Logic Probe

138

02:00:05

 High Logic Led on,   Low Logic Led off.  
SPST switch is there to simulate High and Low . Based around two 2n2222 NPN Transistors

published 10 years ago

tehmarkster

10 years ago

Is the beta helper necessary for a simple switch?

rbrtkurtz

10 years ago

Kind of, yes. The point is that you a high impedance, as you don't want to want to draw much current from the signal you are probing. So using a Darlington configuration like this, you only need to draw 1uA to turn on the 10mA LED, rather than the 100uA you would need to draw using only 1 transistor. (FYI, these numbers are bare minimums, assuming a ß of 100 on the transistors. You'll probably want to draw more like 5-10uA to ensure the transistors get turned on fully.)

rbrtkurtz

10 years ago

Also, for all intents and purposes, a MOSFET might a wise choice to use in this application, since they are voltage controlled devices, and will draw even less current from the signal. E.g., you cab probe it and see what's going on, and disturb the original signal very little.

rbrtkurtz

10 years ago

Lots of typos, sorry. Y'all should get the point...

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