PeiZen, Inc.
1227 Flynn Road,
Suite 302
Phone: (805)
383-8855
E-mail: ml@peizen.com
Website: www.peizen.com
Technical Bulletin
Q-501-300
Released on
3/22/04
Switch Debounce Filter
When
a typical mechanical switch is switched, the contacts can take up to 25
milliseconds to settle down. The result
is switch bounce noise. Electronics
engineers have come up with many successful schemes to overcome the switch
bounce problem. However, when one has to
deal with a large number of switching circuits, such as those in control
consoles and test sets, cross-talk between circuits is another problem. As the switch is opened or closed, glitches
can be coupled over to neighboring circuits.
It is much better to get rid of the noise glitches at the source before
they cause problems. By using a filter
such as that one shown in Figure 1 near the switch, the switching noise and
cross-talk can be eliminated.
Figure 1: This switch debounce filter clips the
switching voltage and slows down the rise and fall times of the switching
signal to eliminate cross-talk.
In
a typical switching circuit, a pull up or pull down resistor is connected to
the wiper of the switch. In our case, a
20K-ohm resistor is connected between the switch wiper and 24V. The logic input circuit used in our test
equipment is the Systran DID48 logic input module. This module has an input debounce, but it
cannot filter out the switching noise from switches mounted at 10 feet
away. Our approach to fix the problem is
to install the Q-501-300 Switch Debounce Filter close to the switches. The DID48 can accept input signal levels of +
and 32 volts. The logic 1 is defined
as +2.0V to +32V, and logic 0 is defined as 32V to +0.8V. The power supply used in our equipment is
+/- 24 volts. Since the input doesnt
need the full swing of the power supply, the input voltage can be clamped down
with 5-volt zener diodes, VR1 and VR2 plus the corresponding blocking diodes
CR2 and CR3. This combination has an
inherent AC noise rejection because of the low dynamic resistance in the zener
and blocking diodes. CR1 is used to
bypass the resistor R1 when the switch is switched to 0 so to allow the
capacitor C1 to discharge through the external 20 K-ohm resistor, R101. The capacitor C1 and resistor R1 form a
filter that sets the rise and fall times to around 20 milliseconds. By slowing down the rise and fall times, the
switching signals can no longer couple over to the neighboring circuits. Typically, the capacitance between wires in
an unshielded cable is typically 13 picofarads/ft, which is not capacitance
enough for cross-talk for signals with 20 ms rise and fall times.
Figure 2. By using surface mounted components, a 60-channel switch debounce filter can be fitted onto this PWB this is 6 X 15.75.
Figure
2 shows a 60-circuit switch debounce filter that is assembled on a four-layer
printed wiring board (PWB) with surface mount components and ribbon cable
connectors. The cable used between the
switch and the input logic circuits is a 10-foot ribbon cable with no
shielding. By using this filter, we were
able to eliminate the switching noise and cross-talk problems.