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