lQ
C
HARGE
-T
RANSFER
T
OUCH
S
ENSOR
QProx™ QT113
Projects a proximity field through air or any insulator
Less expensive than many mechanical switches
Sensitivity easily adjusted
Consensus filter for noise immunity
100% autocal for life - no adjustments required
2.5 to 5V, 600µA single supply operation
Toggle mode for on/off control (strap option)
10s, 60s, infinite auto-recal timeouts (strap options)
HeartBeat™ health indicator on output
Only one external part required - a 1¢ capacitor
Lead-Free package
Opt1
Opt2
3
4
Vdd
Out
1
2
8
7
6
5
Vss
Sns2
Sns1
Gain
QT113
APPLICATIONS -
Light switches
Prox sensors
Appliance control
Security systems
Access systems
Pointing devices
Elevator buttons
Consumer devices
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, and 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.
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 requires only a common inexpensive capacitor
in order to function.
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.
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 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
-40
0
C to +85
0
C
0
AVAILABLE OPTIONS
SOIC
-
QT113-ISG
8-PIN DIP
QT113-DG
-
Copyright 1999-2004 QRG Ltd
R1.05/0405
1 - OVERVIEW
Figure 1-1 Basic Circuit Configuration
+2.5 to 5
SENSING
ELECTRODE
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 a basic circuit using the device.
1
2
3
4
OUTPUT=DC
TIMEOUT=10 Secs
TOGGLE=OFF
GAIN=HIGH
Vdd
OUT
SNS2
7
5
6
C
s
R
SERIES
1.1 BASIC OPERATION
The QT113 employs bursts of charge-transfer cycles to
acquire its signal. Burst mode permits power consumption in
the microamp range, dramatically reduces RF emissions,
lowers susceptibility to EMI, and yet permits excellent
response time. Internally the signals are digitally processed
to reject impulse noise, using a 'consensus' filter which
requires three consecutive confirmations of a detection
before the output is activated.
The QT switches and charge measurement hardware
functions are all internal to the QT113 (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 ADC is designed to
dynamically optimize the QT burst length according to the
rate of charge buildup on Cs, which in turn depends on the
values of Cs, Cx, and Vdd. Vdd is used as the charge
reference voltage. Larger values of Cx cause the charge
transferred into Cs to rise more rapidly, reducing available
resolution; as a minimum resolution is required for proper
operation, this can result in dramatically reduced apparent
gain. Conversely, larger values of Cs reduce the rise of
differential voltage across it, increasing available resolution
by permitting longer QT bursts. The value of Cs can thus be
increased to allow larger values of Cx to be tolerated (Figures
4-1, 4-2, 4-3 in Specifications, rear).
The IC is responsive to both Cx and Cs, and changes in Cs
can result in substantial changes in sensor gain.
Option pins allow the selection or alteration of several special
features and sensitivity.
OPT1
GAIN
10nF
C
x
OPT2
Vss
SNS1
8
1.2 ELECTRODE DRIVE
The internal ADC treats Cs as a floating transfer capacitor; as
a result, the sense electrode can in theory be connected to
either SNS1 or SNS2 with no performance difference.
However the electrode should only be connected to pin SNS2
for optimum noise immunity.
In all cases the rule Cs >> Cx must be observed for proper
operation; a typical load capacitance (Cx) ranges from
10-20pF while Cs is usually around 10-50nF.
Increasing amounts of Cx destroy gain; therefore it is
important to limit the amount of stray capacitance on both
SNS terminals, for example by minimizing trace lengths and
widths and keeping these traces away from power or ground
traces or copper pours.
The traces and any components associated with SNS1 and
SNS2 will become touch sensitive and should be treated with
caution to limit the touch area to the desired location.
A series resistor, Rseries, should be placed inline with the
SNS2 pin to the electrode to suppress ESD and EMC effects.
