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Limit sensing of almost any fluid or powder
Two-tier level sensor - Hi / Low limits with one probe
Only one external part required - a 5¢ capacitor
Uses internal probes or external electrodes
Active high or active low outputs
Slosh filter averages response of moving fluids
LED drive capable on both outputs
2.5 to 5V 20µA single supply operation
HeartBeat™ health indicator on both outputs
RoHS compliant package
QT114
QL
EVEL
™ S
ENSOR
IC
Vcc
Out1
Out2
Filt
1
8
Gnd
Sns2
Sns1
Pol
QT114
2
3
4
7
6
5
APPLICATIONS -
!
Process controls
!
Vending machines
!
Consumer appliances
!
Automotive fluids
!
Medical fluid sensing
!
Soil moisture sensing
DESCRIPTION -
The QT114 QLevel™ charge-transfer (“QT”) sensor IC is specifically designed to detect point level in fluids and
powders. It will project a sense field through almost any dielectric, like glass, plastic, or ceramic, to sense level on
the inside of a vessel, from its exterior. It has the unique capability of independently sensing two trip points when
used with structured electrodes having two tiers.
The QT114 does not have sensing timeouts, drift compensation, or other functions which would interfere with level
sensing. Its threshold levels are fixed, and the amount of signal required to exceed a threshold is dependent on
circuit gain and electrode size and loading, all of which are under the control of the designer.
The QT114 requires only a single inexpensive capacitor in order to function. One or two LEDs can also be added to
provide a visual sensing indication.
Power consumption is under 20mA in most applications, allowing operation from Lithium cells for many years. In
most cases the power supply needs only minimal regulation.
The QT114 employs numerous signal acquisition and processing techniques pioneered by Quantum. No external
switches, opamps, or other analog components aside from CS are required.
A unique feature is the 'slosh filter', a detection integrator which averages detections over a rolling 15 second
interval before activating or deactivating the OUT pins. This filter allows use of the QT114 with violently moving
fluids, for example in a moving vehicle, that would otherwise cause the outputs to flicker between two states.
The device also includes selectable output polarity, allowing both output lines to be made either active-high or
active-low. It also includes the Quantum-pioneered HeartBeat™ signal, allowing a host controller to monitor the
health of the QT114 continuously if desired. By using the charge transfer principle, the IC delivers a level of
performance clearly superior to older technologies. It is specifically designed to replace electromechanical devices
like float switches, thermistors, and conductance probes.
T
A
0 C to +70
0
C
-40
0
C to +85
0
C
0
AVAILABLE OPTIONS
SOIC
-
QT114-ISG
8-PIN DIP
QT114-DG
-
l
q
Copyright © 1999-2006 QRG Ltd
QT114 R1.04/1106
The QT114 is a digital burst mode charge-transfer (QT)
sensor designed specifically for point level sensing; it
includes all hardware and signal processing functions
necessary to provide stable level sensing under a wide
variety of changing conditions. Only a single external
capacitor is required for operation.
Figure 1-1 shows a basic QT114 circuit using the device, with
conventional OUT drives and power supply connections. The
sensing electrode can be connected to a single-tier or 2-tier
electrode as required.
Calibration is done by design, through adjustment of the
electrode sizes and the Cs capacitor. Only under rare
situations do QT114 circuits require calibration on an
individual basis, and the circuit can make provision for that.
To 10x Scope Probe
Vcc
1
OUT 1
OUT 2
2 OUT1 SNS2 7
3 OUT2 SNS1 6
4 FILT
POL 5
8 Gnd
V
dd
FILTER
POLARITY
POL: 1 = Active High
FILT: 1 = Slosh Filter
V
dd
2M
Ω
(optional)
To Electrode(s)
Cs
1M
Ω
multi-turn
pot (optional)
1 SIGNAL ACQUISITION
The QT114 employs a short, low duty cycle burst of
charge-transfer cycles to acquire its signal. Burst mode
permits power consumption in the low microamp range,
dramatically reduces RF emissions, lowers susceptibility to
EMI, and yet permits excellent response time. Internally the
signals are digitally processed to generate the required
output signals.
