Example Applications (continued)

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ELM409

ELM409DSA

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

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The following are a a few circuits that use the

ELM409, but may not be what comes to mind when

you think of debouncing. They take advantage of the

internal timers in the ELM409 as well as the ability to

change options while the circuit is operational, to

provide some interesting functions.

The first (Figure 8) is a circuit that simply connects

the debounce input to the positive supply. With the

debounce period set for 500 msec, the output will

initially be low, then go high 500 msec after the power

is turned on. This delay may be useful if trying to

sequence the power to different circuits, or enable the

outputs (audio power amplifiers often need this delay

to allow the supplies to settle before connecting the

amplifier outputs to the speakers).

Figure 9 shows a circuit that provides a single

50 msec wide pulse at the output, 25 msec after power

has been applied. It relies on the fact that the

ELM409’s pin 7 output is always at a low level on

power up (and pin 6 is always high), so the initial

options setting will then be 010, for a 50 msec pulse

after a 25 msec debounce period. Once the output

pulse begins, however, the connection from pin 7 to

pin 4 causes the options to change to 011, giving a

debounce period of 500 msec (which can never be

met by a 50 msec pulse). The output remains low from

that time on.

This circuit works because the option pins are

always read just before a 25 msec debounce period is

about to expire (allowing for changes ‘on the fly’). In

this case, the pulse output is at its midpoint when the

pins are read, so the new latched setting becomes

011, and the debounce period changes to 500 msec.

As the pulse never lasts that long, it is ignored by the

500 msec debounce circuit, and the output never

changes again.

The final circuit also uses the output to modify the

input, and the debounce period. The result is a series

of 25 msec pulses with 500 msec gaps in between,

that continually repeat. Low frequency oscillators such

as this may be useful on the test bench, or it may

simply be used to drive an LED in order to attract

attention.

These are just a few of the many applications

possible with the ELM409… Enjoy!

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Figure 8. Power on delay

Figure 9. Power on pulse

Figure 10. Low frequency oscillator