Elm Electronics – Circuits for the Hobbyist

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ELM405DSA

ELM405

Debouncing Circuits

Any time that two metal surfaces meet, as they do

inside a rotary encoder, there will be a tendency for

the moving one to bounce, which causes the electrical

connection to make and break. The duration of this

bouncing action may be very short, but it is usually fast

enough to cause multiple counts to be recorded by

connected electronic circuits. As the number of

bounces can not be predicted, a means of removing

them is necessary. Circuits that remove the bounce

are usually called ‘debouncing’ circuits.

Many debouncing circuits employ a simple timer to

determine if an input is stable. This generally works

well if the two contacts meet and then remain still. With

a rotary encoder however, one of the contacts meets

the other then usually continues sliding over the

surface of the stationary contact. This will produce

noise while the contact is sliding, occasionally enough

to make it look like there are more inputs.

The ELM405 employs a two stage system to

remove the bounce and the sliding noise from the

input signal. A block diagram of the stages are shown

in figure 3. The first stage is a digital filter circuit that is

used to determine the average value of the waveform

over a time. If a long enough time is chosen, short

duration pulses will have little effect on the overall

average. If the time chosen is too long, however, the

circuit will be slow to respond, and may in fact average

out some legitimate inputs. Choosing the time period

(or time constant of the circuit) is thus very important in

determining how effective the filtering function will be.

We have found that with typical rotary encoder

specifications (usually 3.0 msec of bounce or noise,

maximum), the ELM405 debounce circuit works quite

well.

After the signal has been filtered, it is compared to

some reference levels, and the output of these

comparators are used to control a simple timer. The

timer is used to ensure that the output of the filter is

stable, and not just a momentary transient, while the

use of two comparator levels provides hysteresis, so

that some variation in the filter output can be tolerated.

Once the signals from the rotary encoder have

been debounced, they may be used by the direction

logic circuitry. The following section shows what the

ELM405 is able to produce from these signals.

Figure 3. Internal Debouncing Logic

tc = 1.7 msec

3 msec

timer

timer

filter

rotary

encoder

input

to

output

logic