Introduction: Spark Gap Transmitter/Marconi Receiver - the Simplest Wireless Transceiver
For me, it is communication that fascinated me and made me study electronics. I always asked myself: how is it possible to send our voices hundreds of miles away without wires? it is this question that encouraged me to study Electrical and Electronics Engineering. The exiting thing about knowledge is that the more you learn, the more you have new questions in mind. Going deeply into electronics generated new questions: how did Marconi send his first wireless message before inventing transistors and amplifiers? it is possible to use only passive components (resistors, capacitors, and inductors) to transmit and receive messages? how? The answer on these questions made me respect a passive component that I had underestimated its importance before: the inductor.
In this project, I am making a homemade spark gap transmitter, along with a coherer based receiver. The coherer is a simple device that detects electromagnetic waves, and used in the early years of wireless communication.
More on the dawn of the wireless technology can be found in the following webpage:
Step 1: The Spark Gap Transmitter
History and science
Spark gap transmitters are the first devices able to transmit Radio Frequency (RF) pulses (actually, damped signals). The first spark gap transmitter was made by the famous scientist Heinrich Rudolf Hertz. Hertz proved experimentally in 1887 the existence of the electromagnetic waves proposed by James Clerk Maxwell at 1865.
The genius scientist Maxwell (also presented the first durable color photograph in 1861) demonstrated theoretically that electric and magnetic fields travel together through space as waves moving at the speed of light. This proposition was left without experimental proof until Hertz' work. Hertz thought that if the electric and magnetic fields travel together, it will be possible to detect the magnetic field of an electric signal generated across the room.
Hertz uses a relay to transform a DC voltage of a battery into an AC voltage. He then uses a high voltage transformer to raise the voltage signal to Kilo-volt values. It is known that the isolation of the air can be broken with high voltage, and it requires 1 KV to break the isolation of a 1 mm air gap. As a result, Hertz generates electric sparks that travel the air gap between the transmitter's spark balls. These electric sparks generate a magnetic field that can be detected from a distance. Hertz detects the magnetic field by placing a ring with a gap in front of the transmitter. The receiver receives the signal and re-generates sparks that jump between the balls of the micrometer air gap of the receiver.
Concerning the Project:
I made this project to encourage my Analog communication systems course students; I believe it is nice to show them a reproduction of the very early experiments done by communication pioneers.
I made the transmitter using the electronics of an electric lighter. This lighter uses the 1.5v of an AA battery to produce sparks that start fire. The electronics of the lighter (actually it is an oscillator) transforms the 1.5 DCV into alternating voltage, then the HV transformer increases the amplitude of the alternating voltage to kilovolt values. The very high amplitude signal breaks the isolation of the air gap producing sparks. The spark gap and the antenna are made of segments of steel ropes.
Another alternative for an easy and simple transmitter is the use of piezo electric spark generator of a lighter.
I was thinking not to show the transmitter here because I am using "new" components to generate the spark. Actually, I was thinking to use the original components used by Hertz and Marconi: a relay interconnected to form an oscillator, and a car ignition coil for the HV transformer. Unfortunately, I didn't find enough time to acquire these components yet. You can find such a transmitter in the following link:
NOTE: to broadcast using different channels, one has to add an LC Band Pass Filter (BPF) that indicates the channel frequency to both the transmitter and the receiver. The channel frequency is equal to the natural frequency (or resonance frequency) of the LC filter: f = 1/(2*pi*sqrt(L*C)). Note that receiving the signal requires that the receiver is tuned on the same channel (it uses the same LC BPF).
Step 2: The Receiver
History and science
When Hertz made his experiment in 1887, he reproduced the spark in a gap many steps ahead of the transmitter using a metal ring that acts as a filter. Hertz believes that electromagnetic waves have no practical application. What a spark can do? he said:
“I do not think that the wireless waves I have discovered will have any practical application.”
Heinrich Rudolf Hertz
It is not until Marconi that electromagnetic signals became transmitting useful messages safely. Marconi uses a coherer which is a device invented by the French physicist Edouard Branly that is able to detect electric signals. From wikipedia, a coherer is a " device consisting of a tube or capsule containing two electrodes spaced a small distance apart with loose metal filings in the space between. When a radio frequency signal is applied to the device, the metal particles would cling together or "cohere", reducing the initial high resistance of the device, thereby allowing a much greater direct current to flow through it".
Concerning the Project:
I made my own coherer using a plastic tube (sometimes made of an old pen), iron filing collected from a neighbor blacksmith, and some nails and screws as cathodes. I made some examples with different distances between the cathodes, and after some experiments I ended up using the best working coherer. The coherer is placed in series with a 9v battery, an ON/OFF switch, and an LED. n antenna is also connected to one end of the coherer. Make sure that you connected the LED in the right direction, else it will never go on.
More about coherers can be found in the following pages:
Step 3: Sending/Receiving a Message... Further Applications
Before you start "playing" with the transceiver, make sure that the iron filling between the coherer cathodes is not too squeezed between the screws. Note that the resistance of the iron filling decreases when the iron particles are squeezed, which allows great current to flow across the circuit burning out your LED. You might add a resistance (1K for example) in series with the coherer to ensure that the total resistance of the circuit will not decreases to dangerous values.
Turn ON the transmitter and the receiver circuits. If the LED is ON initially, tap gently on the coherer with your finger until the LED goes OFF. Place the transmitter and the receiver several steps away, and press the button on the transmitter; you should see and hear the sparks across the transmitter gap. If the receiver is in range, the LED turns ON indicating that an RF signal is received. Try to move the receiver closer to the transmitter if the LED stays OFF. Note that the LED remains ON after the reception of the first message, and you have to reset it by tapping again gently with your finger. On the Marconi receiver, the inventor uses an electric bell to indicate the reception of the signal, and another electric bell apparatus to shake the coherer and reset it (the second bell has no cone). Your receiver is now ready to receive the next message.
The genius inventor Nicolay Tesla invented a radio controlled boat in 1898 using spark gap transmission:
In the 1950s, a Japanese company called RADICON produced spark gap controlled car, bus, boat, and robot toys. Check the following exciting videos
I wish I had time to make such toys... If you have time, it would be nice to realize such projects; and please post them in instructables.
More about Marconi and the invention of wireless communication