QProx™ QT113 / QT113H
™
C
HARGE
-T
RANSFER
T
OUCH
S
ENSOR
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Projects a proximity field through air
Less expensive than many mechanical switches
Sensitivity easily adjusted via capacitor value
Turns small objects into intrinsic touch sensors
100% autocal for life - no adjustments required
2.5 to 5V, 600µA single supply operation
µ
Toggle mode for on/off control (strap option)
10s, 60s, infinite auto-recal timeout (strap options)
Gain settings in 2 discrete levels
HeartBeat™ health indicator on output
Active-low (QT113) or active-high outputs (QT113H)
Only one external part required - a 1¢ capacitor
Vdd
O ut
O pt1
O pt2
1
8
Vss
Sn s2
Sn s1
Gain
Q T113
2
3
4
7
6
5
APPLICATIONS -
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Light switches
Prox sensors
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Appliance control
Security systems
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Access systems
Pointing devices
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Elevator buttons
Toys & games
The QT113 charge-transfer (“QT’”) touch sensor is a self-contained digital IC capable of detecting near-proximity or touch. It will
project a proximity sense field through air, via almost any dielectric, like glass, plastic, stone, ceramic, and most kinds of wood. It can
also turn small metal-bearing objects into intrinsic sensors, making them responsive to proximity or touch. This capability coupled with
its ability to self calibrate continuously can lead to entirely new product concepts.
It is designed specifically for human interfaces, like control panels, appliances, toys, lighting controls, or anywhere a mechanical
switch or button may be found; it may also be used for some material sensing and control applications provided that the presence
duration of objects does not exceed the recalibration timeout interval.
The QT113 requires only a common inexpensive capacitor in order to function.
Power consumption is only 600µA in most applications. In most cases the power supply need only be minimally regulated, for example
by Zener diodes or an inexpensive 3-terminal regulator.
The QT113’s RISC core employs signal processing techniques pioneered by Quantum; these are specifically designed to make the
device survive real-world challenges, such as ‘stuck sensor’ conditions and signal drift. Even sensitivity is digitally determined and
remains constant in the face of large variations in sample capacitor C
S
and electrode C
X
. No external switches, opamps, or other
analog components aside from C
S
are usually required.
The option-selectable toggle mode permits on/off touch control, for example for light switch replacement. The Quantum-pioneered
HeartBeat™ signal is also included, allowing a host microcontroller to monitor the health of the QT113 continuously if desired. By
using the charge transfer principle, the IC delivers a level of performance clearly superior to older technologies in a highly
cost-effective package.
T
A
0 C to +70
0
C
0
0
C to +70
0
C
-40
0
C to +85
0
C
-40
0
C to +85
0
C
0
AVAILABLE OPTIONS
SOIC
QT113-S
QT113H-S
QT113-IS
QT113H-IS
8-PIN DIP
QT113-D
QT113H-D
-
-
Copyright Quantum Research Group Ltd
R1.10/0104
Quantum Research Group Ltd
1 - OVERVIEW
The QT113 is a digital burst mode charge-transfer (QT)
sensor designed specifically for touch controls; it includes all
hardware and signal processing functions necessary to
provide stable sensing under a wide variety of changing
conditions. Only a single low cost, non-critical capacitor is
required for operation.
Figure 1-1 shows the basic QT113 circuit using the device,
with a conventional output drive and power supply
connections.
Figure 1-1 Standard mode options
+2.5 to 5
1
2
3
Vdd
OUT
SNS2
SENSING
ELECTRODE
7
5
C
s
10nF
OPT1
GAIN
1.1 BASIC OPERATION
C
x
4
6
The QT113 employs bursts of charge-transfer cycles to
OPT2
SNS1
acquire its signal. Burst mode permits power consumption in
OUTPUT=DC
Vss
the microamp range, dramatically reduces RF emissions,
TIMEOUT=10 Secs
8
lowers susceptibility to EMI, and yet permits excellent
TOGGLE=OFF
GAIN=HIGH
response time. Internally the signals are digitally processed
to reject impulse noise, using a 'consensus' filter which
requires three consecutive confirmations of a detection
1.2 ELECTRODE DRIVE
The internal ADC treats Cs as a floating transfer capacitor; as
before the output is activated.
