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                  ReadMe for the Component Tester Hardware

         (c) 2013 by Markus Reschke (madires@theca-tabellaria.de)

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Last edit:  2013-03-22


* About

The Component Tester is based on the project of Markus Frejek [1&2] and the
successor of Karl-Heinz Kbbeler [3&4].


* Safety Advice

The Component Tester is no DMM! It's a simple tester for components capable
of measuring several things. The probes aren't protected in any way and
won't survive higher voltages than 5V. Don't use the tester for live circuits!
Just use it for unsoldered electronic components! If you test a capacitor make
sure it's discharged before connecting the probes. This isn't just the Safety
Sally, your life may be at risk if you connect the probes to a live circuit
or a power supply (or even mains). 


* What's different?

Karl-Heinz has done a realy great documentation of the tester. I recommend
to read it. Therefore I'll tell you just about the differences to the
official release.


* What's in the box?

Following files are in the archive:
- this README
- schematics in PDF format
- schematics and PCB layout for Eagle 6.4.0
- pictures of my Component Tester


* Circuit

The circuit was extended with a current limiting PWM driver for the LED
backlight of the LCD module. The PWM circuit is based on an idea of
Eberhard Haug [5]. You'll find the details of the current sensing resistor
for the current limiter in the schematics. Calculate the proper resistor
values for the LCD module you're using.

The tester has two power supply inputs, one for battery operation (9V) and
another for an external power supply. The LCD backlight is powered by the
latter input only. The supply voltage shouldn't exceed 12VDC because the
low drop voltage regulator supports 13.2V at maximum (add 0.7V for the
reverse protection diode).


* Layout

The layout is optimized for a benchtop-type box and home lab PCB production
without through-hole plating. I recommend to use precision sockets for both
ICs. They allow to solder the pins at the top layer without much trouble.
You might need to grind the top layer pads for the crystal to prevent any
short circuit of the leads via the metal case. Also the drill diameter of
the pads for the ISP pinheader connector could require some rework.


* References

[1] AVR-Transistortester, Markus Frejek,
    Embedded Projects Journal, 2011-11

[2] http://www.mikrocontroller.net/topic/131804
    thread of Markus Frejek, Forum, 2009

[3] http://www.mikrocontroller.net/articles/AVR-Transistortester
    Online documentation of the Transistortester, Online Article, 2009-2011

[4] http://www.mikrocontroller.net/articles/AVR_Transistortester
    Short description of the TransistorTester, Karl-Heinz Kbbeler,
    Online Article, 2012

[5] http://www.led-treiber.de/html/leds_grundlagen.html#PWM-Dimmer
    Description of a LM339 based PWM circuit, Eberhard Haug.



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