Low-component-count logic probe works with TTL and CMOS logic

摘要

The circuit in Figure 1 uses the LM358 dual op amp running as a comparator, plus a few other inexpensive components, to make a TTL (tran-sistor-transistor-logic)/CMOS-logic probe. The circuit gets its power from the circuit under test, which lets it work with TTL or CMOS logic. The IC_(1A) and IC_(1B) op amps come in an LM358 package. Switch S_1 selects the TTL or the CMOS mode of operation. The green LED shows logic low, and the red LED shows logic high. The noninverting input of IC_(1A) and the inverting input of IC_(1B) connect to the test probe. The circuit uses 90% of the power-supply voltage as CMOS-logic high and 2.7V as TTL high. It uses 0.7V as logic low for both TTL and CMOS because their logic-low levels approach 0.7V. Voltage divider R_3/R_4 divides voltage from 2.7V zener diode D_1' providing 1.35 V at IC_(1A)'s noninverting input and IC_(1B)'s inverting input. Diode D_2 is forward-biased, and the 0.7V voltage across D_2 becomes the lower limit, which represents logic low. You set this voltage on the noninverting input of IC_(1B).