Figure 2-2 Powering From a CMOS Port Pin
Figure 2-3 Damping Piezo Clicks with R
s
+2.5 ~ +5
PORT X.m
0.01µF
1
CMOS
RE
Vdd
2
3
4
7
5
6
SENSING
microcontroller
OUT
SNS1
GAIN
SNS2
ELECTRODE
Vdd
OPT1
OPT2
PORT X.n
OUT
QT110
Rs
Cx
Vss
Vss
8
On-Duration expires, whichever occurs first. If the latter occurs
first, the sensor performs a full recalibration and the output
becomes inactive until the next detection.
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.
In this mode, two Max On-Duration timeouts are available: 10
and 60 seconds.
2.2.2 TOGGLE
M
ODE
O
UTPUT
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 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 2-3).
This makes the sensor respond in an on/off mode like a flip
flop. It is most useful for controlling power loads, for example in
kitchen appliances, power tools, light switches, etc.
Max On-Duration in Toggle mode is fixed at 10 seconds. When
a timeout occurs, the sensor recalibrates but leaves the output
state unchanged.
Note that the piezo drive does not operate in Pulse mode.
2.2.5 HEARTBEAT™ OUTPUT
Table 2-1 Output Mode Strap Options
The output has a full-time HeartBeat™ ‘health’ indicator
superimposed on it. This operates by taking 'Out' into a 3-state
mode for 350µs once before every QT burst. This output state
can be used to determine that the sensor is operating properly,
or, it can be ignored using one of several simple methods.
Tie
Pin 3 to:
Tie
Pin 4 to:
Max On-
Duration
Vdd
Vdd
Gnd
Gnd
Vdd
Gnd
Gnd
Vdd
10s
60s
10s
10s
DC Out
DC Out
Toggle
Pulse
The HeartBeat indicator can be sampled by using a pulldown
resistor on Out, and feeding the resulting negative-going pulse
into a counter, flip flop, one-shot, or other circuit. Since Out is
normally high, a pulldown resistor will create negative
HeartBeat pulses (Figure 2-4) when the sensor is not detecting
an object; when detecting an object, the output will remain
active for the duration of the detection, and no HeartBeat pulse
will be evident.
2.2.3 PULSE
M
ODE
O
UTPUT
This mode generates a negative pulse of 75ms duration with
every new detection. It is most useful for 2-wire operation, but
can also be used when bussing together several devices onto
a common output line with the help of steering diodes or logic
gates, in order to control a common load from several places.
If the sensor is wired to a microcontroller as shown in Figure
2-5, the controller can reconfigure the load resistor to either
ground or Vcc depending on the output state of the device, so
that the pulses are evident in either state.
Max On-Duration is fixed at 10 seconds if in Pulse output
mode.
Electromechanical devices will usually ignore this short pulse.
The pulse also has too low a duty cycle to visibly activate
LED’s. It can be filtered completely if desired, by adding an RC
timeconstant to filter the output, or if interfacing directly and
only to a high-impedance CMOS input, by doing nothing or at
most adding a small non-critical capacitor from Out to ground
(Figure 2-6).
Note that the beeper drive does not operate in Pulse mode.
2.2.4 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.
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
D
RIVE
The QT110’s output is active low ; it can source 1mA or sink
5mA of non-inductive current.
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,
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
LQ
5
QT110 R1.04/0405
QUANTUM [ QUANTUM RESEARCH GROUP ]
