The circuit of the TransistorTester in figure 2.1 is based on the circuit of Markus F. released in Abb. 1of AVR-Transistortester report [1]. Changed or moved parts are marked with green color, optionalparts are marked with red color.Some changes are made because the electronical power switch make problems in some implemen-tations. Therefore the resistor R7 is reduced to 3.3kΩ. The capacitor C2 is reduced to 10 nF andR8 is moved so that the PD6 output does not try to switch the C2 capacitor directly. Additionalblocking capacitors are added and should be placed near the power connection of the Atmega andnear the Voltage regulator.Because the PD7 input and PC6 (RESET) are the only pins, where pull up resistors where needed,one extra 27kΩ resistor is added to the PD7 (pin 13) input. With this modification the software candisable all internal pull up resistors of the ATmega.The additional crystal with its 22pF capacitors are optional added. The accuracy of a crystalhas the benefit of more stable time measurement for getting the capacitor values.New software version can use a voltage scale switch of the ADC. The speed of switching isreduced by the external capacitor C1 at the AREF (21) pin of the ATmega. To avoid slowing downthe measurement speed more than necessary, the value of this capacitor should be reduced to 1nF.Removing of the capacitor C1 is also possible. For adapting the software to the actual circuit takea look to the Makefile options in the configuring chapter 4 at page 47.Some different versions of R11 / R12 resistor combinations circulates in the internet. I haveadapted my software to the original of Markus F. [1] with 10kΩ and 3.3kΩ. The voltage ration ofthe divider can be set with Makefile options.The additional 2.5V precision voltage reference connected at pin PC4 (ADC4) can be used tocheck and calibrate the VCC voltage, but is not required. You can use a LM4040-AIZ2.5 (0.1%), aLT1004CZ-2.5 (0.8%) or a LM336-Z2.5 (0.8%) as voltage reference. If you don’t install the precisionvoltage reference and you don’t add the relay extension, you should install a pull up resistor R16 toPC4 with a higher resistance value (47kΩ). This helps the software to detect the missing voltagereference. A optional ISP connector has been added to easier load new software versions to the tester.