Searching \ for '[EE]: Need help with faulty El Cheapo' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/index.htm?key=need+help+with+faulty
Search entire site for: 'Need help with faulty El Cheapo'.

Exact match. Not showing close matches.
PICList Thread
'[EE]: Need help with faulty El Cheapo'
2001\09\01@093417 by Jim Brunner

picon face
(sorry for the repost - forgot the ":" in the subject)

I'm building the El Cheapo programmer from Myke's book and I'm having
difficulty with a hardware problem.

I have the latest software and parts list.

Going through the build steps, things look good up until step 9,

At step 2, voltage across C1 = 17.0V
At step 3, voltage at R1 = 13.08V
At step 4, voltage across 1 & 14 of U3 = 4.99V
At step 6, voltage at pin 13 of U2, when high = 4.89V, when low = .794V
At step 7, voltage at pin 12 of U2, when high = 4.95V, when low = .04V
At step 8, voltage at R6, when high = 12.35V, when low = .002V

At step 9, voltage always reads HIGH.

I pulled transistor Q2 and checked the input voltage.  The delta
between the 2 resistors that connect to the base and emitter is
13.05V when high, and .682V when low.  It looks like the .682V is
enough to drive the transistor.

Tracing backwards in the schematic, it looks like this input is a 10K
pull-up resistor to +12V pulled down by the 2N7000 MOSFET.  There
should be +12 on both the emitter and base unless the MOSFET pulls
down the base.

Is it possible that the MOSFET is "leaking"?  Could it be pulling the
circuit down by .7V when not powered?

(I've checked and double checked the orientation on all of the
transistors.  I've downloaded the data sheets and cross checked
against the schematics.  I noticed the emitter/collector error in the
book on Q2 (which is corrected in the schematic shown by the
software.)
--
-----------
Jim Brunner

_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com


--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2001\09\01@105918 by Mike Blair

picon face
Hola Mate,

I had the same problem.

The 2N7000 is not shown correctly on the PCB layout.  The flat side will need
to face the rounded part of the drawing.  If you admire it (schematic) you
will see that the drain would be connected to ground if you put it in like
the picture.  Reading from the data sheet the pins are (from the top) DGS,
the board is arranged SGD if you used the picture.

See page 601 of the book, you will also need to correct the schematic
concerning the transistor Q2.  The schematic is incorrect, the collector is
connected to pin 4 of U2 (18-pin socket) and the emitter is connected to the
7812 13V supply.  The drawing for the pin layout of the 2N3906 is also wrong.
The pins are from the top CBE vice the EBC shown.

Hope this helps.





Michael Blair
WP3K
Gringo en Puerto Rico!
Grid: FK68vk

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2001\09\01@175845 by Jim Brunner

picon face
(oops again - the online reply came out with a null subject.  I can
feel the flames already.)

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).

--
-----------
Jim Brunner

_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com


--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2001\09\01@220728 by Jim Brunner

picon face
OK.  Progress.  I've tuned down the power supply a bit.  The spec
calls for a >14V power supply.  I was using a 24V (which is greater
than 14).  The actual voltage was closer to 30V which is near the
limit for the 78L12 regulator.  Using a 15V supply, the board is now
passing all of the self test functions.

Unfortunately, when I attempt to program a PIC 16F84, I get the
following error from Myke's programming utility:

   Programming Failure at address 0x00
   0x0186 was Written, but 0x07F was Read Back

(I also got this error last night when the board wasn't working properly.)

Any ideas?
--
-----------
Jim Brunner

_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com


--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


More... (looser matching)
- Last day of these posts
- In 2001 , 2002 only
- Today
- New search...