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Smoke Detector

Bill takes apart a household smoke detector, showing how it uses a radioactive source to determine the presence of smoke. He also discusses the MOSFET used in the detection circuit.

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Transcript I find smoke detectors a marvel of engineering. Let me show you how it works. This black cylinder has louvers which guide air into the detector. Now, it hides the essential part of the device.

Tucked in here lies about one micro-curie of radioactive Americium 241 - that's about 0.29 micrograms. That tiny bit of radioactive material generates a small current that makes the detector work. Let me explain how.

Air flows between these two electrodes, now of course air doesn’t conduct electricity, but when alpha particles from the radio-active Americium slam into the oxygen and nitrogen molecules that make up air they knock electrons about leaving charged charge gas molecules. The 9 volt battery causes these ions to move thus creating a current. Now, here its tiny: Something like 100 picoamps, about one hundred billionth the current flowing in your home.

When smoke enters the chamber the ions attach to it and slow down and often lose their charge, both events cause the current to stop, which triggers the alarm.

To make such a compact thing work with only a nine-volt battery required the solid state revolution of the 1960s. Let me show you. If I remove the two electrodes you can see a small integrated circuit.

It incorporates a marvelous device called a MOSFET, which can detect those very small changes in current. In the detector it serves as an on-off switched triggered by the tiny current between the electrodes. Like every transistor, its operation depends on being able to make diodes from semiconductors.

Recall that a diode allows current to pass in only one direction because it uses two types of semi-conductors - a type that uses negative charge carriers and one that uses positive charge carriers. Flip that battery and the flow of charge stops.

Now, to "build" a MOSFET we take two such diodes and put them together so that each is reversed. Now this seems useless because no current will ever flow through such an arrangement, but engineers embed this diode sandwich into the same type of semiconductor as the diode ends that touch. Then they place metal contacts at the ends of the diodes and on the block of semiconducting material. Next they coat the diodes with a thin layer of silicon dioxide. Now unlike metals or semiconductors this doesn't conduct electricity at all.

On that they place one more metal contact called the gate, which opens or closes the current channel between the source and the drain. When we create a voltage difference between the the gate and the source it generates a field through the insulating layer that draws the free positive charge carriers toward the gate, opening a channel that now allows current to flow.

In a circuit we use the 9-volt battery to create a potential difference between the source and the gate. Its “biases”the gate so that the current flows through the MOSFET thus turning on the horn, so we counter that with a current flowing from the detector. Here is where the ionized gas creates the tiny current between the two electrodes I showed you earlier. That current passes through a large resistor and creates a voltage that opposes the battery and it shuts down current flow through the MOSFET. If smoke enters the chamber, the tiny current stops, the MOSFET allows current to flow in this section of the circuit which triggers the horn.

To me this is engineering at it best: Simple, reliable, and inexpensive ... and saving countless lives. I'm Bill Hammack, the engineer guy