1.3 ELECTRODE DESIGN
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
can be made into a sparse mesh
(Figure 1-3) having lower Cx than a
Figure 1-2 Internal Switching & Timing
R esult
Single-Slo pe 14-bit
Switched Capacitor ADC
E LE C TRO DE
S NS 2
Burst Controller
C
s
C
x
S NS 1
S tart
Do ne
C ha rge
Amp
lQ
2
R1.05/0405
solid plane. Sensitivity may even remain the same, as the
sensor will be operating in a lower region of the gain curves.
1.3.2 K
IRCHOFF
’
S
C
URRENT
L
AW
Like all capacitance sensors, the QT113 relies on Kirchoff’s
Current Law (Figure 1-3) 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.
equally well. 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 to reduce loading and keep
gain high.
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 and capacitance, electrode 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.
Sensitivity can often be increased by using a bigger
electrode, reducing panel thickness, or altering panel
composition. Increasing electrode size can have diminishing
returns, as high values of Cx will reduce sensor gain (Figures
4-1 to 4-3). The value of Cs also has a dramatic effect on
sensitivity, and this can be increased in value with the
tradeoff of reduced response time. 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. Panel material can also be changed to
one having a higher dielectric constant, which will help
propagate the field. Metal areas near the electrode will
reduce the field strength and increase Cx loading.
1.3.3 V
IRTUAL
C
APACITIVE
G
ROUNDS
When detecting human contact (e.g. a fingertip), grounding
of the person is never required. The human body naturally
has several hundred picofarads of ‘free space’ capacitance to
the local environment (Cx3 in Figure 1-3), which is more than
two orders of magnitude greater than that required to create
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 1-3) between it and the
environment to complete the return path. If the QT113 circuit
Ground planes around and under the electrode and its SNS
ground cannot be earth grounded by wire, for example via
the supply connections, then a ‘virtual capacitive ground’ may trace will cause high Cx loading and destroy gain. The
possible signal-to-noise ratio benefits of ground area are
be required to increase return coupling.
more than negated by the decreased gain from the circuit,
A ‘virtual capacitive ground’ can be created by connecting the and so ground areas around electrodes are discouraged.
QT113’s own circuit ground to:
Keep ground away from the electrodes and traces.
- A nearby piece of metal or metallized housing;
1.3.5.2 Decreasing Sensitivity
- A floating conductive ground plane;
In some cases the QT113 may be too sensitive, even on low
- Another electronic device (to which its output might be
gain. In this case gain can be lowered further by decreasing
connected anyway).
Cs.
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
Figure 1-3 Kirchoff's Current Law
crumpled into a ball. Virtual ground planes are more effective
and can be made smaller if they are physically bonded to
other surfaces, for example a wall or floor.
C
X2
1.3.4 F
IELD
S
HAPING
The electrode can be prevented from sensing in undesired
directions with the assistance of metal shielding connected to
circuit ground (Figure 1-4). 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
S e nse E le ctro de
S EN SO R
C
X 1
C
X3
Su rro und in g e nv iro nm en t
lQ
3
R1.05/0405
Figure 1-4 Shielding Against Fringe Fields
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
the converse. Note that the positive and negative drift
compensation rates are different.
2.1.2 T
HRESHOLD
C
ALCULATION
The internal threshold level is fixed at one of two setting as
determined by Table 1-1. These settings are fixed with
respect to the internal reference level, which in turn will 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.
Sense
wire
Sense
wire
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
operation. To prevent this, the sensor includes a timer which
monitors detections. If a detection exceeds the timer setting,
the timer causes the sensor to perform a full recalibration
(when not set to infinite). This is known as the Max
On-Duration feature.
U ns hielded
Electrode
S hielded
E lectrode
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.
Table 1-1 Gain Setting Strap Options
Gain
High - 6 counts
Low - 12 counts
Tie Pin 5 to:
Vdd
Vss (Gnd)
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.
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.
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
After the Max On-Duration interval, the sensor will once again
function normally to the best of its ability given electrode
conditions. There are two finite timeout durations available
via strap option: 10 and 60 seconds (Table 2-1).
