Low-component-count Logic Probe Works with TTL and CMOS Logic

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 (transistor-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 S₁ selects the TTL or the CMOS mode of operation. The green LED shows logic low, and the red LED shows logic high.


FIGURE 1

The noninverting input of IC1A and the inverting input of IC1B connect to the test probe. The circuit uses 90 percent 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 R₃/R₄ divides voltage from 2.7V zener diode D₁, providing 1.35V at IC_1A’s noninverting input and IC_1B’s inverting input. Diode D₂ is forward-biased, and the 0.7V voltage across D₂ becomes the lower limit, which represents logic low. You set this voltage on the noninverting input of IC_1B.

In TTL mode, the voltage at the inverting input of IC_1A is 2V. In CMOS mode, the voltage at the inverting input of IC_1A is nearly 90 percent of the input voltage through voltage divider R₆/R₇. When the probe is in its high-impedance state in either CMOS or the TTL mode, IC_1A’s inverting input voltage is greater than its 1.3V noninverting input voltage. IC_1A’s output is low. IC_1B’s 1.3V inverting input voltage is greater than its 0.7V noninverting input voltage. The output of IC_1B is also low, and both LEDs are off.

In TTL mode, when measuring logic high, IC_1A’s 2.7V inverting input voltage is less than its noninverting input voltage, which is the probe voltage. IC_1A's output is high. IC_1B’s inverting input voltage, which is the probe voltage, is greater than its 0.7V noninverting input voltage. IC_1B’s output is, therefore, low. The red LED turns on, indicating logic high.

When measuring logic low, IC_1A’s 2.7V inverting input voltage is greater than the voltage at its noninverting input, which is the probe voltage. Thus, IC_1A’s output is low. IC_1B’s inverting input voltage, which is the probe voltage, is greater than its 0.7V noninverting input voltage. Thus, IC_1B’s output is high. The green LED turns on, indicating a logic low.

In CMOS mode, when measuring logic high, IC_1A’s inverting input voltage, which is 90 percent of the supply voltage, is greater than the voltage at its noninverting input. The output is thus high. IC_1B’s inverting input voltage, which is the probe voltage, exceeds that of its 0.7V noninverting input voltage, and IC_1B’s output is low. The red LED turns on, indicating logic high.

When measuring logic low, IC_1A’s inverting input voltage, which is 90 percent of the supply voltage, exceeds the voltage at its noninverting input. IC_1A’s output is low, and IC_1B’s output is high because its inverting input voltage is higher than 0.7V at its noninverting input voltage. The green LED turns on, indicating logic low. When the probe’s pin is pulsing, both LEDs alternately turn on and off at the pulse frequency.