a direct result, the sense electrode can be connected to
The QT switches and charge measurement hardware either SNS1 or SNS2 with no performance difference. In both
functions are all internal to the QT113 (Figure 1-2). A 14-bit cases the rule Cs >> Cx must be observed for proper
single-slope switched capacitor ADC includes both the operation. The polarity of the charge buildup across Cs
required QT charge and transfer switches in a configuration during a burst is the same in either case.
that provides direct ADC conversion. The ADC is designed to
dynamically optimize the QT burst length according to the It is possible to connect separate Cx and Cx’ loads to SNS1
rate of charge buildup on Cs, which in turn depends on the and SNS2 simultaneously, although the result is no different
values of Cs, Cx, and Vdd. Vdd is used as the charge than if the loads were connected together at SNS1 (or
reference voltage. Larger values of Cx cause the charge SNS2). It is important to limit the amount of stray capacitance
transferred into Cs to rise more rapidly, reducing available on both terminals, especially if the load Cx is already large,
resolution; as a minimum resolution is required for proper for example by minimizing trace lengths and widths so as not
operation, this can result in dramatically reduced apparent to exceed the Cx load specification and to allow for a larger
gain. Conversely, larger values of Cs reduce the rise of sensing electrode size if so desired.
differential voltage across it, increasing available resolution The PCB traces, wiring, and any components associated with
by permitting longer QT bursts. The value of Cs can thus be or in contact with SNS1 and SNS2 will become touch
increased to allow larger values of Cx to be tolerated (Figures sensitive and should be treated with caution to limit the touch
4-1, 4-2, 4-3 in Specifications, rear).
area to the desired location. Multiple touch electrodes can be
The IC is responsive to both Cx and Cs, and changes in Cs used, for example to create a control button on both sides of
an object, however it is impossible for the sensor to
can result in substantial changes in sensor gain.
distinguish between the two touch areas.
Option pins allow the selection or alteration of several special
features and sensitivity.
1.3 ELECTRODE DESIGN
Figure 1-2 Internal Switching & Timing
ELE C TRO DE
R esult
1.3.1 E
LECTRODE
G
EOMETRY AND
S
IZE
There is no restriction on the shape of
the electrode; in most cases common
sense and a little experimentation can
result in a good electrode design. The
QT113 will operate equally well with
long, thin electrodes as with round or
square ones; even random shapes are
acceptable. The electrode can also be
a 3-dimensional surface or object.
Sensitivity is related to electrode
surface area, orientation with respect
to the object being sensed, object
composition, and the ground coupling
quality of both the sensor circuit and
the sensed object.
If a relatively large electrode surface is
desired, and if tests show that the
electrode has more capacitance than
the QT113 can tolerate, the electrode
SNS2
Single -Slo pe 14-bit
Switched Cap acito r AD C
Bu rst Controller
C
s
C
x
SNS1
Start
Do ne
C ha rg e
Am p
-2-
can be made into a sparse mesh (Figure 1-3) having lower crumpled into a ball. Virtual ground planes are more effective
Cx than a solid plane. Sensitivity may even remain the same, and can be made smaller if they are physically bonded to
as the sensor will be operating in a lower region of the gain other surfaces, for example a wall or floor.
curves.
