I want to build a logic probe. What is the best solution to detect a high-impedance state?

  • 2
    \$\begingroup\$ While I cannot advise as to best, I once built a system to detect hi-z by biasing the node to halfway between the logic levels and using an ADC to measure that. Software compared the measurement to a tolerance range and classified it as hi-z \$\endgroup\$
    – Bryan
    Sep 4 at 20:45
  • \$\begingroup\$ That depends as impedance can be voltage sensitive with clamp diodes \$\endgroup\$ Sep 4 at 20:47
  • \$\begingroup\$ You would have to define how high impedance is the threshold between determining high impedance input and for example input that is pulled down or up to some voltage with a resistor. And how much the probe is allowed to affect the circuit so that it does not disturb the normal operation? Then, when you have a specification, the best way can be determined. Or at least a few good ones, as the best is usually opinion-based or too complex and expensive, so likely you just want good enough solution that fits the specs. \$\endgroup\$
    – Justme
    Sep 4 at 21:01
  • \$\begingroup\$ @justme : just a good solution to test logic gates at hobby level. \$\endgroup\$ Sep 4 at 21:05
  • 2
    \$\begingroup\$ @ElianRussel OK, but you still need to define what is and what is not considered high impedance. Also, what types of logic gates? 5V TTL, 5V CMOS, 3.3V CMOS? Any others? Do you want to have a tester for each logic type or universal for many types? \$\endgroup\$
    – Justme
    Sep 4 at 21:09

2 Answers 2


Logic probes detect voltage either high or low. High impedance is not voltage. Generally the probe is biased to mid point voltage through a resistance so it can be "pulled" high or low. High-Z is no action, but could be confused with a valid mid point bias although that should not be the case with logic circuits.

To detect high-Z, a small current is applied then the state is measured. Current into the probe point would measure as high. For your indicators you need a way to discriminate between the Logic high, and the High-Z high.

The current must be small enough not to disturb the operation of the circuit being probed.

EDIT1: An approach that I have used is to use a switch to select between TTL and CMOS levels. TTL levels are fixed to 2V high and 0.8V low. So any thing in between is unknown. Use a window comparator with two outputs: One turns on a red light for hi and the other turns on a green light for low. All other voltages the lights are off. So biasing in the middle will leave the lights off.

For CMOS, the levels depend on the supply voltages, so you need a supply voltage reference to the window comparator then set the thresholds to 70% and 30%.

  • 1
    \$\begingroup\$ So, the idea is to connect "a middle voltage" and then verify the final state of the pin. I got your point: if the pin is in Z-state it will not drive the line. Should the "mid point voltage" depend on the logic family? Can I fix it at Vcc/2? \$\endgroup\$ Sep 4 at 21:03
  • \$\begingroup\$ @ElianRussel: Yes the mid point is based on the logic family. See my edit1 \$\endgroup\$
    – RussellH
    Sep 4 at 21:29

Always start with full specs or functional expectations as one addition might be implemented differently such as "open Circuit detect"

  • Logic Probes may use Schmitt triggers to detect the valid logic state.

  • Dynamic states or glitches are detected and displayed by a <= 1 second toggle.

  • Invalid levels may be detected with a dual Op Amp amp window comparator.

  • Vdd may be supplied by an internal regulator with switch options or alligator clips to D.U.T.

  • Open Circuit may detected by dual OA window comparator with the probe biased to mid-Vdd

    • the probe could also be used to inject voltage at high impedance for CMOS with a 1Hz clock.
    • Indication can be both LEDs dim or OFF or as you specify.
  • Input impedance ought to be >=100k and input capacitance < 10 pF.

This is a partial solution without glitch detection. You can switch to simulate an open circuit and then slide the voltage to the probe. Simulation

enter image description here

  • \$\begingroup\$ Is the oscillator required or I can provide a fixed "mid point" voltage by another solution? OFF TOPIC: The link is broken, too many schemes :) "https://%20http://ww". \$\endgroup\$ Sep 5 at 8:27
  • \$\begingroup\$ Fix link. Nothing is required unless you specify it, like I did. It was just a novelty to inject pulses while being high impedance and open circuit detection in one circuit \$\endgroup\$ Sep 5 at 10:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.