Logic Probes

[Peter Snowberg]

By now everybody here should have an audio probe in their tool kit. When you play with digital the DMM becomes much less useful because of their slow update rate. You can always use an scope to watch digital lines, but that can be inconvenient and overkill. Every digital builder has another tool at their disposal.... The Logic Probe.

A logic probe is a high impedance tester that shows if the line under test is at a high or low state with LEDs. Fancier probes also have the capability to show that a line is oscillating between V+ and GND and that's a very nice feature.

There are lots of plans on the net for probes so I'll list a few here. You can also make one with a CMOS inverter, a pair of LEDs (use RED for maximum versatility), and three resistors.

http://www.swansontec.com/sprobe.htm (good basic probe; use a comparator like LM339; NE5532 will NOT work here)
http://www.mitedu.freeserve.co.uk/Circuits/Testgear/lprobe.html  (Includes pulse LED)
http://www.solorb.com/elect/logprobe/index.html  (included HI and LOW pulse LEDs, but designed for +5V use)
http://www.uoguelph.ca/~antoon/circ/probe1.htm  (+5V design including pulse LED)
http://electronickits.com/kit/complete/meas/ck103.pdf (incl. pulse with audio too) Kit form here

[gaussmarkov]

thanks, peter!    i was going to suggest putting your earlier response in a faq--should we start one?

[Peter Snowberg]

You're welcome. 

A digital FAQ is a great idea . I'll start one in the next few days for things like this and links to a bunch of the posts that are already here. I'll also try to add some things to the Wiki.

[Dave_B]

I'd add this one to the list, since I've always wanted to build one.  The number of features makes me a little dizzy. 
http://members.cox.net/berniekm/super.html

Logic Probe
Logic Pulser
Frequency Counter
Event Counter
Voltmeter
Diode Junction Voltage
Capacitance Measurement
Signal Generator
NTSC Video Pattern Generator
Serial Ascii Generator
Midi Note Generator
R/C Servo
Square Wave Generator
Pseudo Random Number Generator
PWM Generator
Loan Consolidation
Etc.