The diode and continuity tester is a multifunctional module consisting of mainly 3 elements:

  1.  4 LEDs for checking if the voltages still are active
  2.  the continuity tester (mainly software)
  3.  the diode tester for measuring voltages of diodes

I don’t think that I have to explain the LEDs… so let’s move on to the contionuity tester.

The continuity tester works like this:
A digital pin from the Arduino is clocking out a signal of ~100Hz. Another digital pin checks if the signal from the first pin is coming through the terminals on the prontpanel back to the second digital pin. If so, there likely is a continous path. If the pin receives the signal, the Arduino is enabling a small logic circuit which puts the output of an astable multivibrator on a piezo element making a sound at about 5kHz. Also a signal is sent to the mainboard causing the screen to flicker white/black. So just in case you went deaf you still can see the testing result.

Via control on the TFT the module can be put in diode testing mode. A relais is disconnecting the continuity signal and connects to a constant current source consisting of a transistor. The transistor can switch between 3mA and 15mA via another relais controlable on the TFT screen. The voltage across the output terminals is measured and calculated by the Arduino.