Using the LM3909 integrated circuit can be made a continuity tester using a few simple electronic components. Dimensions that can be achieved this continuity tester is extremely low. Miniature speaker sounds when the resistance of the probe is between 0 and 100 ohms. Differences of 5 ohms are translated into corresponding variations in output volume.
The battery has a relatively long life because the power consumption of continuity tester is only 30 mA when the input terminals are shorted.
This TTL and CMOS signals tester circuit is very simple logic tester that allows analysis of TTL and CMOS signals. Circuit diagram shows that T1, R3, R4, D5 and D6 form a current source for LEDs D3 and D4. As a result, the current through the LED will be about 12 mA, regardless of supply voltage, LED cathodes are connected to ground either N1 or N2, respectively.