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When testing circuits with a logic probe , it is sometimes difficult to watch the LEDS on the probe to determine the logic state. With this probe the logic states are audible. This probe is designed for TTL circuits only but could be modified for CMOS. The way it works is as follows. The 5 volt power source will be the circuit under test. Clip the ground input of the probe to the ground of the circuit being tested. The other input lead is used to probe the different chips of the circuit being tested. Any input greater then 2 volts will be high and output a high tone through the speaker. Any input less then .8 volts will be low and produce a low tone through the speaker. Audible Logic Probe  Circuit Diagram

This is a Digital Complementary metal–oxide–semiconductor (CMOS) logic Probe Circuit Diagram.The logic probe can indicate four input states, as follows: floating input—all LEDs off logic 0 input— D2 switched on (D3 will briefly flash on); logic 1 input— Dl_ switched on; pulsing input—D3 switched on, or pulsing in the case of a low frequency input signal (one or both of the other indicators will switch on, showing if one input state predominates). Digital Cmos logic Probe Circuit Diagram