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I saw this video: https://youtu.be/KiJH1lgaHrY?t=1m49s (at 1:49) where the guy touched the test pen with his fingers and the test pen lights up.

How could it possible? I tried that on my test pen and it doesn't light up. Is it a special test pen?

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  • \$\begingroup\$ Google "amplification" \$\endgroup\$ – PlasmaHH Oct 18 '17 at 11:27
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What I see in the video is (1) a button battery (coin cell) inside, with access by unscrewing the metal battery cover/cap (using slot type screwdriver at large diameter end of probe), and (2) the fuse continuity measurements that are shown require that you make contact with the battery cap with one finger while holding the fuse-to-be-tested at one end of the fuse and then touching the other end of the fuse to the probe tip. Thus the "test" shows that current (provided by the coin cell) can flow through your body from one hand to the other and through the fuse to the probe tip. That current is very small and is effectively amplified by circuitry in the device to light the LED using power supplied by the button cell. The slotted cap (with battery) is best seen at about 1:15 minutes and the contact via both hands is demonstrated at about 1:50 minutes.

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The finger is sensed, then the sense signal is used to tell another circuit to turn on the light.

You might as well ask how it is possible for just a finger flick to turn on the light in a room when the finger did nothing electrical at all. Even more mysterious, the light persists after the finger is removed!

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  • \$\begingroup\$ So the test pen has touch sensor on the metal tip or is it the pressure on the metal tip that trigger the sensor? Also, I see the light is very strong, does the electric current from the touched human body really power up the light ? \$\endgroup\$ – null Oct 18 '17 at 15:10

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