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3-LED
LOGIC
PROBE
Ever been chasing a problem on a digital
logic board and wasted a lot of time because
you were too lazy to get the scope out and
plug it in? What, you don’t even own one?
This logic probe will prove invaluable in
digital fault finding and only costs a few
dollars.
By RICK WALTERS
All right. So what is a logic probe?
A logic probe is a small hand-held
device which indicates the logic
state at its input probe. The logic
level should only be ground (low)
or at the positive supply (high) but a
faulty device can have an output level
somewhere around half the supply.
Ideally, a logic probe should indicate
all three circuit states and that is what
this simple design does.
The probe has three LEDs which are
readily visible whether you are right
32 Silicon Chip
or left-handed. The red one indicates
a low level, the green one a high level
and the yellow one is lit whenever the
level changes from high to low.
You may wonder why we bothered
with the yellow indication. We have
just stated that if the level is low, the
red LED will light, if the level is high
the green one will be lit, and if the
level is changing from high to low
then obviously both will light.
The fault condition described above
can sometimes cause both LEDs to
come on and this would give us a false
indication. The yellow LED needs a
full high-low transition to light it,
thus eliminating any false indication.
How does it work?
As you can see from the circuit
of Fig.1 there is not much to it. A
4001 quad 2-input NOR gate is used
as it lets us make a monostable by
cross-coupling two gates. We’ll get
to that in a moment, so let’s start at
the input.
The probe tip is connected directly
to pins 5 & 6 of IC1b. The 10MΩ resistor holds those pins low and prevents
the input capacitance being charged
and staying high when the probe en
counters a momentary high level. The
output of IC1b is fed to pins 1 & 2 of
IC1a which in turn, drives the LEDs.
Note that since each gate effectively
inverts its input and there are two
signal inversions via these gates, the
output of IC1a is in phase with the
input.
Thus when the input is low, the
Fig.1: the circuit uses a 4001 quad 2-input NOR gate to
indicate high, low or fault logic conditions.
output of IC1a is low and the red LED
will be lit. When the input goes high,
the red LED will go out and the green
one will light.
The output of IC1b is also coupled
through a .001µF capaci
tor to one
input of IC1c. This input is held
low by the 10kΩ resistor to ground.
IC1c’s output, pin 10, is coupled via
the 0.18µF capacitor to the inputs of
IC1d. These inputs are held high by
the 100kΩ resistor which means the
output at pin 11 will be low.
A low to high transition at the
output of IC1b will pull pin 8 of IC1c
high and consequently pin 10 will go
low. This will pull pins 12 & 13 low,
taking pin 11 high and thus turning on
LED3. As pin 11 is also connected to
pin 9 of IC1c, it will hold the output
of IC1c low even after the initial logic
signal at pin 4 has charged the .001µF
capacitor.
The yellow LED will stay lit until
the voltage on the 0.18µF capacitor,
which is charging through the 100kΩ
resistor, reaches the switching threshold of IC1d. When it is reached, the
output of IC1d will go low, the yellow
LED will extinguish and the output of
IC1c will go high again.
Thus each high to low input transition will flash the yellow LED for
18ms. At low frequencies this is
readily apparent but as soon as the
input frequency is high enough, the
LED will appear to be lit continuously.
So just to sum up, if the red or
green LED is on, the logic circuit
being measured is indicating a valid
condition (ie, low or high), although
if you want a high and you get a low
you obviously have a problem.
Power for the Logic Probe comes
from the circuit being measured and
can be anywhere between 5V and
15V DC. Diode D1 protects the logic
probe if you accidentally make the
wrong supply connections (ie, wrong
polarity) to the circuit.
PC board assembly
We made the PC board as small
as possible, so you could fit it into a
smaller case than the one we used, if
you have one. We would have preferred a slightly narrower rectangular
case but the one we used is readily
available and inexpensive. On the
positive side, if you have large hands,
the size and shape of the specified
case is quite convenient to handle.
The assembly details for the Logic
Probe are shown in Fig.2 and are
quite straightforward. Don’t use an
IC socket for the 4001 as there is
Fig.2: not shown on this wiring
diagram are the positive and
negative supply leads which clip
onto the circuit being measured.
Fig.3: actual size artwork
for the PC board.
not much depth in the case we have
specified. Use the PC stakes as they
are a convenient connection for the
LED leads. Keep the wires close to the
PC board when you solder them and
cut the top off the stakes or else they
will prevent you from assembling the
case properly.
Drill the three holes in the case for
May 1998 33
This is the view inside the Logic Probe case. Note that the leads to the three
LEDs must be sleeved to avoid the possibility of shorts.
the LEDs and file a notch in the end
panel to bring the power wires out.
Make it small enough so that the wires
are lightly clamped when the case is
screwed together.
We secured the board inside the
case by using a small self-tapping
screw into one of the integral pillars.
But the pillar is very short and you
must be careful not to tighten the
screw too much otherwise it will
penetrate right through the case. If
you look closely at the inside photo
of the Logic Probe you will note that
we have placed a black fibre washer
underneath the screw head to avoid
this problem.
Another point to note about the
inside photo is that the LEDs should
have sleeving on their leads to avoid
A slot is cut in one of the end pieces of
the case for the power supply leads.
34 Silicon Chip
the possibility of shorts.
We used a probe from an old multimeter lead as the input prod but
failing this, a nail or a small gauge
screw with a filed point could be
pressed into service. I’m sure your
ingenuity won’t fail you here.
Testing
Connect the power leads to 5-12V
and the red LED should immediately light. If it doesn’t, you probably
have its leads reversed. Don’t worry
though, just make the connections
correctly and it should work properly.
Use your multimeter to measure the
voltage at pin 3 of IC1a. It should be
at ground potential; ie 0V.
Now put the probe on the positive
supply. This should extinguish the
red LED and light the green one.
As you remove the probe from the
supply, you should see the yellow
LED flash briefly. Tap the probe on
and off a few times until you see it.
The beauty of this device is that if
you connect it to a logic PC board with
a 5V supply, all the functions work
as described. But it can be connected
to any supply up to 15V with safety
and the logic thresholds will move to
track the supply.
It will work with all “C” & “HC”
devices as well as the older TTL range.
The upper frequency depends on the
Parts List
1 PC board, code 04104981, 50
x 26mm
1 small plastic case, Jaycar HB6030 or equivalent
1 red crocodile clip
1 black crocodile clip
3 5mm LED bezel clips
8 PC stakes
1 6mm long self-tapping screw
1 fibre washer (see text)
0.5m red hookup wire
0.5m black hookup wire
Semiconductors
1 4001 quad 2-input NOR gate
(IC1)
1 1N914 small signal diode (D1)
1 5mm red LED (LED1)
1 5mm green LED (LED2)
1 5mm yellow LED (LED3)
Capacitors
1 0.18µF MKT polyester
1 0.1µF MKT polyester or
monolithic ceramic
1 .001µF MKT polyester
Resistors (0.25W, 1%)
1 10MΩ
1 10kΩ
1 100kΩ
3 1kΩ
supply voltage. With a 5V supply the
4001 should indicate up to 2-3MHz
and around three times this frequency
with a 15V supply.
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