Don

The first thing to check is whether you are actually able to wiggle the
parallel port lines.  When you click on "Low" does pin 16 of the parallel
port actually change?  Recent versions of Windows are very unfriendly about
allowing programs to play with I/O devices.

Once you see pin 16 wiggle, trace the signal through to the base of Q1.
Once you can be sure you have an input signal, it should be pretty simple
business to trace the signal step by step through the circuit until you come
to a short/bad solder joint/bad component, whatever.  But first be sure you
have a stimulus.

--McD

----- Original Message ----- 
From: "Don or Pepper Hewitt" <hewitt_10@comcast.net>
To: <piclist@mit.edu>
Sent: Friday, May 27, 2005 1:44 PM
Subject: PIC: Help with ElCheapo


> I am new to PIC micros and I am trying to build Myke's ElCheapo from the
> website (the home built version using 74LS05). I am using the windows
> version of the programmer.  I have gone through all the build/test steps
> and they all work except for step 9 as follows:
>
> --------------------------------------------------------------------------
---------------------------------------------------------
> Step 9: Check the VPP Driver
>
> 1.  Solder:
> a.  Solder 10k Base Current Limiting Resistor (R8) to R9/U3 Pin 6
> Connection.
> b.  Solder in 2N3906 PNP Bipolar Transistor (Q1) as shown in the diagram
> to the left.  Leave the Transistor's Collector floating (unconnected).
> c.  Connection from Collector of Q1 to Pin 4 of U2 (PICmicro MCU to be
> programmed).
>
> 2.  Connect El Cheapo to AC/DC ('Wall Wart') Power Supply and Measure
> Voltage *output from 74LS05* by connecting the Positive Output of the
> DMM and Ground as shown in diagram to left.
> a.  Click on 'Low' Below.  If Voltage at Q1's Collector does not change,
> Check wiring at at base current limiting resistor resistor (R8).  The
> voltage may only drop to 5 or 6 volts.  You may want to repeat this test
> after the part number socket is wired in with a PICmicro MCU installed
> to make sure that this line drops to less than 0.3 Volts
> b.  Click on 'High' Below.  If Voltage at Q1's Collector is less than 13
> Volts, Check wiring at base current limiting resistor (R8) and
> connection between Q1's Emitter and 13 Volts.
>
> When you have appropriate voltages out of Q1's Collector for 'Low' and
> 'High', Disconnect Power and Click on
>
> --------------------------------------------------------------------------
---------------------------------------------------------- 
>
>
> With the P16F84A not connected; the voltage at the collector of Q1 is ~
> 13 volts when I Click on Low  it is also at ~ 13 volts.  If I understand
> Step 9 correctly; the collector should drop 5 or 6 volts when I click
> Low.  I did connect the collector to Pin 4 of the 16F84 just to see and
> it went to 8 volts when I clicked low.
>
> My knowledge of transistors is very limited.
>
> Am I understanding Step 9 okay?  Do you think I am not actually
> programming the 16F84A?
>
> Please help.  I am very frustrated.
>
> Thanks,
>
> Don Hewitt
>
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