APPLICATIONS -
Elevator buttons
Toys & games
Access systems
Pointing devices
Appliance control
Security systems
Light switches
Industrial panels
The QT118H charge-transfer ("QT'") touch sensor is a self-contained digital IC capable of detecting near-proximity or touch. It will
project a sense field through 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 respond 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 IC requires only a common inexpensive capacitor in order to function. A bare piezo beeper can be connected to create a
`tactile' feedback clicking sound; the beeper itself then doubles as the required external capacitor, and it can also become the
sensing electrode. An LED can also be added to provide visual sensing indication. With a second inexpensive capacitor the device
can operated in 2-wire mode, where both power and signal traverse the same wire pair to a host. This mode allows the sensor to
be wired to a controller with only a twisted pair over a long distances.
Power consumption is under 20
A in most applications, allowing operation from Lithium cells for many years. In most cases the
power supply need only be minimally regulated.
The IC'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 Cs and electrode Cx. No external switches, opamps, or other
analog components aside from Cs are usually required.
The device includes several user-selectable built in features. One, toggle mode, permits on/off touch control, for example for light
switch replacement. Another makes the sensor output a pulse instead of a DC level, which allows the device to 'talk' over the
power rail, permitting a simple 2-wire interface. The Quantum-pioneered HeartBeatTM signal is also included, allowing a host
controller to monitor the health of the QT118H 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.
lq
1999-2000 Quantum Research Group
R1.03 / 0302
lQ
QProxTM QT118H
C
HARGE
-T
RANSFER
T
OUCH
S
ENSOR
Sns2
Vss
Sns1
Gain
O pt2
O pt1
O ut
Vd d
1
2
3
4
5
6
7
8
QT
1
1
8
H
-
QT118H-IS
-40
0
C to +85
0
C
QT118H-D
QT118H-S
0
0
C to +70
0
C
8-PIN DIP
SOIC
T
A
AVAILABLE OPTIONS
Less expensive than many mechanical switches
Projects a `touch button' through any dielectric
Turns small objects into intrinsic touch sensors
100% autocal for life - no adjustments required
Only one external part required - a 1 capacitor
Piezo sounder direct drive for `tactile' click feedback
LED drive for visual feedback
3V 20
A single supply operation
Toggle mode for on/off control (strap option)
10s or 60s auto-recalibration timeout (strap option)
Pulse output mode (strap option)
Gain settings in 3 discrete levels
Simple 2-wire operation possible
HeartBeatTM health indicator on output
1 - OVERVIEW
The QT118H 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 QT118H circuit using the device,
with a conventional output drive and power supply
connections. Figure 1-2 shows a second configuration using
a common power/signal rail which can be a long twisted pair
from a controller; this configuration uses the built-in pulse
mode to transmit the output state to the host controller.
1.1 BASIC OPERATION
The QT118H employs short, ultra-low duty cycle bursts 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 reject impulse noise, using
a 'consensus' filter which requires four consecutive
confirmations of a detection before the output is activated.
The QT switches and charge measurement hardware
functions are all internal to the QT118H (Figure 1-3). 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 highly tolerant of changes in Cs since it computes
the threshold level ratiometrically with respect to absolute
load, and does so dynamically at all times.
Cs is thus non-critical; as it drifts with temperature, the
threshold algorithm compensates for the drift automatically.
A simple circuit variation is to replace Cs with a bare piezo
sounder (Section 2), which is merely another type of
capacitor, albeit with a large thermal drift coefficient. If C
piezo
is in the proper range, no other external component is
required. If C
piezo
is too small, it can simply be `topped up'
with an inexpensive ceramic capacitor connected in parallel
with it. The QT118H drives a 4kHz signal across SNS1 and
SNS2 to make the piezo (if installed) sound a short tone for
75ms immediately after detection, to act as an audible
confirmation.
Option pins allow the selection or alteration of several
special features and sensitivity.
1.2 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. In both
cases the rule Cs >> Cx must be observed for proper
operation. The polarity of the charge buildup across Cs
during a burst is the same in either case.
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 SNS1 (or
SNS2). 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 touch sensitive and should
be treated with caution to limit the
touch area to the desired location.
Multiple touch electrodes can be
used, for example to create a
control button on both sides of an
lq
1
Figure 1-1 Standard mode options
SENSING
ELECTRODE
C
s
2nF - 500nF
3
4
6
5
1
+2.5 to 5
7
2
OUT
OPT1
OPT2
GAIN
SNS1
SNS2
Vss
Vdd
OUTPUT = DC
TIMEOUT = 10 Secs
TOGGLE = OFF
GAIN = HIGH
C
x
8
Figure 1-2 2-wire operation, self-powered
object, however it is impossible for the
sensor to distinguish between the two
touch areas.
