swing, with an intermediate count at about 200 between the

two. Thus, the lower electrode level should cause a signal

swing that (when 'dry') starts at 300 or more and when

covered ends at about 200. The upper electrode when

covered should generate a signal level of 100 or less.

There is a hysteresis of 3 counts around both T1 and T2.

The signal can be viewed for setup purposes with an

oscilloscope via a 10x or FET probe connected to a 2M ohm

resistor as shown in Figure 1-1; the resistor is required to

reduce the loading effect of the scope probe capacitance.

When viewed this way the signal will appear as a declining

slope (Figure 3-1). The duration of the slope corresponds to

the burst length: each count of burst takes approximately 7

microseconds on average. The ‘low level’ threshold at 250

counts is at 1750 microseconds from the start of the

waveform, while the 150 count ‘upper’ threshold is at about

1050 microseconds from the start, at 3 volts Vcc. These trip

points can be easily observed by monitoring the OUT lines

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.

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.

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.

3.3 INTERFACING

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.

The OUT lines can sink up to 5mA of non-inductive current.

If an inductive load is used, like a small relay, the load

should be diode clamped to prevent device damage.

POL strapping can be changed 'on the fly'.

Cycling and Stiction: Care should be taken when the QT114

and the loads are powered from the same supply, and the

supply is minimally regulated. The QT114 derives its internal

references from the power supply, and sensitivity shifts can

occur with changes in Vcc, as happens when loads are

switched on. This can induce detection ‘cycling’, whereby a

trip point is crossed, the load is turned on, the supply sags,

the trip is no longer sensed, the load is turned off, the supply

rises and the trip point is reacquired, ad infinitum. To prevent

this occurrence, the outputs should only be lightly loaded if

the device is operated from a poorly regulated supply.

Detection ‘stiction’, the opposite effect, can occur if a load is

shed when an Out line becomes active.

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.

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).

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.

- 7 -

Figure 3-1 Burst Waveform at 2M Pickoff Resistor