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Radio wave sensor, coherer detector (receiver in the early days of radio) and rain sensor
Children may end up with "Hmm" when looking at the sensor. That's because you can't see the inside. So this time it can easily see inside and understand mechanism. Introducing the coherer detector and rain sensor. When a radio wave is received, a current flows through the circuit, and when it gets wet with water, electricity passes through, both of which act as sensors.
Experiment 1: Make a coherer detector
you need : Battery, LED, film case, aluminum foil
Turn off the aluminum foil to 10 cm square. Make it a ball with a diameter of about 1.5 cm. It takes a lot of power. In addition, attach two electrodes to the plastic cup.
The aluminum sphere rounded to the cup You have put about 10 pieces. Connect LED / 3V power supply in series. If the LED lights up, shake the cup lightly to prevent current from flowing. If one of the electrodes is extended into the air to make an antenna, the sensitivity will increase.
If you ignite an electronic lighter nearby, the LED will light up.
Aluminum is very sensitive to oxygen, and there is always a 10 Å oxide film on its surface. This film is a substance called Al2O3 that does not conduct electricity, but it will break if voltage or force is applied. The voltage of the discharge of the electronic lighter broke the coating and caused current to flow, and the LED turned on. When you shake the cup, an oxide film is formed on the surface again and the LED goes out.
Coherers generally used metal powder. In this way, wireless communication that receives radio waves has become possible. In addition, it seems that the famous Titanic also transmitted SOS with this communication device.
Experiment 2: Let's make a rain sensor
you need : Aluminum foil, tissue, salt, lead wire
This sensor? It's not so much, but when it gets wet, the saline solution conducts electricity and sounds a buzzer.
Make a circuit that sounds the buzzer, and arrange the aluminum foil slightly apart. Place tissue paper wrapped in salt on it. When the tissue gets wet, the inside becomes a saline solution and an electric current flows. (If only water is used, it will hardly flow at around 3V)
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