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
QT118H 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
QT118H can tolerate, the electrode can be made into a
sparse mesh (Figure 1-4) having lower Cx than a 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 QT118H relies on Kirchoff's
Current Law (Figure 1-5) 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.
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 1-5), which is more
than two orders of magnitude greater than that required to
create a return path to the QT118H via earth. The QT118H's
PCB however can be physically quite small, so there may be
little `free space' coupling (Cx1 in Figure 1-5) between it and
the environment to complete the return path. If the QT118H
circuit 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 QT118H's own circuit ground to:
(1) A nearby piece of metal or metallized housing;
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2
Figure 1-3 Internal Switching & Timing
C
s
C
x
SNS2
SNS1
ELECTRODE
S
i
ngl
e
-
S
l
o
pe 14
-
b
i
t
S
w
i
t
c
hed
Cap
aci
to
r
A
D
C
C ha rge
A m p
Bu
rst C
ontrol
l
e
r
R esult
Do ne
S tart
Figure 1-5 Kirchoff's Current Law
S e n s e E le c tro d e
C
X2
Su rro und in g e nv iro nm e n t
C
X3
SENSO R
C
X1
(2) A floating conductive ground plane;
(3) A nail driven into a wall when used with small
electrodes;
(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
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.
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 1-6). 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 QT118H, the sensitivity is low enough that
'walk-by' should not be a concern if the product
has more than a few millimeters of internal air
gap; if the product is very thin and contact with
the product's back is a concern, then some form
of rear shielding may be required.
1.3.5 S
ENSITIVITY
The QT118H can be set for one of 3 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.
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, 4-2). 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.
Pin 7
Low
Pin 6
Medium
Floating
High
Tie Pin 5 to:
Gain
Table 1-1 Gain Setting Strap Options
1.3.5.2 Decreasing Sensitivity
In some cases the QT118H 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 1-4), or by decreasing Cs.
lq
3
Figure 1-6
Shielding Against Fringe Fields
Sen se
wire
Sense
wire
Unshielded
Electrode
S hielded
Electrode
Figure 2-1 Drift Compensation
T hre sh old
S ig na l
H ys te res is
R efer ence
Ou tpu t
2 - QT118H SPECIFICS
2.1 SIGNAL PROCESSING
The QT118H processes all signals using 16 bit precision,
using a number of algorithms pioneered by Quantum. The
algorithms are specifically designed to provide for high
survivability in the face of all kinds of adverse environmental
changes.
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 QT118H 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 QT118H'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 touching the sense pad. 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.
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 QT118H 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
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.
This is known as the Max On-Duration feature.
After the Max On-Duration interval, the sensor will once
again function normally, even if partially or fully obstructed,
to the best of its ability given electrode conditions. There are
two timeout durations available via strap option: 10 and 60
seconds.
10s
Vdd
Gnd
Pulse
10s
Gnd
Gnd
Toggle
60s
Gnd
Vdd
DC Out
10s
Vdd
Vdd
DC Out
Max On-
Duration
Tie
Pin 4 to:
Tie
Pin 3 to:
Table 2-1 Output Mode Strap Options
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 QT118H 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. The required count is 4.
The Detection Integrator can also be viewed as a 'consensus'
filter, that requires four detections in four successive bursts
to create an output. As the basic burst spacing is 75ms, if
this spacing was maintained throughout all 4 counts the
sensor would react very slowly. In the QT118H, after an
initial detection is sensed, the remaining three bursts are
spaced about 18ms apart, so that the slowest reaction time
possible is 75+18+18+18 or 129ms and the fastest possible
is 54ms, depending on where in the initial burst interval the
contact first occurred. The response time will thus average
92ms.
2.1.5 F
ORCED
S
ENSOR
R
ECALIBRATION
The QT118H has no recalibration pin; a forced recalibration
is accomplished only when the device is powered up.
However, the supply drain is so low it is a simple matter to
treat the entire IC as a controllable load; simply driving the
QT118H's Vdd pin directly from another logic gate or a
microprocessor port (Figure 2-2) will serve as both power
and 'forced recal'. The source resistance of most CMOS
gates and microprocessors is low enough to provide direct
power without any problems. Note that most 8051-based
microcontrollers have only a weak pullup drive capability
and will require true CMOS buffering. Any 74HC or 74AC
series gate can directly power the QT118H, as can most
other microcontrollers. A 0.01uF minimum bypass capacitor
close to the device is essential; without it the device can
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4
Figure 2-2 Powering From a CMOS Port Pin
0.01 F
C M O S
m ic rocon troller
OUT
P ORT X. m
P ORT X. n
V d d
V ss
QT11 8
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