"This Latching Continuity Tester can help you locate those difficult-to-find
intermittent short and opens that other testers always seem to miss. It has been
part of my workbench for many years and performs superb. I have solved many
intermittend problem with this highly flexible unit."
Latching Continuity TesterA continuity tester is a must on every service
bench for testing cables, pcboards, switches, motors, plugs, jacks, relays, and
many other kinds of components. But there are times when a simple continuity
test doesn't tell the whole story. For example, vibration-induced problems in
automobile wiring can be extremely difficult to detect because a short or open
is not maintained long enough for a non-latching tester to respond.
This
latching continuity tester detects intermittent (and steady state) opens and
shorts. The tester will detect and latch on an intermittent condition with a
duration of less than a millisecond. In addition, it provides both visual and
(defeatable) audio indicators, uses only one inexpensive and easy-to-find IC,
and can be built from all new parts for about $35, or less if you have a
well-stocked junkbox.
Circuit Elements:Schmitt Trigger The heart of the
circuit is a 4093 quad tow-input NAND Schmitt trigger, one gate of which is
shown in Fig. 1-a. The gate functions as shown in Fig. 1-b. Nothing happens
until the enable input goes high. When that happens, the output responds to the
input as follows.
As long as the input voltage stays between VH and VL, the output stays high.
But when the input goes above VH, the output goes low. The output will not go
high again until the input goes below VL. That characteristic is what gives the
Schmitt trigger its ability to "square-up" a slowly changing input signal. The
Schmitt trigger is ideally suited for our application because it is not
dependent on edge triggering, and because both slow and fast signals trigger it
when either threshold is exceeded.
We use two gates of the 4093 as a
combination detector and latch. The gates are cross connected to form an SR
(Set-Reset) flip-flop. When pin 12 goes low, pin 11 will go high. That high may
be used to enable an LED or other indicator. Switch S1 is used to select whether
the tester will provide ouput when it detects an open or a short. In the OPEN
position, pin 12 is held low, so the output of the gate is normally high. When
the test leads are connected across a short, pin 12 is pulled high, so the
output drops low. The circuit works in the converse manner when S1 is in the
CLOSED position.
As shown in Fig. 2-a, we use another Schmitt trigger to
build a gated astable oscillator. A gated astable oscillator produces output as
long as the GATE input is high. Fig. 2-b shows the waveforms that are present at
various points in the circuit. When the pin-8 input goes high, pin 10 goes low,
and C1 starts discharging through R1. When VC falls below VL, the output of the
gate goes high, so C1 starts charging through R1. When VC exceeds VH, the output
again drops low. Oscillation continues in that way as long as the gate input
remains high. The frequency of oscillation is given by a fairly complex equation
that can be simplified, for purposes of approximation, as F = 1 / R1C1.
Putting it all together:The complete circuit is shown in
Fig. 3. In that circuit, IC1-a and IC1-b funtion as the flip-flop/detector. The
output of IC1-a is routed through S4, AUDIO. When that switch is closed, IC1-d
is enabled and an audio tone will be output by BZ1. The frequency of that tone
can vary from 1000Hz to well above the audio range (100KHz), according to the
setting of R4. In addition, R4 varies frequency and volume simultaneously, so
you can set it for the combination that pleases you best. Originally we used a
PM (Permanent Magnet) speaker. Whe the detector has not been tripped, the full
power-supply voltage is across the buzzer, but no current is drawn. The reason
is that the piezo element is like a capacitor and does not conduct DC current.
Whe the circuit is oscillating, the buzzer consumes a current of only about 0.5
milliamp. The output of the flip-flop/detector circuit also drives IC1-c. If S2
is in the AUTO position, the output of IC1-c will automatically reset the
flip-flop after a period of two to six seconds, depending on the position of R7.
If S2 is in the MANUAL position, the LED will remain lit (and the buzzer will
continue buzzing, if S4 is on) until maual RESET switch S3 is pressed
Construction:Astable OscillatorThe circuit may be built
on a piece of perforated construction board or Vero-board, or on a PCB. The PCB
is designed to take board-mounted switches, which makes a neat package and
eliminates a rat's nest.
Referring to Fig. 4, mount and solder the components
in this order: diodes, fixed resistors, IC-sockets, capacitors, variable
resistors, and then the pcb mounted switches. The regular ones will work too it
just means more wire. Mount the buzzer and the LED last as described below.
Trimmer potentiometer R7 is manufactured by Piher (903 Feehanville Drive, Mount
Prospect, IL 60056); it has a shaft that extends through the panel. If the Piher
pot is unavailable, an alternate is available from Digi-Key (701 Brooks Ave,
South, P.O. Box 677, Thief River Falls, MN 56701). The disadvantage of the
alternate is that it has no shaft, so it must be adjusted using a miniature
screwdriver.
The circuit board is helf approximately 1/2-inch from the cover
by the shafts of the switches. The LED and the buzzer should be inserted in the
approproate holes in the pcb now. Then install the top cover, and adjust the
height of the LEDso that it protrudes through the top cover. Then solder its
leads. Attach the buzzer to the top cover, using silicone rubber adhesive (RTV
or double side foam tape.
We mounted a pair of banana jacks on the top of our
prototype's case, but you could solder the wires directly to the appropriate
points on the circuit board, tie strain reliefs in the wires, and then solder
alligator clips to the ends of the wires. However, a set of good leads are
really all that expensive and it does give the tester more flexible usage as you
have the opportunity to use a variety of different leads to suit your
purpose.
The nine-volt battery is secured to the side of the case with a clip
or use a holder. Your completed pcb should appear as in Fig. 5.
Usage
Hints:Set S1 for short or open depending on the condition to be
tested. Then connect the test leads across the circuit to be tested. If an
intermittent condition is detected, the LED will illuminate, and the buzzer will
sound (if S4 is on). If you don't remove the test leads (assuming if S2 is set
for AUTO Reset, the LED will flash and audio will warble at a rate determined by
the reset circuit.
It is very important that the test leads make a positive
connection with the circuit to be tested. In fact, clips should be used instead
of test leads. There are good test leads available for about $15 which are
hardened stainless-steel and have sharpened points which were my personal
choice. This detector is so sensitive that, when it is initially connected
across a long length of parallel wires or traces, it may latch due to
capacitance between the wires. As a matter of fact, it happens with my model all
the time. Just press the reset switch S3 when that occurs.
PartsR1 10K
R2,R3 470K
R4 100K potentiometer
R5
Not used
R6 1.8K (1800 ohm)
R7 1M Trim pot
R8 10M
C1 0.1?F,
ceramic
C2,C4 0.01?F ceramic
C3 4.7?F, 16V, Elec. .
IC1 4093B Quad Nand
Schmitt Trigger (NTE4093B/ECG4093B)
D1,D1 1N914 or 1N4148
(NTE519/ECG519)
LED1 Red, 5mm, High Brightness
BZ1 Piezzo Buzzer
S1
DPDT, miniature toggle, pcb mount
S2,S4,S5 SPDT, miniature toggle, pcb
mount
S3 SPST, momentary push, normally open
Additionally: IC socket,
plastic case (4.75" x 2.5" 1.5"),
banana jacks, wire, solder, battery
clip,couple cold beers
Copyright and Credits:The original project is copyright
? by Eldon L. Knight (1986). Document updates & modifications, all diagrams,
PCB/Layout by Tony van Roon using Paint Shop Pro deLuxe.
Source:
http://www.hqew.net/circuit-diagram/Latching-Continuity-Tester_5084.html
