Figure 2-3
Getting HB pulses with a pullup resistor when not active
Figure 2-4
Using a micro to obtain HB pulses in either output state
greatly reduced by placing a 470K resistor Rs in parallel with
the resonator; this acts to slowly discharge the resonator,
attenuating of the harmonic-rich audible step (Figure 2-6).
2.2.5 PIEZO
A
COUSTIC
D
RIVE
A piezo drive signal is generated for use with a piezo sounder
immediately after a detection is made; the tone lasts for a
nominal 95ms to create a ‘tactile feedback’ sound.
Note that the piezo drive does not operate in Pulse mode.
The sensor drives the piezo using an H-bridge configuration
for the highest possible sound level. The piezo is connected
across pins SNS1 and SNS2 in place of Cs or in addition to a
parallel Cs capacitor. The piezo sounder should be selected
to have a peak acoustic output in the 3.5kHz to 4.5kHz
region.
2.2.6 OUTPUT
The QT118H’s output is active high and it can source or sink
1mA of non-inductive current.
D
RIVE
Care should be taken when the IC and the load are both
powered from the same supply, and the supply is minimally
regulated. The device derives its internal references from the
power supply, and sensitivity shifts can occur with changes in
Vdd, as happens when loads are switched on. This can
induce detection ‘cycling’, whereby an object is detected, the
load is turned on, the supply sags, the detection is no longer
sensed, the load is turned off, the supply rises and the object
is reacquired, ad infinitum. To prevent this occurrence, the
output should only be lightly loaded if the device is operated
from an unregulated supply, e.g. batteries. Detection
‘stiction’, the opposite effect, can occur if a load is shed when
Out is active.
Since piezo sounders are merely high-K ceramic capacitors,
the sounder will double as the Cs capacitor, and the piezo's
metal disc can even act as the sensing electrode. Piezo
transducer capacitances typically range from 6nF to 30nF in
value; at the lower end of this range an additional capacitor
should be added to bring the total Cs across SNS1 and
SNS2 to at least 10nF, or possibly more if Cx is above 5pF.
Piezo sounders have very high, uncharacterized thermal
coefficients and should not be used if fast temperature
swings are anticipated, especially at high gains. They are
also generally unstable at high gains; even if the total value
of Cs is largely from an added capacitor the piezo can cause
periodic false detections.
3 - CIRCUIT GUIDELINES
The burst acquisition process induces a small but audible
voltage step across the piezo resonator, which occurs when
SNS1 and SNS2 rapidly discharge residual voltage stored on
the resonator. The resulting slight clicking sound can be
3.1 SAMPLE CAPACITOR
When used for most applications, the charge sampler Cs can
be virtually any plastic film or good quality ceramic capacitor.
The type should be relatively stable in the anticipated
Figure 2-5 Eliminating HB Pulses
Figure 2-6 Piezo Sounder Circuit
+2.5 ~ +5
GATE OR
MICRO INPUT
1
RE
Vdd
2
3
4
7
5
6
SENSING
2
3
4
7
5
6
OUT
SNS1
GAIN
SNS2
C M OS
OUT
SNS2
GAIN
SNS1
ELECTRODE
Co
OPT1
OPT2
100pF
Rs
OPT1
OPT2
Cx
Vss
8
lq
6
QT118H R1.08 / 0405
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