Download Switch Debounce Filter

PeiZen, Inc.
1227 Flynn Road, Suite 302
Camarillo, CA 93012
Phone: (805) 484-2454, Toll-free: 1-866-484-2454
E-mail: [email protected]
Websites: 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 doesn’t 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.