Example Applications
The following pages show a few circuits that may
help you get started with the ELM334. They show
everything from a minimal circuit to a typical full
implementation that you may wish to try. We
encourage experimentation, and hope that you enjoy
doing so.
be a magnetic switch, or a mechanical contact). When
the door opens then, the contact does too, and the pin
6 input goes high. With the pin 7 input low, the green
LED will turn off (see Figure 1 for the pin logic). If we
had connected a dual red/green LED, the red LED
would turn on when the door opens.
The first circuit (Figure 2) shows the absolute
minimum required to use the ELM334 IC. A single
(normally closed) contact is connected between pin 6
and circuit common, while a single green LED is
connected between pins 2 and 3. We do not use a
current limiting resistor in series with the LED, as the
ELM334 has fairly high output resistance when
operated with a 3V supply, and will limit the LED
current. Two ‘AA’ batteries in series provide the 3V
source to operate the circuit.
Figure 2 would likely work well if experimenting on
the bench, but really requires more circuitry to make it
more reliable for ‘real world’ operation. Figure 3 shows
the recommended additions to make a more reliable
circuit. Note that we have now shown a ground symbol
to represent the circuit common (battery negative), just
to simplify the schematic. This new circuit shows a
capacitor across the IC supply terminals, to prevent
noise generated internally by the ELM334 from
affecting the supply voltage, and thus the complete
circuit. We’ve also added a few resistors on both the
input and the output sides of the IC, mostly to protect it
from the effects of induced voltages and currents
which can cause a phenomenon called ‘latch-up’ in
some CMOS circuits. If the door closed switch or the
LED is more than a few feet away from the ELM334,
these extra resistors should be added.
Normally, the door closed switch is closed when
the door is, and opens when the door does (this could
3V
1
2
3
4
8
7
6
5
Functionally, the circuit in Figure 3 operates in
exactly the same manner as the one in Figure 2.
green
door
closed
Figure 2. Absolute Minimum ELM334 Circuit
+3V
3V
0.1µF
+3V
100W
100W
1
2
3
4
8
7
6
5
green
5.1KW
2.2KW
door
closed
+3V
Figure 3. A Better Minimal ELM334 Circuit
ELM334DSC
Elm Electronics – Circuits for the Hobbyist
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