The "Toiletuna" Crystal Set

Guess where the coil former came from!

Both radio enthusiasts and radio amateurs have been advocates of 'Kitchen Table Technology' since the earliest days of radio. The crystal set has always been a very 'homebrew' item of equipment and all sorts of domestic items (breadboards etc.) have been ingeniously used in persuit of those elusive signals. When it was decided that a novelty crystal set with a toilet roll coil former might be a laugh to make, it was mentioned that back in the Fifties, it was always 'Sixty-five turns on a bog roll' for a crystal set - It was an accepted standard! Now in the early days of the 21st Century, the good old loo roll is still with us and makes a great (and plentiful) coil former for a crystal set. The layout and circuit is much the same as our 'Clementswood' receiver except for the home-wound coil.

Components and construction

The coil is roughly 50 close-wound turns of 0.3mm (32 SWG or 30 AWG) enamelled copper wire. With a 200pF capacitor this tuned around 900kHz - 1400kHz. We noticed that different antennas and earthing arrangements affected the tuning range slightly, so once again a little experimenting may be required! The coil could probably benefit from being replaced with a larger one with spaced turns to reduce the self-capacity of the windings (watch this space!) but as far as simplicity goes, this one is hard to beat.

The choices of coil and capactior that you can use are all open to experimentation. The tuning capacitor was purchased from a Maplin Electronics store (Stock code FT78K) and has a maximum value of around 230pF. If you can get hold of an old broadcast variable capacitor of about 365pF then all the better as you'll extend your frequency coverage. It might even be an interesting idea to replace the coil with a proper AM band ferrite rod antenna later on - feel free to experiment!

Above is a suggested layout for the receiver. Layout is an entirely personal thing as nothing is really critical and this set was assembled on a small piece of hardboard (fiberboard). The connections for the antenna and earth were made with small copper hardboard pins which were tapped into the board. The earpiece socket and tuning capacitor were fixed down with double-sided adhesive tape and the coil was glued to two cardboard spacers which were then glued down to the board.

The "Toiletuna" Shopping list

1000pF (1nF) ceramic disk capacitor (polyester types are fine too)

100pF Ceramic disk capacitor

200pF variable capacitor (365pF is better - see below)

OA47 diode (OA91 and 1N34 are also ideal)

100K resistor (0.25W 5% carbon or metal film)

32 SWG or 30 AWG wire for coil (You know where to get a coil former from!) :o)

3.5mm mono jack socket (An enclosed type is useful for sticking to the board)

Crystal earpiece with 3.5mm jack plug (This MUST be a high impeadance type!)

365pF variable capacitors are worth tracking down as you'll be able to tune much lower in frequency than with the 200pF capacitor. There are still a few to be found cheaply in the UK and The Crystal Set Society in the USA advertises them along with other related items for sale on their website. A good supplier in the UK of capacitors and all kinds of components is J.Birkett, 25 The Strait, Lincoln LN2 1JF Tel. 01522 520767. Most of our parts came from a local Maplin Electronics store, but there are many other mail order suppliers around - have a look through the pages of any electronics magazine.

Antennas & earthing

Don't forget that a good antenna and ground connection are essential. A long piece of wire will be fine for the antenna (I use 60 feet of wire strung down the garden) and a copper tube, or better still a proper earth rod buried in the soil will be fine for a ground. Don't attempt to connect your ground to the AC mains supply earth/ground connection. High impeadance headphones will be needed for this receiver too, but are very scarce, so an inexpensive high impeadance crystal earpiece will be best (Something like Maplin Electronics Stock code LB25C).

And finally...