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In & out of circuit LED tester
The idea for this circuit came to me
when I was having difficulty locating
faulty LEDs in a string, as used in an
LED light bulb or infrared illuminator.
Assuming that you’re prepared to
spend the time, repairs can be made
or good sections isolated to repair
other units. Probing with a multimeter works, but you have to reverse the
probes quite often. So I built this simple circuit around a 4093 quad schmitt
trigger input NAND gate IC (one of my
favourite chips for quick and dirty
solutions).
IC1d is configured as a simple
oscillator. I have tied input pin 13 to
ground via a 4.7MW resistor, so the
siliconchip.com.au
oscillator will not function until this
pin goes high, accomplished with a
simple touchpad made from a piece
of Veroboard. Until this is activated,
power consumption is virtually nil.
When it does oscillate, it drives
two gates in series out-of-phase (IC1c
& IC1b), so their outputs continually
swap polarities. These outputs are connected by the device under test (DUT)
via a 22W resistor and LED2/LED3.
These LEDs are connected in parallel
with opposite polarity.
LED2 and LED3 can only light when
the DUT passes current. In the case
of a functional device, the LED that
turns on must have the same polarity
Australia’s electronics magazine
as the DUT. If both turn on, you have
a short; if neither do, then there is an
open circuit.
A low resistance across the DUT
could cause both LEDs to flash, but
that is uncommon. The DUT can be
connected by a simple socket or a
pair of probes, used to check devices
in-circuit.
The accompanying breadboard diagram shows how you could build this
circuit on a breadboard, or an IC style
protoboard.
Graham P. Jackman,
Melbourne, Vic. ($80)
June 2021 63
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