TTL Three - State Logic Probe PCB

Posted on Feb 4, 2014

A TTL logic probe is an indispensable tool for digital circuit troubleshooting. Various methods can be used to design a logic probe. The most common designs employ opamps, logic (OR, NOT, XOR) gates, and transistors. The circuit presented here uses NAND logic gates of Hitachi HD series IC HD74LS00, which is a quad-NAND IC. Special technique has be

TTL Three - State Logic Probe PCB
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en employed to obtain three-state operation using just a single IC. In the circuit Gate N1 is wired such that when the output of gate N1 is at logic 0` (i. e. when its input is at logic 1`), LED1 will glow, to indicate high state of the point being probed. Gate N3 is wired to light LED3 when the output of gate N3 is high or when the point being tested is at logic 0` level. At power-on, the output of If any other type of IC (e. g. 74HCT00 or 74LS00) is used, diodes D2-D3-D4 should be added or deleted as necessary; for example, when using HD74LS00, one diode D2 is required.

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