ELM409
Example Applications
This section shows several different ways to use
the ELM409. The first example (Figure 6) is actually a
variation of the circuit shown in Figure 4.
Figure 6 adds two components that should always
be considered in every ELM409 design. The first is a
supply bypass capacitor. Digital circuits will often have
transients (‘spikes’) in their current flow due to the
internal switching process. By adding a capacitor very
close to the IC, much of the current for these spikes
can be supplied by the capacitor rather than through
the (inductive) leads from the supply. The net result is
that there will be less noise generated at the supply
by the ELM409. A value of 0.01µF works well.
0.01µF
4
5
3
2
1
8
409
6
7
4.7KW
Digital
Circuit
The other component that was added in Figure 6
is a series resistor at the input (pin 5). This resistor is
recommended when the electrical contact or switch is
any more than a few feet away from the ELM409 IC.
Placing the resistor in series with the input serves to
limit induced currents that might otherwise damage the
ELM409. A minimum value of a few hundred ohms is
recommended, and a larger value (such as the 4.7KW
shown), is preferred.
Figure 6. Debouncing the pushbutton revisited
The next example (shown in Figure 7) has the
pushbutton connected between circuit common and
the input, rather than from the positive supply. An
arrow is shown at the output, symbolizing the output
signal, which is then connected as appropriate.
The one difference with this circuit is that the input
voltage will go from a high level to a low level when the
switch is pressed. To accommodate this, the options
pins have been set to 110, providing a toggle function
that changes state on each high to low transition. The
circuit will provide alternating high and low outputs
which change state each time the switch is pressed.
0.01µF
4
5
3
2
1
8
409
If the options pins had been set to 000 (ie all tied
low), the circuit would provide an output that follows
the switch presses, but might seem inverted. That is,
pin 7 would normally be at a high level, and would go
to a low level when the switch is pressed. If an output
that is normally low and goes high when the switch is
pressed is required, simply use pin 6 for the output
rather than pin 7.
6
7
4.7KW
This section concludes on page 8, with three more
example circuits and a short description of each.
Figure 7. Toggle circuit with the pushbutton to common
ELM409DSA
Elm Electronics – Circuits for the Hobbyist
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