# Logic probe vs multimeter

Joined Jan 12, 2021
40
Hello folks!
I was just reading about the workings of a "Logic Probe". It supposedly tells us when a certain wire is logic HIGH or logic LOW.
My question is, why do we need this probe if I can just use a multimeter to find the voltage and then deicde if it is HIGH or LOW based
on the displayed voltage value. If the voltage is between 0 Volts and 0.8 Volts ( it will be a logic LOW ) versus 2 Volts and 5 Volts ( which will be a logic HIGH).
Thank you

#### LowQCab

Joined Nov 6, 2012
1,096
Because becoming cross-eyed trying to read constantly changing Digital numbers is
much more difficult and time consuming than noticing if an LED is illuminated or not.
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#### crutschow

Joined Mar 14, 2008
27,728
The logic probe will respond to much higher frequency signals.
The multimeter is only good for DC levels, or very slowly changing signals.

#### MrChips

Joined Oct 2, 2009
24,194
You need to be careful here. Not all logic probes are designed equally.

1) A DMM can show absolute voltages of steady or slowly changing signals,
A DMM cannot accurately show fast changing signals. You could use the frequency option if the DMM has it.
A DMM cannot show pulses of short duration.

2) A properly designed logic probe is good for quick verification of logic levels.
It can also indicate oscillating logic signals of a designed frequency range.
It can indicate the presence of short duration pulses.

The proper tool for diagnostic work is an oscilloscope.

Edit: cc beats me to it.

#### dl324

Joined Mar 30, 2015
13,119
My question is, why do we need this probe if I can just use a multimeter to find the voltage and then deicde if it is HIGH or LOW based
on the displayed voltage value. If the voltage is between 0 Volts and 0.8 Volts ( it will be a logic LOW ) versus 2 Volts and 5 Volts ( which will be a logic HIGH).
It depends on the logic probe design. Some have hardwired threshold voltages and some work for TTL or CMOS and will adjust logic thresholds appropriately.

The HP545A will also indicate floating nodes, will capture single pulses, and, for fast pulses, will blink the indicator at a slower rate.