[SOLVED] how to use a logic probe

[zooterman]

Hi my first post.
I have been given a Lp 2800 logic probe, can someone tell me how to use it, like where do I connect the 2 wires with crocodile clips. I need simple instructions on how to use it.
thanks.

 

[FoxyRick]

Congratulations on a useful piece of kit. Logic probes seem to be a disappearing tool these days, but they are still very much worth having on your work bench.

Connect the two crocodile clips to the power supply of the circuit you want to test - red to positive, black to negative or ground. Don't connect to more than about 15V though. Generally any logic circuit will be much less than that.

Then, the probe tip can be touched to any terminal, integrated circuit pin, etc. that you want to check the logic level of.

Logic probes do not tell you a voltage, like a multimeter, and they are not a tool for using on an analogue circuit with op-amps or transistors (usually). Instead they work on a logic circuit (with TTL or CMOS logic gate chips) and tell you if the level is a logic high (1) or a logic low (0). That is above about 70% of the supply for a high, or below 30% of the supply for a low. If you connect to a rapidly alternating pin, the lights will flash.

Note that some pins do not like to be probed, for example ones with a crystal oscillator. That will usually crash the board. Just reset the power then. Also, fast or very low voltage logic circuits might not like being probed (like a PC motherboard for instance), and the probe might not work on them anyway. That's why they are not used as much as they used to be when things were simpler and slower.

Edit to add: Just a cautionary note: Make sure not to slip with the probe and short two pins together. Also, make sure that the crocodile clips don't slip and do the same. I once blew an old computer up when that happened to me and shorted the power rail I had clipped on to, to a couple of signal lines right next to it

 

[bigdogguru]

The alligator clips are typically for power, simply attach the red lead to Vcc (+) and the black lead to GND.

The tip of the probe is then applied to the test point, a red LED indicates a logic 1 or high, a green LED indicates a logic 0 or low and a yellow LED indicates a pulse train.


BigDog

 

[zooterman]

Thanks guys.
I have a reel to reel deck that is logic controlled, so I will be using it on that. just to confirm please, testing is done on a unpowered board, correct.

 

[bigdogguru]

just to confirm please, testing is done on a unpowered board, correct.

No, the logic board must be powered for the logic probe to detect the various states of component pins.

Just make sure you dealing with logic voltage levels, not higher voltages.

I would recommend scouting out the power levels with a digital voltmeter (DVM) before attempting to use the logic probe.

BigDog

 

[zooterman]

Thanks bigdoguru & others for your help.