1.3.4 F
IELD
S
HAPING
1.3.2 K
IRCHOFF
’
S
C
URRENT
L
AW
Like all capacitance sensors, the QT113 relies on Kirchoff’s
Current Law (Figure 1-4) to detect the change in capacitance
of the electrode. This law as applied to capacitive sensing
requires that the sensor’s field current must complete a loop,
returning back to its source in order for capacitance to be
sensed. Although most designers relate to Kirchoff’s law with
regard to hardwired circuits, it applies equally to capacitive
field flows. By implication it requires that the signal ground
and the target object must both be coupled together in some
manner for a capacitive sensor to operate properly. Note that
there is no need to provide actual hardwired ground
connections; capacitive coupling to ground (Cx1) is always
sufficient, even if the coupling might seem very tenuous. For
example, powering the sensor via an isolated transformer will
provide ample ground coupling, since there is capacitance
between the windings and/or the transformer core, and from
the power wiring itself directly to 'local earth'. Even when
battery powered, just the physical size of the PCB and the
object into which the electronics is embedded will generally
be enough to couple a few picofarads back to local earth.
The electrode can be prevented from sensing in undesired
directions with the assistance of metal shielding connected to
circuit ground (Figure 1-5). For example, on flat surfaces, the
field can spread laterally and create a larger touch area than
desired. To stop field spreading, it is only necessary to
surround the touch electrode on all sides with a ring of metal
connected to circuit ground; the ring can be on the same or
opposite side from the electrode. The ring will kill field
spreading from that point outwards.
If one side of the panel to which the electrode is fixed has
moving traffic near it, these objects can cause inadvertent
detections. This is called ‘walk-by’ and is caused by the fact
that the fields radiate from either surface of the electrode
equally well. Again, shielding in the form of a metal sheet or
foil connected to circuit ground will prevent walk-by; putting a
small air gap between the grounded shield and the electrode
will keep the value of Cx lower and is encouraged. In the
case of the QT113, sensitivity can be high enough
(depending on Cx and Cs) that 'walk-by' signals are a
concern; if this is a problem, then some form of rear shielding
may be required.
1.3.3 V
IRTUAL
C
APACITIVE
G
ROUNDS
When detecting human contact (e.g. a fingertip), grounding
1.3.5 S
ENSITIVITY
The QT113 can be set for one of 2 gain levels using option
pin 5 (Table 1-1). This sensitivity change is made by altering
the internal numerical threshold level required for a detection.
Note that sensitivity is also a function of other things: like the
value of Cs, electrode size, shape, and orientation, the
composition and aspect of the object to be sensed, the
thickness and composition of any overlaying panel material,
and the degree of ground coupling of both sensor and object.
1.3.5.1 Increasing Sensitivity
In some cases it may be desirable to increase sensitivity
further, for example when using the sensor with very thick
panels having a low dielectric constant.
Figure 1-3 Mesh Electrode Geometry
Sensitivity can often be increased by using a bigger
electrode, reducing panel thickness, or altering panel
composition. Increasing electrode size can have diminishing
of the person is never required. The human body naturally returns, as high values of Cx will reduce sensor gain (Figures
has several hundred picofarads of ‘free space’ capacitance to
the local environment (Cx3 in Figure 1-4), which is more than
two orders of magnitude greater than that required to create
Figure 1-4 Kirchoff's Current Law
a return path to the QT113 via earth. The QT113's PCB
however can be physically quite small, so there may be little
‘free space’ coupling (Cx1 in Figure 1-4) between it and the
environment to complete the return path. If the QT113 circuit
C
X2
ground cannot be earth grounded by wire, for example via
the supply connections, then a ‘virtual capacitive ground’ may
be required to increase return coupling.
A ‘virtual capacitive ground’ can be created by connecting the
QT113’s own circuit ground to:
(1) A nearby piece of metal or metallized housing;
(2) A floating conductive ground plane;
(3) A nail driven into a wall;
(4) A larger electronic device (to which its output might be
connected anyway).
Free-floating ground planes such as metal foils should
maximize exposed surface area in a flat plane if possible. A
square of metal foil will have little effect if it is rolled up or
S e n se E le ctro de
S EN SO R
C
X1
Su rro und in g e nv iro nm e n t
C
X3
-3-
Figure 1-5
Shielding Against Fringe Fields
Drift compensation (Figure 2-1) is performed by making the
reference level track the raw signal at a slow rate, but only
while there is no detection in effect. The rate of adjustment
must be performed slowly, otherwise legitimate detections
could be ignored. The QT113 drift compensates using a
slew-rate limited change to the reference level; the threshold
and hysteresis values are slaved to this reference.
