swing, with an intermediate count at about 200 between the The OUT lines can sink up to 5mA of non-inductive current.
two. Thus, the lower electrode level should cause a signal If an inductive load is used, like a small relay, the load
swing that (when 'dry') starts at 300 or more and when should be diode clamped to prevent device damage.
covered ends at about 200. The upper electrode when
POL strapping can be changed 'on the fly'.
covered should generate a signal level of 100 or less.
Cycling and Stiction: Care should be taken when the QT114
There is a hysteresis of 3 counts around both T1 and T2.
and the loads are powered from the same supply, and the
The signal can be viewed for setup purposes with an supply is minimally regulated. The QT114 derives its internal
oscilloscope via a 10x or FET probe connected to a 2M ohm references from the power supply, and sensitivity shifts can
resistor as shown in Figure 1-1; the resistor is required to occur with changes in Vcc, as happens when loads are
reduce the loading effect of the scope probe capacitance. switched on. This can induce detection ‘cycling’, whereby a
When viewed this way the signal will appear as a declining trip point is crossed, the load is turned on, the supply sags,
slope (Figure 3-1). The duration of the slope corresponds to the trip is no longer sensed, the load is turned off, the supply
the burst length: each count of burst takes approximately 7 rises and the trip point is reacquired, ad infinitum. To prevent
microseconds on average. The ‘low level’ threshold at 250 this occurrence, the outputs should only be lightly loaded if
counts is at 1750 microseconds from the start of the the device is operated from a poorly regulated supply.
waveform, while the 150 count ‘upper’ threshold is at about Detection ‘stiction’, the opposite effect, can occur if a load is
1050 microseconds from the start, at 3 volts Vcc. These trip shed when an Out line becomes active.
points can be easily observed by monitoring the OUT lines
3.3.2 HEARTBEAT™ OUTPUT
while watching the signal on a scope, by increasing Cx
loading until each OUT line activates in turn. FILT should be
off to speed up response during testing.
Both OUT lines have a full-time HeartBeat™ ‘health’
indicator superimposed on them. These operate by taking
both OUT pins into a 3-state mode for 350µs once before
every QT measurement burst. This state can be used to
determine that the sensor is operating properly, or, it can be
ignored using one of several simple methods.
The QT114's internal clock is dependent on Vcc; as a result,
the threshold points in terms of delay time from the start of
the burst are also substantially dependent on Vcc, but they
are always fixed in terms of signal counts. A regulated power
supply is strongly advised to maintain the proper calibration
points.
If active-low polarity is selected, the HeartBeat indicator can
be sampled by using a pulldown resistor on one or both OUT
lines, and feeding the resulting negative-going pulse(s) into a
counter, flip flop, one-shot, or other circuit (Figure 3-2). In
this configuration, the pulldown resistor will create
negative-going HeartBeat pulses when the sensor is not
detecting fluid; when detecting fluid, the OUT line will remain
low for the duration of the detection, and no pulse will be
evident. Conversely, a pull-up resistor will show HeartBeat
pulses when the line is low (detecting).
If active-high OUT polarity is selected, the pulses will only
appear if there is a pull-up resistor in place and the fluid is
not present (no detection, low output), or, if there is a
pull-down resistor and the output is active (high output).
Potentiometer adjustment:
The external potentiometer
shown in Figure 1-1 is optional and in most cases not
required. In situations where the electrode pickup signal is
weak, trimming may be necessary on a production basis to
make the device sensitive enough. Trimming affects the
baseline reference of the signal, and thus effects the amount
of change in the signal required to cause a threshold
crossing.
Potentiometer trimming is not a substitute for a good choice
of Cs. In low signal situations Cs should still be determined
by design to allow the baseline signal to be just beyond T1
as viewed on a scope. The trimmer should then be added
and the baseline adjusted to the necessary final resting
point.
The trimmer should never be adjusted so that the resistance
from ground to SNS1 or SNS2 is less than 200K ohms. If the
resistance is less than this amount, the gain of the circuit will
be appreciably reduced and it may stop functioning
altogether. A 200K resistor from the wiper to ground can
be added to limit trim current at the extremes of wiper
travel.
If the sensor is wired to a microprocessor as shown in Figure
3-3, the microprocessor can reconfigure the load resistor to
either ground or Vcc depending on the output state of the
QT114, so that the pulses are evident in either state with
either POL setting.
3.3 INTERFACING
3.3.1 OUT LINES AND POLARITY SELECTION
The QT114 has two OUT pins, OUT1 and OUT2, which
correspond to the crossings of signal at T1 and T2
respectively. Each output will become active after the
threshold is crossed, and after the slosh filter (if enabled)
has settled to its final state. The polarity of the OUT lines
is determined by pin 5, 'POL', as follows:
POL = Gnd
Outputs active low
POL = Vcc
Outputs active high
There is no timeout on these outputs; the OUT lines will
remain active for as long as the thresholds are crossed.
Figure 3-1 Burst Waveform at 2M Pickoff Resistor
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