Simple Crystal radio

[Peter]

hi.

Just though I'd post this. Its a very simple crysal radio. The nice thing is its cheep.. No tuning cap. Diode yes...

The coil is 5" diamiter with 40 turns of 24 swg space of 24 swg. Bit ricky but stick to it. Now at 5 turns tap off. keep going 5 turns tap and so on. For tuning just latch on the diode to a tap for station.

As they say brill...

Pete

[Ed_Dinning]

Hi Pete, nice and simple, to improve performance wind on cardboard tube or stick matchsticks to plastic tube to space wir off the plastic (about every 2").
This reduces the coil self capacitance and increases it's "Q".

Ed

[Peter]

hi

Yes what a good idea. Simple,

Thanks

[Pete_kaye]

I would like to make one of these but i STR that I need high impdance phones .Roughly what value would that be?I have various headphones and earpieces of about 4 to 20 ohms but if the headphones are reduced to min vol I get 50 ohms .I have an old microphone which can be used as a quiet earphone (500ohm);would this do?Does experimenting with aerial length reduce effect of loud stations ?Is a proper earth necessary?Is an OA70 diode ok?

[Aerodyne]

You are unlikely to need a volume control system. High impedance phones are essential - dunno whether a step-down output transformer would create such inefficiencies as to prevent the receiver functioning. Might be worth a try. Reducing headphones to min volume would not work - all you are doing is adding resistance, not matching up or increasing impedance.
Remember: a crystal set is powered only by the received signal so a good aerial and earth are prerequisites and yes, I believe an OA70 should work OK. (Not that I've tried one!)
-Tony

[Station X]

ISTR that I need high impdance phones

You don't need high impedance headphones. I didn't believe it myself until I tried it, but it's true.

Maplin do cheap Crystal Earpieces which work well and are more comfortable than headphones:-

http://www.maplin.co.uk/Module.aspx?...256&doy=24m11D

Graham.

[Aerodyne]

You don't need high impedance headphones. I didn't believe it myself until I tried it, but it's true.

Surely, a crystal earpiece will be high impedance?
-Tony

[mikelect]

Originally Posted by G4ILN

You don't need high impedance headphones. I didn't believe it myself until I tried it, but it's true.

Maplin do cheap Crystal Earpieces which work well and are more comfortable than headphones.

Graham.

Crystal earpieces are usually about 1kohm and are considered high impedence


Mike

[Station X]

I seem to have confused the issue by mentioning crystal earpieces.

Low impedance headphones do work. I've just tried some ex WD C-LRs of 120 ohms total on my Gecophone and they work fine.

Graham.

[YC-156]

Hi Pete,

Originally Posted by Pete_kaye

I would like to make one of these but i STR that I need high impdance phones .Roughly what value would that be?

5000 ohm or higher. The best for maximum sensitivity is the old type of 'crystal earplugs', which are 1Meg ohm impedance. The oldie headphones in your photo is around the 5000 ohm mark.

Originally Posted by Pete_kaye

I have an old microphone which can be used as a quiet earphone (500ohm);would this do?

Everything will work, but very poorly indeed. Even 500 ohm is far from good enough. You need either the old headphones or find a crystal earpiece.

Originally Posted by Pete_kaye

Does experimenting with aerial length reduce effect of loud stations ?

In a receiver as simple as the one you show, everything influences reception. Everything else being equal, then a longer antenna equals stronger reception.

Originally Posted by Pete_kaye

Is a proper earth necessary?

Yes, absolutely. If you live in an old house, you may be able to use the central heating pipes as an earth, assuming the pipes are metal.

Originally Posted by Pete_kaye

Is an OA70 diode ok?

Perfect.

Regards

Frank N.

[jim_beacon]

Basically, the higher the headphone impedance, the better the selectivity. The headphones impedance damps the tuned circuit and lowers the Q of the coil. You can offset this by using a matching transformer.

The reason for better selectivity with a leaky-grid detector is the very high input impedance (~10M) of the valve, bootstrapped by another 1M or so for the grid leak, in the case of a homemade set, the Q is then down to how well you made your coil.

If you are after more sensitivity, you can try biasing the detector to just below the point of conduction - some early crystal sets have a battery for this purpose.

Jim.

[Station X]

This webpage may be of interest to anyone wanting to build a crystal set.

http://earlywireless.com/crystal_sets.htm

Graham.

[Aub]

When i was a lad, the first crystal set i built used a pair of ex-WD DLR No5 headphones which, if I recall correctly, were balanced armature. I don't know what impedance they were but they worked very well.

Building these little sets is great fun!

73's

Aub