Once an object is sensed, the drift compensation mechanism
ceases since the signal is legitimately high, and therefore
should not cause the reference level to change.
Sen se
wire
Sense
wire
U nshielded
Electrode
S hielded
E lec trode
The QT113's drift compensation is 'asymmetric': the
reference level drift-compensates in one direction faster than
it does in the other. Specifically, it compensates faster for
decreasing signals than for increasing signals. Increasing
signals should not be compensated for quickly, since an
approaching finger could be compensated for partially or
entirely before even approaching the sense electrode.
However, an obstruction over the sense pad, for which the
sensor has already made full allowance for, could suddenly
be removed leaving the sensor with an artificially elevated
reference level and thus become insensitive to touch. In this
latter case, the sensor will compensate for the object's
removal very quickly, usually in only a few seconds.
With large values of Cs and small values of Cx, drift
compensation will appear to operate more slowly than with
4-1 to 4-3). The value of Cs also has a dramatic effect on
sensitivity, and this can be increased in value (up to a limit).
Also, increasing the electrode's surface area will not
substantially increase touch sensitivity if its diameter is
already much larger in surface area than the object being
detected. The panel or other intervening material can be
made thinner, but again there are diminishing rewards for
doing so. Panel material can also be changed to one having
a higher dielectric constant, which will help propagate the
field through to the front. Locally adding some conductive
material to the panel (conductive materials essentially have
an infinite dielectric constant) will also help; for example,
adding carbon or metal fibers to a plastic panel will greatly
increase frontal field strength, even if the fiber density is too
low to make the plastic bulk-conductive.
1.3.5.2 Decreasing Sensitivity
In some cases the QT113 may be too sensitive, even on low
gain. In this case gain can be lowered further by a number of
strategies: making the electrode smaller, making the
electrode into
a
sparse mesh
using
a
high
space-to-conductor ratio (Figure 1-3), or by decreasing Cs.
Table 1-1 Gain Setting Strap Options
Gain
High - 6 counts
Low - 12 counts
Tie Pin 5 to:
Vdd
Vss (Gnd)
the converse. Note that the positive and negative drift
compensation rates are different.
2.1.2 T
HRESHOLD
C
ALCULATION
Unlike the QT110 device, the internal threshold level is fixed
at one of two setting as determined by Table 1-1. These
setting are fixed with respect to the internal reference level,
which in turn can move in accordance with the drift
compensation mechanism..
The QT113 employs a hysteresis dropout below the
threshold level of 17% of the delta between the reference and
threshold levels.
2.1.3 M
AX
O
N
-D
URATION
If an object or material obstructs the sense pad the signal
may rise enough to create a detection, preventing further
2 - QT113 SPECIFICS
2.1 SIGNAL PROCESSING
The QT113 processes all signals using 16 bit
math, using a number of algorithms pioneered by
Quantum. The algorithms are specifically
designed to provide for high 'survivability' in the
face of numerous adverse environmental
changes.
Figure 2-1 Drift Compensation
S ign a l
H yste resis
T hr es ho ld
R e fe re nce
2.1.1 D
RIFT
C
OMPENSATION
A
LGORITHM
Signal drift can occur because of changes in Cx
and Cs over time. It is crucial that drift be
compensated for, otherwise false detections,
non-detections, and sensitivity shifts will follow.
O u tpu t
-4-
operation. To prevent this, the sensor includes a timer which increasing levels of Cs reduce response time. Figure 4-3
monitors detections. If a detection exceeds the timer setting, shows the typical effects of Cs and Cx on response time.
the timer causes the sensor to perform a full recalibration
(when not set to infinite). This is known as the Max
2.2 OUTPUT FEATURES
On-Duration feature.
