busted radio, a LM386N audio amplifier IC from radioshack, and a small breadboard. It is pretty loud on the speaker i have on it which is a 1 or 2W speaker, i dont know the exact spec. It works pretty damn well and is quite loud for a bedroom. But anyway, here is the pic and right after it ill explain the issues:

Ok.... the 1st issue is, well it is quite loud, but not as loud as the speaker can really go. BUT when i place my finger on the output capacitor(470uf), on the metal on top of it, it actually amplifies the signal a lil, then if i push it down jus a lil, it really drives the signal through the speaker and makes it much louder. is there any way I can make it to where it is as loud as when i push on it without actually having to push on it(Maybe change the cap to a different value)?

Also, the Variable capacitor does not work, it will not tune the radio to different frequencies, and I dont know if this has something to do with the layout or the cap itself, because my variable cap is a bit different looking and I do not know how to hook it up and/or if i did it right:

Can somebody please help me fix this issue? also any suggestions to this probably crappy yet well working circuit will be very appreciated! this circuit runs off a 9v battery powering/grounding the Amplifier chip which is an LM396, and also grounding off the speaker and diode. I know the diode is not germanium but that is all I can get right now because I have to wait till i order any germanium diodes.

Thank you!

 

[(*steve*)]

Well, it's a very interesting circuit. Have you tried drawing a circuit diagram, or did you just wire it up? (Of course you had a circuit diagram to work from -- show us)

I expect that the reason you're not seeing anything happen as you vary the capacitance is that you've not wired it up correctly.

I'm also interested in the coil you're using. How do you know it's the correct inductance to work with that capacitor? Did they come from the same radio?

The position of the diode also seems very suspect.

I suspect that your finger acts as what we call "an antenna"

 

[jaki54321]

Well, it's a very interesting circuit. Have you tried drawing a circuit diagram, or did you just wire it up? (Of course you had a circuit diagram to work from -- show us)

I expect that the reason you're not seeing anything happen as you vary the capacitance is that you've not wired it up correctly.

I'm also interested in the coil you're using. How do you know it's the correct inductance to work with that capacitor? Did they come from the same radio?

The position of the diode also seems very suspect.

I suspect that your finger acts as what we call "an antenna"

I got the inductor coil from a different radio because it had more winds in it an i didnt kno how much I needed... The wire on top of the picture is the antenna(Which in real life is a long green wire, works well too.) The variable cap did not come from the same radio, and the diode position is also somethin tat concerned me as well...

 

[jaki54321]

Here i attempted to sketch this schematic of it, i hope i did it right:

1st schematic ive ever made all by myself... Used ExpressSCH

 

[davenn]

ok
get rid of the 18pF cap for a start
a) its not needed
b) its much a too small value to be used at broadcast freqs anyway.

here's a cct for a basic crystal radio.....

cheers
Dave

 

[davenn]

Now the 1N914 (1N4148) may work ok as the diode (I havent tried one of those in a crystal set) but if possible get a OA95, 1N34 or some other similar signal diode.

the diode should be inline with the input to the IC and orientated around the other way as shown in my cct above. Im guessing one input to the IC will have the signal from the diode the other will go to negative supply/ground <<< correct that if necessary, Steve thanks. in my day the signal was fed to a hi impedance crystal earpiece no such things as IC's

the cct will also work much better with an earth (yes... a, to real ground, earth)

gosh.... I used to love building crystal radios as a kid.... more yrs ago than I care to remember

cheers
Dave

Jaki.....

the inductor you are using ....
is the coil wound on a ferrite rod ?
roughly how many turns ? most small transistor radios had 2 sets of coils on the ferrite rod. you want to choose the one with the greatest number of turns

 

[jaki54321]

Now the 1N914 (1N4148) may work ok as the diode (I havent tried one of those in a crystal set) but if possible get a OA95, 1N34 or some other similar signal diode.

the diode should be inline with the input to the IC and orientated around the other way as shown in my cct above. Im guessing one input to the IC will have the signal from the diode the other will go to negative supply/ground <<< correct that if necessary, Steve thanks. in my day the signal was fed to a hi impedance crystal earpiece no such things as IC's

the cct will also work much better with an earth (yes... a, to real ground, earth)

gosh.... I used to love building crystal radios as a kid.... more yrs ago than I care to remember

cheers
Dave

Jaki.....

the inductor you are using ....
is the coil wound on a ferrite rod ?
roughly how many turns ? most small transistor radios had 2 sets of coils on the ferrite rod. you want to choose the one with the greatest number of turns

Ok I am going to get rid of the 18 Picofarad cap. Here is a picture of the coil:

Honestly i dont know if it even is right, I mean it works quite well but I dont know much about inductors, im still new to electronics, and this is the 5th thing ive tried building(the other 4 projects have all worked but are COMPLETELY different). Would getting rid of the 18pf cap make the variable cap start working? and what would happen if i change the output cap to a cap that is higher/lower in value?

