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Simple radio receiver circuit problems (fixed)

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Alex M

Sun Jul 08 2012, 03:44AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Hi,

I am attempting my first radio circuit project and am using this circuit here

The problem is that I cannot pick up any radio stations at all.

I am using a 360pF variable capacitor and a ferrite rod and pre-wound coil from maplin

The only time I can get something is when I bring the circuit close to my laptop where I can get a buzzing sound, and when I connect and disconnect the battery I get a "pop" sound from the ear piece.

What could be wrong? I don't have a crystal earpiece so am using some earbuds (just one channel), not sure if that is the problem. The resistance of the ear piece speaker is 25 ohms. But if that is then I should at-least get some sort of audio from it right?

Again I know nothing about radios and this is my first venture into this.

Thanks.

Ben Solon

Sun Jul 08 2012, 04:21AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

I'm not going to ask if you have a scope. The piezo earphones are perfect for radios like that because of their high impedence of around 20k and good sensitivity. You could put an op amp to beef the signal up to electromagnetic earphone levels though. I believe that you are indeed getting signal output but your "amp" and "speaker" are mismached to the extreme.

Sulaiman

Sun Jul 08 2012, 06:43AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A crystal earpiece is a useful thing to have when experimenting with radios
Maplin sell one

You can make a Crystal Radio with one.

The circuit you link to does not seem to have any carrier de-modulation
(e.g. a non-linear part, diode etc.)
so I'm obviously missing something subtle.

Steve Conner

Sun Jul 08 2012, 07:20AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think it is some sort of reflex or regenerative circuit that uses positive feedback along with the non-linear characteristics of the transistors themselves. The 2.7k resistor may need adjusted for best results.

Alex M

Sun Jul 08 2012, 07:58AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Thanks all for the replies and advice.

I have replaced the 2.7k resistor with a 4.7k pot and took a walk around the garden and was able to pick up 1 radio station (this is using my earbuds). It was very faint but I put this down to not having the crystal earpiece, the song I heard was one by Queen (somebody to love). No idea what the station was though.

I will try and head upto maplins at some point and pick up one of those crystal ear pieces, hopefully then I will be able to hear any other weaker signals that there might be. The variable capacitor is very sensitive to movements too so I am going to put a smaller one in parallel with it for a bit of fine adjustment.

I can sort of see why people find ham radio fun now ( :

Alex M

Sun Jul 08 2012, 08:52AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Update: I built a little amplifier using a 2n3904 transistor and its much louder ( :

Got a couple more stations now, but its very fiddly.

It picks up lots of 50hz hum lol.

Another update: Its Gold radio I am picking up.

radiotech

Mon Jul 09 2012, 08:19AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The radio circuit 'detects' in TR1. If the coil and tuning capacitor are correct, my guess
is that somehow the connections, E B C , for TR1 & TR2 have got
ten mixed up.
This will render the circuit only marginally functional. Otherwise
the gain of TR1-TR2 is about 50 dB (very high).

Alex M

Wed Jul 11 2012, 04:35PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

radiotech wrote ...

The radio circuit 'detects' in TR1. If the coil and tuning capacitor are correct, my guess
is that somehow the connections, E B C , for TR1 & TR2 have got
ten mixed up.
This will render the circuit only marginally functional. Otherwise
the gain of TR1-TR2 is about 50 dB (very high).

I picked up a crystal earpiece today from maplins and can now use it without the amplifier. Everything is defiantly connected correctly (triple checked).

Just wondering, does AM radio tend to have lots of hissing and background noise when compared to FM and DAB radio?

Conundrum

Wed Jul 11 2012, 09:17PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Yes, lots of noise is normal.

Have you checked that your tuned circuit is resonant at the correct band? a good way to check this is to put an IR remote control near the ferrite rod and press the button(s)
If working then there should be repeating buzz-beep noises around the halfway point of the dial.
If its way off to one end or the other try adding 100pF across the tuning capacitor to see if this fixes it.

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