AN-833
APPLICATION NOTE
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Using the AD7142 and a Capacitive Sensor to Develop a
Single-Push Digital Shutter Button
by Wayne Palmer
INTRODUCTION
SHUTTER BUTTON AREA REQUIREMENTS
This application note describes how the AD7142 can be
combined with a capacitive sensor ring to implement a singleposition digital camera shutter button similar to a traditional
35 mm SLR camera. In addition to easier control of the shutter
button activation, other benefits include faster prefocus times
and less expensive shutter buttons.
A 1.0 mm keep-out area must be maintained between the edge
of the capacitive sensor ring and any metal object such as the
metal camera casing, metal shutter button, or conductive metal
paint. This ensures there is no false sensing due to the capacitive
field being coupled to the metal casing at a virtual ground level
when the camera is handled by the user. Figure 2 outlines the
metal keep-out area where d1 = d2 = 1.0 mm minimum.
Figure 1 shows the block diagram of a single position shutter
button encircled by a capacitive sensor ring connected to the
AD7142 and HOST μC. The combination of the capacitive
sensor ring, which emits an electric field around the shutter
button, and the AD7142 would provide early detection of a
user’s finger approaching the shutter button, and alert the
HOST μC by asserting the INT pin. The HOST controller could
then begin the necessary processing to complete the prefocus
steps before the user actually makes contact with the shutter
button.
INT
AD7142
HOST
µC
SERIAL
INTERFACE
METAL
CASING
d2
05878-002
OPERATION
d1
Figure 2. Metal Keep-Out Area
SHUTTER BUTTON SENSITIVITY
Figure 3 shows a user’s finger hovering over the shutter
button at a distance (D) above the capacitive sensor ring.
The sensitivity of the capacitance sensor ring determines when
the user’s finger is detected as it approaches the shutter button.
If the sensitivity level is set too high, the autofocus motor is
enabled longer than necessary, thus creating extra drain on the
battery. To avoid this, the sensitivity should be set at a low level,
requiring the user to make partial contact with the shutter
button prior to the AD7142 asserting the interrupt output.
SINGLE POSITION
SHUTTER BUTTON
CAPACITIVE SENSOR RING
(LOCATED UNDER CAMERA BODY)
05878-001
D
05878-003
Figure 1. Capacitive Sensor Shutter Button Block Diagram
Figure 3. Finger Position “D” Above Shutter Button
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AN-833
USER’S FINGER IN PROXIMITY
OF SHUTTER BUTTON
31500
TIME (ms)
0
31500
40
60
80
100
USER’S FINGER IN PROXIMITY
OF SHUTTER BUTTON
4
3
31000
2
1
30500
THRESHOLD LEVEL
4
0
3
2
INT
05878-005
31000
DISTANCE D
CDC OUTPUT CODES
20
DISTANCE D
CDC OUTPUT CODES
The AD7142 can be configured for different threshold levels—
that is, sensitivities. This feature can be used to set a very low
sensitivity threshold level to meet the requirements previously
described. Figure 4 shows an example of how the CDC output
code changes when a user’s finger is at various distances (D)
from the shutter button. By setting a low interrupt threshold
level, the user’s finger has to make partial contact with the
shutter button to cross the interrupt threshold level and assert
the AD7142 interrupt output.
SERIAL
READ
BACK
1
30500
Figure 5. Example of Interrupt Output Timing
for Multiple Shutter Button Contacts
THRESHOLD LEVEL
0
COMMON QUESTIONS
0
20
40
60
80
05878-004
30000
100
TIME (ms)
Figure 4. Example of CDC Output Code vs. Distance D
INTERRUPT OUTPUT TIMING
Figure 5 shows an example of the AD7142 interrupt timing
when a user’s finger hovers over the shutter button for an
extended period of time. There are three asserted interrupts
when the user’s finger is close enough to the shutter button to
cause the CDC output code level to go below the interrupt
threshold level. Once the threshold level is exceeded, the
AD7142 asserts the interrupt output, at which time the
HOST μC must read back the interrupt register to clear the
interrupt. The interrupt output pin remains low until this
register is read back.
Does the user have to contact the shutter button to assert the
AD7142 INT output?
No. The AD7142 threshold levels for the capacitive sensor ring
can be programmed for different sensitivity levels. For example,
a high sensitivity level detects the user’s finger a few millimeters
away from the shutter button, whereas a lower sensitivity level
requires the user to assert slight pressure on the shutter button.
The lens focus motor would be constantly operating if the user’s
finger was close to the shutter button while panning to focus on an
object. This would draw unnecessary power and drain the battery.
Is there a way to prevent this?
Yes. For this application, the sensitivity level and capacitive
sensor would be configured to be under-sensitive. Lifting the
finger slightly off the shutter button avoids detection.
Do the shutter buttons have to have a round form factor?
No. A capacitive filed ring can be custom designed to support
different mechanical form factors such as square, rectangular, or
round shutter buttons.
© 2005 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
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