I think im guna experiment around a bit

 

[jaki54321]

Now the 1N914 (1N4148) may work ok as the diode (I havent tried one of those in a crystal set) but if possible get a OA95, 1N34 or some other similar signal diode.

the diode should be inline with the input to the IC and orientated around the other way as shown in my cct above. Im guessing one input to the IC will have the signal from the diode the other will go to negative supply/ground <<< correct that if necessary, Steve thanks. in my day the signal was fed to a hi impedance crystal earpiece no such things as IC's

the cct will also work much better with an earth (yes... a, to real ground, earth)

gosh.... I used to love building crystal radios as a kid.... more yrs ago than I care to remember

cheers
Dave

Jaki.....

the inductor you are using ....
is the coil wound on a ferrite rod ?
roughly how many turns ? most small transistor radios had 2 sets of coils on the ferrite rod. you want to choose the one with the greatest number of turns

well the chip i bought was from radioshack, and it came with a diagram on the package behind it explaining to me how it would work.

Pins:
1: Gain
2: Input -
3: Input +
4: GND
5: VOut
6: Vs
7: Bypass
8: Gain

I assumed Vs Meant voltage input, and VOut meant output, which seems to work. i have a 10uF cap across pin 1 and 8 to get 46 dB of gain, which works pretty damn well. I assumed GND meant ground which seems pretty simple, Bypass means... Well bypass. The input - and input + is what got me. Like your saying, the diode should be oriented the other way around as in ur schematic. So i am not exactly sure if i hooked up the diode correctly... I think another method is hooking up the antenna side of the inductor to the Input - of the IC, which i havent tried yet because I then wouldnt know where to hook up the Input +, especially if i reverse the diode as in ur schematic... Oh soosoo many options...

 

[Resqueline]

Hehe, those very first tries..
Your circuit used a parallell resonance in series with the signal (antenna). That has the effect that tuning will remove one station at a time, passing all others through.
You wouldn't be able to notice that, and if you left out the 18pF you'd be listening to mains hum only)..
Dave's circuit has that parallell resonance "shorting" the signal (antenna) to ground. All stations, except the one tuned to, will then be shorted to ground.
Now, with that inductor (a power toroid) it's quite hard to say which band you're going to be tuned into..
Use Dave's circuit (add an 180-330pF capacitor in parallell with R1 - in to pin 3) and you may see results. The amplifier seems to be ok. Good luck experimenting.

 

[jaki54321]

Hehe, those very first tries..
Your circuit used a parallell resonance in series with the signal (antenna). That has the effect that tuning will remove one station at a time, passing all others through.
You wouldn't be able to notice that, and if you left out the 18pF you'd be listening to mains hum only)..
Dave's circuit has that parallell resonance "shorting" the signal (antenna) to ground. All stations, except the one tuned to, will then be shorted to ground.
Now, with that inductor (a power toroid) it's quite hard to say which band you're going to be tuned into..
Use Dave's circuit (add an 180-330pF capacitor in parallell with R1 - in to pin 3) and you may see results. The amplifier seems to be ok. Good luck experimenting.

Ok im going to give it a try and probably sketch a schematic of it. Should i have the ground to Earth or is the - part of the 9v battery ok?

 

[davenn]

Ok im going to give it a try and probably sketch a schematic of it. Should i have the ground to Earth or is the - part of the 9v battery ok?

Try it without an actual ground for a start. if you live in a metro area within say 10km of some radio stations you will probably hear them ok.

That toroid coil isnt really the best. you should be looking in an old portable radio for the ferrite rod type of inductor. some of them have a circular rod ~ 1cm diam and up to ~ 10cm long others may have a flat rectangular rod that has maybe 40 - 50 turns of fine wire on it.

another thing you could do is to get a bit of plastic pipe ~ 3 to 5 cm in diameter and ~ 10 to 15 cm long and wind as many turns of wire on it as possible. For that sized coil you could use the thickness of wire that is on that toroid core you photographed.
ok a couple of pix......

one on left shows a plastic pipe supported inductor and on the right a ferrite rod inductor

Cheers
Dave

 

[jaki54321]

Ok guys I tried everything you all suggested. The radio didnt work at all... i couldnt get it to start or hear anything... Ive tried looking over the circuit and the polarity of the caps... the only difference i made was the output cap i put in a 1000uF cap instead of a 470uF... still it didnt work at all... Plus i broke off one of the pins off the LM386... Thank you all for your help, ill have to wait for a lil while before I can work on this again... Oh well... Thank you all...

 

[(*steve*)]

The important thing about a tuned circuit is that it needs to be tuned to the correct frequency.

Unless you have a tuning capacitor and a coil that together will resonate at the frequency that a signal is broadcasting on, you will hear nothing. You can't just grab 2 random components and expect them to work.

Perhaps you may be better off buying a crystal radio kit and then building your own amplifier.

 

[Resqueline]

All of the above applies, except I mis-stated pin 3 as being the input (should be pin 2).
The amplifier may require an input capacitor on pin 2 (in order to retain its bias levels) and pin 3 is supposed to be connected to pin 4 (as in your first image).

 

[davenn]

are you still using that toroid inductor ?

I suggest you get rid of it and try one of the styles of inductor I showed in the pix

that toroid one is more likely to act as a RF choke and kill any signal


Dave

 

[jaki54321]

are you still using that toroid inductor ?

I suggest you get rid of it and try one of the styles of inductor I showed in the pix

that toroid one is more likely to act as a RF choke and kill any signal


Dave

I broke off one of the pins from the LM386 amplifier... so I cannot use it anymore... Oh well. Next time I am going to order a Germanium diode, a Variable cap, a good Breadboard, an jus a whole bunch of stuff, i really want to get this working. but i need some new parts. As for the Power toroid, I think I will try another coil.

Thank you for your help, i really appreciate it