break into high frequency oscillation, get hot, and stop
working.
2.2.4 HEARTBEAT™ OUTPUT
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.
Option strap configurations are read by the QT118H only on
powerup. Configurations can only be changed by powering
the QT118H down and back up again; a microcontroller can
directly alter most of the configurations and cycle power to
put them in effect.
Since Out is normally low, a pullup resistor will create
positive HeartBeat pulses (Figure 2-3) 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 OUTPUT FEATURES
The QT118H is designed for maximum flexibility and can
accommodate most popular sensing requirements. These
are selectable using strap options on pins OPT1 and OPT2.
All options are shown in Table 2-1.
If the sensor is wired to a microcontroller as shown in Figure
2-4, 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.
2.2.1 DC MODE
OUTPUT
The output of the device can respond in a DC mode, where
the output is active-high upon detection. The output will
remain active for the duration of the detection, or until the
Max 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.
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 2-5).
In this mode, two Max On-Duration timeouts are available:
10 and 60 seconds.
2.2.2 TOGGLE
M
ODE
O
UTPUT
2.2.5 PIEZO
A
COUSTIC
D
RIVE
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.
A piezo drive signal is generated for use with a bare piezo
sounder immediately after a detection is made; the tone lasts
for a nominal 75ms to create a ‘tactile feedback’ sound.
The sensor will drive most common bare piezo ‘beepers’
directly using an H-bridge drive configuration for the highest
possible sound level at all supply voltages; H-bridge drive
effectively doubles the supply voltage across the piezo. The
piezo is connected across pins SNS1 and SNS2. This drive
operates at a nominal 4kHz frequency, a common resonance
point for enclosed piezo sounders. Other frequencies can be
obtained upon special request.
Max On-Duration in Toggle mode is fixed at 10 seconds.
When a timeout occurs, the sensor recalibrates but leaves
the output state unchanged.
2.2.3 PULSE
MODE
OUTPUT
This generates a positive pulse of 75ms duration with every
new detection. It is most useful for 2-wire operation (see
Figure 1-2), 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 desired a bare piezo sounder can be directly adhered to
the rear of a control panel, provided that an acoustically
resonant cavity is also incorporated to give the desired
sound level.
Max On-Duration is fixed at 10 seconds if in Pulse output
mode.
Since piezo sounders are merely high-K ceramic capacitors,
the sounder will double as the Cs capacitor, and the piezo's
Figure 2-3
Getting HB pulses with a pulup resistor when not active
Figure 2-4
Using a micro to obtain HB pulses in either output state
lq
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