Star Tech Journal

9
STAR*TECH JOURNAL/JULY 1983
BUILD YOUR OWN
LOGIC PROBE
By Don Becker, Atari Inc., Somerset, NJ
One of the most useful pieces of troubleshooting equipment available to the field service
tech is the logic probe. Used properly, it is a simple, fast, and effective way to check
logic levels on PC boards. Unfortunately, many techs are unfamiliar with its use and
are, therefore, unwilling to buy one. Most commercial probes cost somewhere between
thirty and seventy dollars (depending on features such as memory, CMOS/TIL
compatibility, etc.) but you can build one for about ten dollars. As a matter of fact, you
probably have all the parts you need already just kicking around in your junk box.
Construction: The probe in Figure 1 consists of just two !Cs, four resistors, three
LEDs, and one cap. All are standard video game parts. IC l is a 74LS14 inverter, IC2
is a 8301 one-shot, the LEDs are standard 5v LEDs you can steal from old game
boards, and the cap and resistor you probably have already.
I built mine on a piece of perf board about ½" x 3 ½" and put the whole thing in a
plastic cigar tube. Parts placement is not critical - I wired mine point to point with 30-
gauge wire. Drill three holes in the cigar tube for the LEDs to peek through, add a probe
tip and two leads with micro-clips for 5v and ground, and you're finished.
+5
IC1 - 74LS14
IC2- 8602
+5
R/C-100ms
Dlgltal Logic Probe
Figure 1
Check Out: Connect the 5v and ground leads to the logic board Touch the probe tip
to ground - the "0" LED should light Touch the probe tip to +5v - the" l" LED
should light. Touch the probe to clock - all three LEDs should light
Use: The logic probe has three LEDs to display logic levels - "0", "l ", and "P" or
pulse. Figure 2 shows what these levels would look like on an ocsilloscope and the
corresponding display on the logic probe. When the logic probe senses a 5v or high logic
level, the "1" LED lights. When the probe senses a 0v or low logic level, the "0" LED
lights. The "P" or pulse LED lights every time the input changes state. If the input
approximates a square wave with equal
0 0
time both high and low, all three LEDs
5 ---------- I 1
will light. If the input is high pulsing low,
0 - - - - - - - - - - - - -- 0 P
the "l" and" P" LEDs will light; likewise,
if the signal is low pulsing high, the "0"
I 0
5
0 1
and "P" LEDs will light The logic probe
is useful for checking reset lines, clock
0 ----------- 0 p
signals, tracing switch input lines, etc.
You can check basic logic functions also.
I 0
I 1
For example, when checking an inverter,
I p
if the input is high pulsing low, you know
the output should be just the opposite or
low pulsing high. The one disadvantage of
0 0
I 1
this logic probe is that it will not find
I p
floating pins - it sees an open as a high or
logic "l ". Some of the more expensive
logic probes will find opens and some can
also catch a fast pulse and latch onto it. As
you gain experience, you may wish to
purchase one of these.
Figure 2
;=i_n_o_
:=nnc
:_JLJLJL__ ~ !
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