# Logic probe

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#### Gregory

##### Member
Can you use a logic probe on a circuit without power to the circuit.
Can a logic probe test resistors , transistors and any IC
How do you use a logic probe, what does it tell you.
Never used one befor

#### Pommie

##### Well-Known Member
A logic probe just tells you if a voltage is present or not. So yes, you can use it on an unpowered circuit but everywhere will read as zero.

Mike.

#### Gregory

##### Member
I wan to test a circuit fo faultes can I use the probe to do this

Mike.

#### Gregory

##### Member
I have a circuit board If I placepower on the board then how do I go about telling what components are faulty.

#### Pommie

##### Well-Known Member
A logic probe is only useful for logic circuits and can't test non logic components like resistors etc.

Mike.

#### Gregory

##### Member
How do I go about testing a Ic and transistor

#### Pommie

##### Well-Known Member
That's like asking "how do I do brain surgery?". Without a lot of study, you don't.

Mike.

#### ronsimpson

##### Well-Known Member
A logic probe can not test resistors or transistors. (most likely)
You need to know how the board works in detail before the probe will help you.

Most logic probes measure if the voltage on the probe tip is low or high or pulsing. That information probably does not tell you anything.

#### Gregory

##### Member
I do not have a circuit diagram.
So I need to test components on the board.
To find the fault

#### Gregory

##### Member
I know how the components work but putting it all together and reading the circuit is my problem .
Could you give me a simple circuit and describe the voltages at each point.
Thank you

#### Les Jones

##### Well-Known Member
You can test static logic like AND gates. OR gates, decoders and multiplexors PROVIDED that you KNOW ENOUGH about how the whole circuit is designed to function and you KNOW HOW to fool it into creating ALL the posible input signal states to the gates you are testing. The logic probe only shows if the signal output is high or low which you can do with a multimeter. A multimeter can give you more information by showing signal levels that are not valid for the logic family that you are testing. Some logic probes have a pulse stretching function that enables you to see very short pulses that you would not see if the LED only lit for a few microseconds. But as others have said you need to FULLY UNDERSTAND how the circuit is expected to work.

Les.

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