ATTL 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 been employed to obtain three-state operation using just a single IC.

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 NAND gate N2 goes low. This is the default state of the gate. The output of gate N2 goes low (indicated by glowing of LED2) during the following situations:

1. Power to the probe is switched‘on’.

2. The probe’s tip is floating, i.e. when it is neither in contact with a point at logic ‘0’ nor at logic ‘1’ state.

3. The probe tip is in contact with a TTL output that is in high impedance state.

The LED indications for various tip levels are summarised in the accompanying table.

S. No. TIP level Output
1. Ground/logic 0 LED3 ON
2. Vcc/logic 1 LED1 ON
3. Floating/or connected
to high impedance LED2 ON

This logic probe is very well suited for use with microcontrollers, microprocessors, EEPROMs, SRAMs etc. Some points to be noted are:
• Use IC1 of type HD74LS00 only.
• 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.
• Use another diode in series with D2 if the LED indication overlaps the float indicator.

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