"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."
A 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:
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:
The
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.
Parts List
R1 = 10K IC1 = 4093B Quad Nand Schmitt Trigger (NTE4093B/ECG4093B) R2,R3 = 470K D1,D1 = 1N914 or 1N4148 (NTE519/ECG519) R4 = 100K potentiometer LED1 = Red, 5mm, High Brightness R5 = Not used BZ1 = Piezzo Buzzer R6 = 1.8K (1800 ohm) S1 = DPDT, miniature toggle, pcb mount R7 = 1M Trim pot S2,S4,S5 = SPDT, miniature toggle, pcb mount R8 = 10M S3 = SPST, momentary push, normally open C1 = 0.1µF, ceramic Additionally: IC socket, plastic case (4.75" x 2.5" 1.5"), C2,C4 = 0.01µF ceramic banana jacks, wire, solder, battery clip, C3 = 4.7µF, 16V, Elec. 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.