ELM334
Circuit Operation
The ELM334 can be thought of as two inverters
between each output and circuit common, but there is
no advantage to this, as it requires an extra wire for
remote indicators. (It is easier and cheaper to wire
separate LEDs ‘back to back’ as shown in the
Example Applications section.)
that have some additional logic connected. The inputs
to the inverters are located on pins 6 and 7, while the
outputs are connected to pins 2 and 3. If pin 7 is
connected to a low level, then pin 2 goes high. If pin 6
is connected to a low level, pin 3 will go high. There
are several additions, however, that make these a little
more than simple inverters.
One other feature of the internal logic is a ‘lamp
test’ that is performed at each power up - the LEDs are
each turned on for 0.5 seconds so that you can be
sure that they are both working (red first, followed by
green).
To make this IC more useful, we have added
‘debounce’ circuits to the pin 6 and 7 inputs. These are
similar to digital integrators - the input must be at one
level for a set time before the output can change. This
may not be necessary for simple LEDs, but if you want
to interface to a computer circuit or other logic, you will
require a clean signal such as this provides. To ensure
that the circuit works reliably even with very poor
quality switches, a debounce period of 0.5 seconds is
used.
The ELM334 also has a special pushbutton circuit
connected between pins 4 and 5. When a low level
input appears on pin 4, a 0.5 second pulse will be
output on pin 5. This is typically used to drive a relay
which interfaces with the garage door control circuit,
but it can also be connected to logic of your choice.
Only one pulse will be generated for each pushbutton
press, no matter how long the button is held for. The
input also provides a debouncing circuit so that a
mechanical pushbutton can be used.
In addition to the debouncing, there is logic
connected to these pins, so that both LEDs are not on
at the same time, etc. (the door is either closed or
open, but not both). Figure 1 shows the outputs that
can be expected for all combinations of the two inputs.
That covers the basics of how the ELM334 IC
operates. The next section (Example Applications)
shows a few examples of how it may be used…
Typically, a dual red/green LED will be connected
between the two output pins, and this is what is meant
by the last column. You may connect separate LEDs
Pin 6
Pin 7
Pin 2
RLED
Pin 3
GLED
Dual (R/G)
LED glows
ClosedSw OpenSw
H
L
L
H
L
L
H
L
H
L
Green
Red
L
H
Green
Alternates between red and green
for 30 seconds, then shows red
H
H
Figure 1. ELM334 LED Logic
ELM334DSC
Elm Electronics – Circuits for the Hobbyist
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