A Modern Crystal Set
This article is intended for the newcomer who wants to make a simple crystal set, it needs no batteries but it does need a big aerial and some patience.
Making a modern crystal set is quite easy and fun, note that the radio bands are much more crowded than they were in radio's early days. The simple circuits do not have good selectivity, now that the radio stations are close together do not be surprised if you can hear two stations at once! That is the poor selectivity of the circuit may not be able to reject stations adjacent to the one you want.
As well as the finished crystal set you will need,
- A strong radio signal
- A long wire aerial
- A ground connection
The aerial needs to be as long as possible and should run outside the house. From a bedroom window to the bottom of a garden would be about right! The wire used is not important but it needs to be well insulated at each end to keep the precious signal from leaking away.
Use a plastic insulator or nylon cord to terminate between the end of the wire and whatever you tie the aerial to.
A copper cold water pipe would be make a good earth connection. DO NOT UNDER ANY CIRCUMSTANCES try to use a connection from the mains electricity supply. You might find a central heating pipe works OK, if you are really keen you could put a metal spike into the ground outside and a run a wire from that.
Avoiding the Aerial and Earth
Alan Lord has kindly passed on some advise as to how he is able to tune the local
medium wave station without the aerial and earth and achieve good selectivity. See how here.
The Radio Circuit
A simple circuit consists of just four components and an ear piece
- Variable Capacitor
- Coil or Inductor
- A Diode
- A fixed capacitor
Variable Capacitor and Inductor
These two components make the tuned circuit which will receive the radio signal. The coil can either be wound on a simple former, like a cardboard tube or a ferrite rod. The ferrite increases the inductance of the coil, so you need less turns on a ferrite rod to realise the same inductance than you would without using ferrite.
The variable capacitor allows for tuning across the band. The values of capacitance and inductance are chosen to allow the desired radio band to be covered.
Easily available modern variable capacitors tend to be around 150pF in value, to match this use 75 turns of 30SWG enamelled copper wire on a ferrite rod say 8mm diameter and 100mm long. If you don't have 30SWG wire to hand use something similar, even fine PVC insulated wire can be used. You may need to add or remove turns to get the desired coverage.
Tape or glue the wire into place. Leave the ends of the wire long enough to connect into the circuit.
The diode replaces the old fashioned detector. It can be any type of germanium diode. Don't use a silicon diode as these need a stronger signal before they conduct, and crystal sets don't have strong signals!
An OA91 diode would be suitable although any other germanium type will also work fine.
A small fixed capacitor is needed to remove the radio signal and leave behind just the audio signal for the headphones...
The audio signal will be very weak and not strong enough to drive either a loudspeaker or normal headphones. Us a crystal ear piece which has a very high impedance and can be driven by a small signal.
You should be able to buy the parts from an electronic components outlet or you can salvage bits from a discarded transistor radio.
Connecting It Up
Start by winding the coil as above. Fasten the coil and capacitor to a piece of board or plastic. Connect the parts together as shown in the diagram.
Need a Detector Kit? Details here
Alan Lord kindly took time to share some of his ideas,
I find that the larger the diameter the coil and the
thicker the wire the better the sensitivity and selectivity is ( form
factor ). I have a crystal set here in my workshop, it drives a standard
telephone earpiece, has no earth and with no aerial the local station
(radio tay) can be heard at good volume and talk uk and virgin can be
heard weakly. if I add a few feet of wire ( hanging out the window) all
three stations can be received easily and separately.
The coil is wound on a piece of plastic drain-pipe 4.5 inches in
diameter , has 55 turns of 4mm conduit cable, this may seem a bit "over
the top" but it has a high "q" factor and excellent signal pick-up. it
is tuned with a tuning cap. rescued from a scrap radio (about 200pf),
uses a germanium diode driving an old sub-min mains transformer
primary,the secondary matches most standard 8 ohm headphones.
Another one I used had a coil of 1 turn wound around the loft, same
gauge of wire , same tuning cap etc. but it received most stations on
the medium-wave band at decent headphone volume , again because of the
area of the single-turn coil no other aerial is needed , but adding an
earth makes it even louder!.As there is no other aerial attached , the
coil is not "damped" so selectivity is good."
On Alan's website you will find a circuit for a transistor T.R.F. set based on the same type of
coil---it works really well with excellent selectivity--in fact
the prototype is permanently connected to a solar panel so it plays all day
(free radio) and has worked like that for a few years!
Who was Capt. Peter Eckersley?
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