The QT switches and charge measurement hardware
functions are all internal to the QT114 (Figure 1-2). A 14-bit
single-slope switched capacitor ADC includes both the
required QT charge and transfer switches in a configuration
that provides direct ADC conversion. The burst length is
inversely proportional to the rate of charge buildup on Cs,
which in turn depends on the values of Cs, Cx, and Vcc. Vcc
is used as the charge reference voltage. Larger values of Cx
cause the charge transferred into Cs to accumulate more
rapidly. The trip points of the sensor can be changed by
altering Cs and Cx, the load capacitance. As a result, the
values of Cs, Cx, and Vcc should be fairly stable over the
expected operating temperature range.
Two fixed thresholds are used, one for low fluid level and the
other for high level; adjusting Cs and Cx to allow these to trip
at appropriate points is required by design, and if required
may be trimmed by an adjustment. Figure 1-1 shows the
optional potentiometer which can be used to fine-tune the
placement of these threshold points relative to the signal.
Figure 1-1 Standard mode options
It is not necessary to use both detection threshold points; if
only single point sensing is desired, only the lower threshold
and OUT1 can be used, while ignoring OUT2.
Two option pins allow the selection of output polarity and the
insertion of a 'slosh filter' before the OUT pins, as shown in
Figure 1-1.
1.1 ELECTRODE DRIVE
The internal ADC treats Cs as a floating transfer capacitor; as
a direct result, the sense electrode can be connected to
either SNS1 or SNS2 with no performance difference. The
polarity of the charge buildup across Cs during a burst is the
same in either case. Cs must be of within a certain range for
proper operation.
It is possible to connect separate Cx and Cx’ loads to SNS1
and SNS2 simultaneously, although the result is no different
than if the loads were connected together at SNS2 (or
SNS1). It is important to limit the amount of stray capacitance
on both terminals, especially if the load Cx is already large,
for example by minimizing trace lengths and widths so as not
to exceed the Cx load specification and to allow for a larger
sensing electrode size if so desired.
The PCB traces, wiring, and any components associated with
or in contact with SNS1 and SNS2 will become proximity
sensitive and should be treated with caution.
1.2 THRESHOLD POINTS
E LEC TRO DE
R esult
Single -Slo pe 14-bit
Switched Cap acito r AD C
S NS2
Bu rst Controller
The QT114 employs twin threshold points set at both
250 (for T1) and 150 counts (for T2) of acquisition
signal. The signal travels in an inverse direction:
increasing amounts of Cx reduce the signal level; the
baseline ('dry') signal should lie at 300 counts or more
under most conditions. Calibration details are
discussed fully in Section 3.2.
C
s
C
x
S NS1
Start
Do ne
2 ELECTRODE DESIGN
The QT114 is designed to operate with a 'plateau'
sensor, having a substantial surface area at each
desired trip point, to create a capacitive 'step'.
C harg e
Amp
Figure 1-2 Internal Switching and Timing
LQ
2
QT114 R1.04/1106
As Figure 2-1 shows, a vertical strip sensor on the outside
of a container (or a vertical, insulated rod in the fluid) will
generate a long sloping signal. The desired trip point 'T' is
subject to a great deal of variation in location if the sensing
signal drifts much, for example due to changes in Cs or Cx
over the operating temperature range.
Figure 2-2 shows the response from a horizontal strip of
the same surface area; the signal exhibits a very rapid rise
in signal between points l1 and l2. Variations in circuit gain
or signal drift have much less of an effect on the trip point
with this orientation.
In some cases (thin walled vessels for example) it may be
sufficient to have a small round or square electrode patch
on the exterior.
