---
title: "Digital Logic Probe"
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published_at: "2026-03-13T00:30:05+00:00"
modified_at: "2026-03-13T00:30:06+00:00"
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excerpt: "Construction guide for a logic probe using half of an LM-339 quad comparator, compatible with both CMOS and TTL logic levels. Two LEDs indicate high/low/pulse states. Parts cost under $10. Includes circuit description and probe tip construction tips."
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  title: "Syntax"
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volume: "3"
issue: "4"
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    title: "Syntax/Syntax ZX80 v3 n4"
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pages: "15"
pubdate: "April 1982"
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---

# Digital Logic Probe

Construction guide for a logic probe using half of an LM-339 quad comparator, compatible with both CMOS and TTL logic levels. Two LEDs indicate high/low/pulse states. Parts cost under $10. Includes circuit description and probe tip construction tips.