The QT113 is designed for maximum flexibility and can
After the Max On-Duration interval, the sensor will once again accommodate most popular sensing requirements. These
function normally, even if partially or fully obstructed, to the are selectable using strap options on pins OPT1 and OPT2.
best of its ability given electrode conditions. There are two All options are shown in Table 2-1.
finite timeout durations available via strap option: 10 and 60
2.2.1 DC M
ODE
O
UTPUT
seconds (Table 2-1).
The output of the QT113 can respond in a DC mode, where
the output is active-low upon detection. The output will
2.1.4 D
ETECTION
I
NTEGRATOR
It is desirable to suppress detections generated by electrical remain active-low for the duration of the detection, or until the
noise or from quick brushes with an object. To accomplish Max On-Duration expires (if not infinite), whichever occurs
this, the QT113 incorporates a detect integration counter that first. If a max on-duration timeout occurs first, the sensor
increments with each detection until a limit is reached, after performs a full recalibration and the output becomes inactive
which the output is activated. If no detection is sensed prior until the next detection.
to the final count, the counter is reset immediately to zero. In
In this mode, three Max On-Duration timeouts are available:
the QT113, the required count is 3.
10 seconds, 60 seconds, and infinite.
The Detection Integrator can also be viewed as a 'consensus'
filter, that requires three detections in three successive bursts
Table 2-1 Output Mode Strap Options
to create an output.
2.1.5 F
ORCED
S
ENSOR
R
ECALIBRATION
The QT113 has no recalibration pin; a forced recalibration is
accomplished only when the device is powered up. However,
supply drain is low so it is a simple matter to treat the entire
IC as a controllable load; simply driving the QT113's Vdd pin
directly from another logic gate or a microcontroller port
(Figure 2-2) will serve as both power and 'forced recal'. The
source resistance of most CMOS gates and microcontrollers
are low enough to provide direct power without problem. Note
that most 8051-based micros have only a weak pullup drive
capability and will require CMOS buffering. 74HC or 74AC
series gates can directly power the QT113, as can most other
microcontrollers.
DC Out
DC Out
Toggle
DC Out
Tie
Pin 3 to:
Vdd
Vdd
Gnd
Gnd
Tie
Pin 4 to:
Vdd
Gnd
Gnd
Vdd
Max On-
Duration
10s
60s
10s
infinite
Infinite timeout is useful in applications where a prolonged
detection can occur and where the output must reflect the
detection no matter how long. In infinite timeout mode, the
designer should take care to be sure that drift in Cs, Cx, and
Vdd do not cause the device to ‘stick on’ inadvertently even
Option strap configurations are read by the QT113 only on when the target object is removed from the sense field.
powerup. Configurations can only be changed by powering
2.2.2 T
OGGLE
M
ODE
O
UTPUT
the QT113 down and back up again; again, a microcontroller This makes the sensor respond in an on/off mode like a flip
can directly alter most of the configurations and cycle power flop. It is most useful for controlling power loads, for example
to put them in effect.
in kitchen appliances, power tools, light switches, etc.
2.1.6 R
ESPONSE
T
IME
Max On-Duration in Toggle mode is fixed at 10 seconds.
The QT113's response time is highly dependent on burst When a timeout occurs, the sensor recalibrates but leaves
length, which in turn is dependent on Cs and Cx (see Figures the output state unchanged.
4-1, 4-2). With increasing Cs, response time slows, while
2.2.3 H
EART
B
EAT
™ O
UTPUT
Figure 2-2 Powering From a CMOS Port Pin
P O RT X .m
0.01µF
The QT113 output has a full-time HeartBeat™ ‘health’
indicator superimposed on it. This operates by taking 'Out'
into a 3-state mode for 300µs once after 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.
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 2-3) when the sensor is not
detecting an object; when detecting an object, the output will
remain low for the duration of the detection, and no
HeartBeat pulse will be evident.
If the sensor is wired to a microcontroller as shown in Figure
2-4, the microcontroller can reconfigure the load resistor to
either ground or Vcc depending on the output state of the
QT113, so that the pulses are evident in either state.
C MO S
m icro controller
V dd
P O RT X .n
O UT
Q T11 0
V ss
-5-
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