Figure 2-3 shows the response from a twin-level external
electrode set. The use of two horizontal electrode planes
or tiers creates well-defined trip points that can be used to
sense both 'low' and 'high' levels. A crossing of threshold
T1 will be reflected in the OUT1 signal, while T2 will be
reflected on OUT2.
Signal
l
2
T
1
l
1
T
1
Level
l
1
l
2
Figure 2-1 Signal vs. Level for an External Vertical Strip
S ignal
2.1 EXTERNAL ELECTRODES
External electrodes should be electrically conductive; metal
foils and conductive carbon are both possible. Care should
be taken that other objects or people near the vessel will
not touch the electrode; in some cases shielding around
the electrode with grounded metal will be required to
prevent disturbances. If used, the shield element should be
spaced apart from the electrode by an air gap or a
low-density foam to reduce Cx loading.
The required surface area of the external electrode will
depend on the amount of signal needed to bracket the
detection threshold, which in turn will depend in part on Cs
and stray Cx. External electrodes sensing through thick
walls and/or sensing low permittivity fluids will require
larger surface areas than those sensing water through thin
plastic, for example. External electrodes are more likely to
require potentiometer trimming to achieve reliable
operation (Figure 1-1, also Section 3.2).
Note that external electrodes used with conductive
solutions (i.e. aqueous liquids) do not measure the
permittivity of the fluid: they actually measure the
permittivity of the vessel wall, between 2 plates: the
electrode (plate 1) and the fluid (plate 2, effectively a
variable-area ground plate): if the fluid were to be replaced
with mercury the signal would be unchanged. A 20%
thickness variation in the vessel wall will therefore introduce
about a 20% variation in the resulting capacitance; if the
vessel wall cannot be controlled accurately enough in
production, serious sensing errors may occur.
When external electrodes are used to sense non-aqueous
substances (like oils or gasoline), the vessel wall dielectric
becomes a lessor contributor to the overall signal, which is
then heavily dominated by the permittivity of the fluid. The
lower the permittivity of the fluid the greater its dominance.
l
1
l
2
T
1
T
1
Level
l
1
l
2
Figure 2-2 Signal vs. Level for an External Horizontal Strip
Signal
T
2
l
3
l
4
T
2
l
1
l
2
T
1
T
1
Level
l
1
l
2
l
3
l
4
Figure 2-3 Signal vs. Level for Twin Horizontal Strips
Numerous types of internal point-level probes are possible.
2.2.1 D
ISC
P
ROBES
The simplest internal geometry is probably a disc probe
(Figure 2-4), having at least one planar surface ('tier') parallel
to the fluid surface. The sensing error can be minimized by
making the tier thin, so that the signal transitions abruptly
higher (see Figure 2-2) as the fluid covers the tier.
A notable difficulty with disc probes is the task of insulating
them with a uniform, repeatable thickness of insulation.
2.2.2 S
PIRAL
W
IRE
P
ROBES
A spiral solid-wire probe is simple to construct (Figure 2-5),
and has the advantage of being pre-insulated in a wide
choice of plastics from inexpensive PVC to PTFE. These
probe types provide a large step-function of capacitance
localized at the desired trip point, and are easy to form.
Spiral wire probes are most effective in water-based fluids;
they are not as effective in oils and other nonconductive
substances.
2.2 INTERNAL PROBES
When used with aqueous fluids or other electrically
conducting liquids, internal probes should be insulated with a
plastic layer. See also Section 2.1 for a discussion of
electrodes when used with conductive fluids. Aqueous
probes should be 100% insulated, even on the cut end of a
wire probe. The slightest pinhole of exposed metal anywhere
on an immersed part of the probe will immediately convert
the probe into a bare-metal probe (see Section 2.2.5).
LQ
3
QT114 R1.04/1106
Figure 2-4 Single Level Internal Planar Probe
Figure 2-5 Single Level Internal Spiral Wire Probe
T
2
T
2
Figure 2-6 Twin-Level Internal Planar Probe
Spiral wire probes have the disadvantage of not being as
rugged as a solid disc probe.
