subject=Re:[EE]:Need help with faulty El Cheapo
source= http://www.piclist.com/postbot.asp?id=piclist\2001\09\01\105918a


Mike, thanks for the reply.  Unfortunately I don't think that covers the problem.  I had already taken care of the 2N7000 and have it properly installed (D to the right [with the board oriented with the DB-25 connector on the left]).  The transistor is working and the output voltage clearly varies from high to low.

I also noticed the book-schematic errors with the 3906 and have installed that properly as well.  (Myke has a proper schematic in his current software.)

The problem is that the 3906 is staying active all the time.  The emitter is tied high (as it should be).  When the 2N7000 is inactive, the base of the 3906 should be high as well (via the pull up resistor R5).  The problem is that the voltage at the end of R5 is .7V "less high" than the voltage at the 3906 emitter.  Since the transistor activates at .7V, it never shuts off.

ASCII art:

+ ------------------+-------- E-3906-C ---- (output)
                    |             B
                   10K            |
                    |             1K
                    |             |
GND --- S-2N7000-D -+-------------+
             G
             |
          pin 16
         (control)

PROBLEM:  With the 2N7000 inactive, the voltage at 3906-B is .7V less than the voltage at 3906-E.  (With the 2N7000 active, there is a large voltage differential - as expected.)

I am considering moving the 1K resistor.  If the 2N7000 is sinking a small amount of current, possibly the 1K can slow it down and allow the 10K a little more of an edge.  Hopefully this would lessen the .7V to a point where the 3906 can go inactive.

The new picture would look like this:

+ ---------------------+--- E-3906-C ---- (output)
                       |        B
                      10K       |
                       |        |
                       |        |
GND -- S-2N7000-D --1K-+--------+
            G
            |
         pin 16
        (control)

I was also considering lowering the 10K resistor (what's a few milliamps between friends).

---

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