Among some of our modern contemporaries, ‘musical’ post-cards evoke strong reactions of astonishment about hyper modern microcontroller technology. However, such ﬂat melody memories would only have elicited a weary smile from our forefathers.
As early as 1928, there are reports that radio cards with the dimensions of a regular postcard and a thickness of only a few millimeters were being made. These cards concealed a basketwork coil with a sliding tap for tuning the frequency of the received signal, a fixed capacitor and a miniscule detector device consisting of a small crystal with a ‘whisker’ contact. A similarly simple circuit can also be implemented using cur-rent resources. For this, you will need an interesting local medium-wave transmitter and a high-impedance headphone (1–2 kΩ), as well as a good aerial (such as a metal downpipe or an earthed radiator). The aerial is connected to an LC resonant circuit tuned to the frequency of the local transmitter, and a diode provides the demodulation. The necessary capacitance following the diode is provided by the cable to the headphone or ampliﬁer.
The coil can be made using a circular piece of stiff cardboard with a diameter of a couple of centimetres. Cut an odd number of slots into the cardboard disc. Then wind enamelled copper wire (diameter 0.15–0.2 mm) back and forth through the slots. Forty turns will give an inductance of around 80 µH. The coil looks like the bottom of a reed basket,which explains its cryptic name in RF jargon. To tune the coil to the frequency of the local transmitter and determine the required frequency of the resonant circuit, connect a dual-gang or multiple-gang variable capacitor (500–1000 pF) to the coil, with the stator sections (the fixed portion of the capacitor plates) connected in parallel.
The rotor sections, which are connected to the shaft of the rotary capacitor, must without fail be connected to ground in order to pre-vent a ‘hand effect’ while tuning. Incidentally, the resonant-circuit formula cannot be used to determine the tuning capacitance, since it ignores the effect of the aerial. After the capacitor has been adjusted, estimate the value of the capacitance (or even better, measure it), dig out a suitable fixed capacitor from your parts box and solder it to the coil at the centre of the cardboard disc, along with a general-purpose germanium diode (AA119, AA112, OA95, etc.). Secure the capacitor and diode with glue. For terminals, you can use 4-mm tubular rivets for miniature plugs, as shown in the photo.
A suitable ‘enclosure’ can be made from ‘customer discount’ cards in credit-card format (you probably already have more than you really need). Use one card as the ‘circuit board’ for the receiver, and cut an opening in a second card to receive the circuitry. Ideally, this card should thick enough to fit the full height of the receiver. The cover is formed by a third card. After a ﬁnal check, glue or rivet the cards together, and your card radio is finished. It’s not high-end, but it has astonishingly good performance for such a simple circuit.
One ﬁnal glimpse into the past: already in the 1930s, such fixed-tuned detector receivers were available in the form of ‘Berlin plugs’, ‘Hamburg plugs’, and so on, for receiving local transmitter signals in various locations.
Author : G. Stabe - Copyright : Elektor