Figure 2-7 Twin-Level Internal Spiral Wire Probe
Coaxial probes are more expensive to make, and can have
problems with vibration if they are not constructed robustly.
The outer cylinder should be perforated at key spots to allow
fluid to fill and drain the cavity without trapping air bubbles
inside. The outer cylinder can also be made of a wire mesh.
The outer cylinder does not have to be coated in plastic, even
when used with water-based fluids. When used with oils, the
inner rod does not require insulation either.
2.2.3 S
IDE
-E
NTRY
P
ROBES
Another type is a side-entry probe (Figure 2-8), which
requires an entry point into the vessel wall, but may have the
advantage of accessibility in certain cases. These can be
made of simple metal rod, insulated in almost any plastic if
required.
2.2.4 C
OAXIAL
P
ROBES
Another type of internal probe is the coaxial probe (Figure
2-10); these are most useful with oils or similar fluids having
a low dielectric constant; the inner rod is connected to the
signal connection, and together with the outer grounded
cylinder forms a capacitor whose dielectric is either air or oil.
Keeping the gap between rod and cylinder to a minimum
increases the 'gain' of the electrode.
2.2.5 B
ARE
M
ETAL
P
ROBES
Bare metal internal probes can be used, for example with
nonconductive fluids like oils, without difficulty. This applies
to all probe types described above.
Bare probes can also be used with aqueous fluids, but in
these cases a 1,000pF (1nF) ceramic NPO capacitor should
be inserted between the probe and the QT114 to block DC
current flows.
LQ
4
QT114 R1.04/1106
T
1
Figure 2-8 Single Internal Horizontal Probe
Figure 2-9 Twin-Level Internal Horizontal Probes
T
2
Figure 2-10 Coaxial Probe For Non-Aqueous Fluids
Figure 2-11 Twin-Level Coaxial Probes For
Non-Aqueous Fluids
A bare internal probe used with conductive fluids and an
in-line blocking capacitor will generate a huge, robust
capacitive response that will not readily permit the use of a
two-level probe due to signal saturation. Even the slightest
amount of bare metal exposed to the fluid will usually
generate an immediate, large response with aqueous fluids.
2.2.7 V
ISCOUS
, C
ONDUCTIVE
F
ILMS
Highly viscous fluids, or those having a high surface tension,
and having substantial conductivity can fool some electrode
designs into thinking that there is fluid present when there is
not. This is a particular problem with external electrodes,
where the residual films of certain types of fluids inside the
container, electrically coupled to the fluid mass below, will
create a substantial capacitive response. Internal probes are
much more resistant to this effect since the fluid surface is
guaranteed to become mechanically disconnected from the
probe when the level drops. Coating the inner vessel surface
with a smooth plastic of polyethylene or PTFE often has a
very beneficial effect on this phenomenon.
2.2.6 S
CALE
B
UILDUP
Scale buildup on internal probes, bare or insulated, is not
generally a problem since the sensor is still measuring
capacitance, not conductance, and a reduction in
conductivity around the probe will have minimal or no effect.
Probe designs should be tested for this to be certain in all
specific cases.
A legitimate concern with bare metal probes is the buildup of
scale or other deposits at the entry point of the probe into the
vessel. Such deposits may create a conductive surface path
(especially if the vessel is made of metal) that may lead to
false-positive trips. If the shank of the probe at the entry point
is insulated enough so that conductive bridging cannot occur,
this problem should be alleviated.
2.3 SINGLE LEVEL SENSING
When sensing for a single trip point, the single electrode can
be a simple horizontal strip on the outside of a nonmetallic
vessel (Figure 2-2), or an internal probe having a substantial
horizontal 'plateau' at the trip point (Figures 2-4, 2-5, 2-8,
2-10). When the strip or plateau is ‘covered’ with fluid the IC
will detect on at least the OUT1 line; OUT2 can be ignored.
LQ
5
QT114 R1.04/1106
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