Comments on: Logic probe kit

By: Eldon Fasci

Eldon Fasci — Fri, 26 Nov 2010 00:30:48 +0000

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By: NOBAMA

NOBAMA — Fri, 14 Aug 2009 18:41:51 +0000

This is WAY too complicated and power-hungry for a logic probe with this level of functionality.

By: Becky

Becky — Fri, 03 Jul 2009 23:59:49 +0000

Thoughtful post and well written. Please write more on this if you have time.

By: dennis123123

dennis123123 — Sun, 29 Jun 2008 23:29:00 +0000

Thankyou wolf! Ill give that a go :)

By: Wolf

Wolf — Sun, 29 Jun 2008 00:23:14 +0000

@dennis

Basically, you just connect the emitter of a common npn transistor (like a pn2222a) to the probe batteries ground, connect an led and a current limiting resistor in series between the transistors collector and the batteries positive terminal (with the led's plus side attached to the battery), then attach a second, high value resistor between the transistors base and the logic probe.

The resistor values will depend on the voltage your probes running on, and the voltage you want it to detect, but for a 9v battery and signals above 2v a 470ohm current limiting resistor, and 20k probe would put the max current through the led at 19ma, and keep the current through the probe to less than 1ma. If you wanted to, you could probably go with even higher resistance on the probe and still keep the transistor saturated, but the highest my pot goes is 20k.

If anyone sees that I'm mistaken on any of this feel free to correct me.

By: adam

adam — Sat, 28 Jun 2008 20:24:32 +0000

A 9V battery to 7805 linear regulator is a really bad idea. It will burn that extra 4V off as heat and it will constantly leak about 2-3ma of current. You’ll be replacing that battery a lot. Find a LDO regulator with a low Iq (quiescent current).

By: Dennis

Dennis — Sat, 28 Jun 2008 19:50:40 +0000

@Wolf (post 3)

Would you mind sharing how those few components make a working logic probe? I had a google and only came up with IC based designs :(

Ty!

By: chr0n1c

chr0n1c — Sat, 28 Jun 2008 07:57:46 +0000

amazing that they did it all without any syringes!

By: Steve

Steve — Sat, 28 Jun 2008 07:50:03 +0000

Perhaps I got the wrong vibe by the end of the synopsis. "Of course, the resulting device is not as versatile as a LogicPort logic analyzer, but it is still useful."

I've owned the logic analyzer from intronix for a few years. I also own a micronta logic probe, HP Logic Dart, and a Fluke 123 scope meter. Each has its own. And to give the logic probes a little more respect. All these tools are great for their intention. But a logic probe has a place too. The logic analyzer, Logic Dart and Scopemeter take far too long to setup for most quickies. A logic probe gets powered and references signals. visual and audible indications are great and there is not setup over head. I once repaired a pinball controll board using only a logic probe- This was the eye opener that each tool does infact have a very special place in terms of use. Within 5 mins the pinball machine was working. It would have taken that long to setup the other instruments consider the broad range of pins and various signals that needed to be checked.

Don't take this the wrong way either. The logic port is an excellent tool. But it should not be compared to a logic probe. Apples and Oranges anyone?

By: Colin

Colin — Sat, 28 Jun 2008 05:48:46 +0000

NIce little project you have put together.Thanks, regards Colin

By: wolf

wolf — Sat, 28 Jun 2008 04:58:23 +0000

So basically, whoever created this has managed to take a device that can be built with an led, 2 resistors, a transistor, and a battery, and created a version thats larger, costs more, is more labor intensive to build, and offers exactly the same functionality.

Am I missing something?

By: Batman

Batman — Sat, 28 Jun 2008 02:28:25 +0000

simple and effective, but I’d think a bicolor led would cost less than the 7-seg. The 7805 will probably drain the 9v batt in no time and get hot doing it.