Benchmarking the Cigar Box Crystal Radio Against the Car AM Radio: Benchmarking is an engineering term for comparing a new product against a well known product of high quality. In this video, the car radio in the home driveway is used to identify the AM radio stations that one could expect to receive on a homemade Cigar Box Crystal Radio. The AM car radio became the benchmark when it identified the frequencies of 10 potential stations in the air.
This video explains how the Cigar Box Crystal Radio started and evolved into a project for teaching in the classroom. At first, the radio did not work for teaching. An amplified speaker added to the radio allowed it to be used for teaching in the classroom. In addition, if the modified radio was attached an external antenna three local radio station were clearly resolved with loud audio. See the two versions previously made and ideas for improving the project in the future.
One of the advantages of the Cigar Box Crystal radio is that is does not need an outdoor antenna and ground rod. For a young scientist, it would be a major feat and cost to string up a 140 foot antenna outside and then also drive a copper rod 10 feet into the ground. For most kids, that would never happen. However, a loop antenna around a cigar box crystal radio is an alternative. However, the signal will need to be amplified
at the speaker.
The Cigar Box Crystal Radio project uses a loop coil around the outside of the box as both the antenna and the tuning inductor. A variable air capacitor is used as the capacitor of the tuning circuit or "tank circuit". One of the principles of radio electronic can be learned while building this crystal radio by measuring the inductance of the coil and the capacitance of the capacitor and calculating the frequencies at which the radio will resonant.
The audio resonance between tuning forks is an analogy for RF resonance between antennas. One can feel and experience the auditory phenomena and then it is easier to understand how radio waves can make an antenna resonate at the same frequency. The same size and material forks are required and that explains why antenna size and material is also important for RF resonance.
Suncatchers as familiar objects in most peoples lives, and crystals collect and refract light in home windows entertaining us with glistening and colorful light patterns to our eye. Light is an electromagnetic radiation that we can see. Radio waves are also an electromagnetic radiation, but we cannot see them. Antennas collect and guide radio waves through wire to receivers like radios, cellphones, and TVs that in entertain us with music, voices, images, and videos. The crystal radio that is going to be built is like an Rfcatcher that captures and reflects voices and music.
Wave animation made by IncrediVFX (https://www.youtube.com/channel/UC4u_N4P_ZCyASxXX3o-QGWg)
This image has water waves moving in the background and a stationary radio wave drawn on top. Radio waves cannot be seen but are similar to water waves in that they too travel with repeating high and low heights. The water waves have highs and lows, but only in the up and down direction. Radio waves have highs and lows in two directions, up and down and sideways, as shown as blue and red. The up and down
radio waves in blue are electric fields, symbolize as E ⃗. The sideways magnetic field are in red and labeled as B ⃗. Water waves are visible and travel slowly. Radio waves are not visible, travel at the speed of light, a million times faster than water waves. Speed is symbolized as v ⃗.
Our class has worked with magnetic field and electric fields in the past, but never both in the form of radio waves. If someone sprinkles iron filing on over a magnet, they can see the shape of the magnetic field in the iron filing ( see left picture). If someone tapes strips of paper to the metal of a Van de Graff generator, they can see the shape of the electric field in the bent paper strips (see right picture). Like magnetic and static electricity have fields that we can’t see without help, radio waves have both electric and magnetic fields that we cannot see that travel through space.
Fifteen soldering practice kits were purchased from Banggood for $17.30 or $1.05 each. The expected class size is 13 student, however it is good practice to purchase a couple of extra units just in case a beginning student ruins their circuit board.
Fifeteen “Ktaxon 14 in 1 40W 110V Soldering Iron Kit Set Station, Include Desoldering Pump, Tweezers, Rosin, Suction Cup, Screwdriver” sets were ordered from Walmart at $12.99 each with free shipping. This is about as cheap as I have ever seen a soldering kit, and if it lasts the 6 weeks of the course than the requirements are satisfied. The soldering iron is available in 3 wattages: 30W, 40W, and 60W. The 40W unit was ordered should provide more than enough heat to melt solder. This class was advertised for student ages 8-10, and there will be a good adult to student ratio for supervision. This is the first time in 26 years of teaching that I have seen so many components available at this low price. Previously, the iron and stand were ordered separately for $9 and $5, respectively.
The students will use their crystal radios to “catch” electromagnetic radiation (EMR) at about 1,000,000 Hz, the AM radio band. There are two types of EMR, natural and man made. Marconi made the first radio station in 1895, before that there was only natural EMR. Naturally, stars are constantly emitting EMR across the entire spectrum, although the majority is in the form of infrared, visible light, and ultraviolet. We can only see visible light, all the rest remains invisible to us. However, the crystal radio can detect man made radio waves from a local station. It is a thrill to make this EMR and detect invisible EMR.
see https://www.youtube.com/watch?v=lwfJPc-rSXw
The crystal radio uses simple components to make the radio: coil, capacitor, diode, and ear phone. These components teach the fundamental concepts of radio electronics of the inductor-capacitor resonance circuit, the diode signal rectifier, and audio transducer. In this project, the inductor is also the small loop antenna for the system that allows the student to avoid needing a long wire antenna and earth ground, major barriers to completing your first radio. The ear phone was later replaced by an audio amplifier and speaker for ease of use. All this construction consists of easy first time project tasks. The project is based on plans by Joseph Cooper, VE3FMQ, called “The Cigar Box Crystal Radio Set.”