In 1896, Marconi successfully covered a distance of about 3 km using electromagnetic waves. A little later, he established radio contact across water between Lavernock Point, South Wales and Flat Holm Island. The transmitter consisted of a spark inductor coupled to a dipole antenna.
At the receiver side, he used another dipole and a glass tube filled with silver and nickel filings, the so-called coherer. The filings enabled the coherer to act as a ‘defined’ bad contact with the RF pulse energy ‘rattling’ the contact and thus driving the audio amplifiers connected to the output.
The circuit shown here operates on he same principle. Admittedly it does not cover the same distance as Marconi’s extensive radio equipment, but then you do not need to start filing away on the family’s silver cutlery! The transmitter consists of an empty (!) disposable cigarette lighter with a piezo spark mechanism. On the gas nozzle of the empty lighter you solder a 30-cm long, 1-mm diameter (20 SWG) solid brass wire which acts as an antenna.
When the spark button is pressed, the antenna briefly radiates electromagnetic energy in the VHF FM frequency band. The effect is easy to verify by holding the transmitter close to an FM radio and ‘firing’ it!
Compared with the primitive transmitter, our receiver is a more up to date design realised in ‘all solid-state’ electronics. A telescopic antenna picks up the transmitter’s RF energy and applies it to a resonant circuit consisting of an inductor and a capacitor. The electrical signal is rectified by a Germanium diode (AAxxx or OAxx) and amplified by a darlington stage.
The collector of the second transistor causes voltage changes at pin 4 of the bistable (flip-flop) IC type 4013. The receiver sensitivity can be adjusted using preset P1. Each time the transmitter sparks away, the flip-flop toggles and switches the LED on or off. As a circuit variation, you may want to use a relay instead of a LED to allow lamps, a TV set, motors or actuators to be switched on and off using your ‘primitive’ spark transmitter. Your friends will be impressed.