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schematic

simulate this circuit – Schematic created using CircuitLab

At a coffee house recently, I saw a napkin drawn schematic at the table before the waitress cleaned up after the previous patron. I do not remember the part number for the P ch JFET, but the N CH was MPF102. I cannot figure out what the point of such a circuit would be. It looks to me like the JFETs are going to be biased off unless the voltage rises past the source drain breakdown voltage. Anything that high would damage the microcontroller input pin. What am I missing here?

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  • \$\begingroup\$ let's just say, if you were a spy following a person of interest, they just gave you the slip :) \$\endgroup\$ – Techydude May 7 '15 at 11:46
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    \$\begingroup\$ Don't believe everthing you see written on a napkin left lying on a table. \$\endgroup\$ – Olin Lathrop May 7 '15 at 12:06
  • \$\begingroup\$ I'd GUESS they were aiming a a current source. \$\endgroup\$ – Russell McMahon May 7 '15 at 16:20
  • \$\begingroup\$ I found this:lcbsystems.com/LambdaDiode.html \$\endgroup\$ – steverino May 8 '15 at 18:30
  • \$\begingroup\$ If GP3 is set to wake on change then the goal may have been to wake a sleeping part when the voltage rises. I have used a zener for this before. But this approach (if properly biased, unlike the circuit I copied) would not eat battery until the wakeup like the zener would. It would make sense that someone would use two FETs and a few resistors in preference to a zener and a resistor if battery life is critical. \$\endgroup\$ – steverino May 8 '15 at 18:48
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The circuit is wrong/useless. Look at the 0V connections - the series combination of J1 and J2 are shorted by 0V. This effectively makes the circuit V1 in parallel with R3: -

enter image description here

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Ignoring that spurious extra ground at the top right (presumably supposed to go to the microcontroller GPIO), the JFETs will present a low impedance near 0V and increase for (poorly controlled) higher voltages. JFETs are depletion devices (normally 'on').

I don't see much point to this circuit as drawn. Sometimes JFETs are useful as low-leakage clamp diodes, but that's not what's happening here.

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