2.1.4 D
ETECTION
I
NTEGRATOR
It is desirable to suppress detections generated by electrical
noise or from quick brushes with an object. To accomplish
this, the QT113 incorporates a detect integration counter that
increments with each detection until a limit is reached, after
which the output is activated. If no detection is sensed prior
to the final count, the counter is reset immediately to zero. In
the QT113, the required count is 3.
The Detection Integrator can also be viewed as a 'consensus'
filter, that requires three successive detections to create an
output.
Figure 2-1 Drift Compensation
S ig na l
H ys te res is
T hre sh old
R efer ence
Ou tpu t
lQ
4
R1.05/0405
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 many 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.
Option strap configurations are read by the QT113 only on
powerup. Configurations can only be changed by powering
the QT113 down and back up again; again, a microcontroller
can directly alter most of the configurations and cycle power
to put them in effect.
Vdd do not cause the device to ‘stick on’ inadvertently even
when the target object is removed from the sense field.
2.2.2 T
OGGLE
M
ODE
O
UTPUT
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.
Max On-Duration in Toggle mode is fixed at 10 seconds.
When a timeout occurs, the sensor recalibrates but leaves
the output toggle state unchanged.
2.2.3 H
EART
B
EAT
™ O
UTPUT
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.
2.1.6 R
ESPONSE
T
IME
The QT113's response time is highly dependent on burst
length, which in turn is dependent on Cs and Cx (see Figures
4-1, 4-2). With increasing Cs, response time slows, while
increasing levels of Cs reduce response time. Figure 4-3
shows the typical effects of Cs and Cx on response time.
Table 2-1 Output Mode Strap Options
Tie
Pin 3 to:
DC Out
DC Out
Toggle
DC Out
Vdd
Vdd
Gnd
Gnd
Tie
Pin 4 to:
Vdd
Gnd
Gnd
Vdd
Max On-
Duration
10s
60s
10s
infinite
2.2 OUTPUT FEATURES
The QT113 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.
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
2.2.1 DC M
ODE
O
UTPUT
HeartBeat pulses (Figure 2-3) when the sensor is not
The output of the QT113 can respond in a DC mode, where
detecting an object; when detecting an object, the output will
the output is active-low upon detection. The output will
remain active-low for the duration of the detection, or until the remain low for the duration of the detection, and no
HeartBeat pulse will be evident.
Max On-Duration expires (if not infinite), whichever occurs
first. If a max on-duration timeout occurs first, the sensor
If the sensor is wired to a microcontroller as shown in Figure
performs a full recalibration and the output becomes inactive 2-4, the microcontroller can reconfigure the load resistor to
until the next detection.
either ground or Vcc depending on the output state of the
In this mode, three Max On-Duration timeouts are available:
10 seconds, 60 seconds, and 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
QT113, so that the pulses are evident in either state.
Electromechanical devices like relays will usually ignore this
short pulse. The pulse also has too low a duty cycle to visibly
affect 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).
Figure 2-2 Powering From a CMOS Port Pin
PORT X.m
0.01µF
2.2.4 O
UTPUT
D
RIVE
The QT113’s `output is active low and can sink up to 5mA of
non-inductive current. If an inductive load is used, such as a
small relay, the load should be diode clamped to prevent
damage. When set to operate in a proximity mode (at high
gain) the current should be limited to 1mA to prevent gain
shifting side effects from occurring, which happens when the
load current creates voltage drops on the die and bonding
wires; these small shifts can materially influence the signal
level to cause detection instability as described below.
Care should be taken when the QT113 and the load are both
powered from the same supply, and the supply is minimally
regulated. The QT113 derives its internal references from the
power supply, and sensitivity shifts can occur with changes in
Vdd, as happens when loads are switched on. This can
CMOS
microcontroller
Vdd
PORT X.n
OUT
QT113
Vss
lQ
5
R1.05/0405
Robotics Development
京公网安备 11010802033920号
EEWORLD
