lQ
C
HARGE
-T
RANSFER
T
OUCH
S
ENSOR
QProx™ QT118H
Less expensive than many mechanical switches
Projects a ‘touch button’ through any dielectric
100% autocal for life - no adjustments required
No active external components
Piezo sounder direct drive for ‘tactile’ click feedback
LED drive for visual feedback
2.5 ~ 5V single supply operation
10µA at 2.5V - very low power drain
µ
Toggle mode for on/off control (via option pins)
10s or 60s auto-recalibration timeout (via option pins)
Pulse output mode (via option pins)
Gain settings in 3 discrete levels
Simple 2-wire operation possible
HeartBeat™ health indicator on output
Pb-Free package
Vdd
Out
Opt1
Opt2
1
8
Vss
Sns2
Sns1
Gain
QT118H
2
3
4
7
6
5
APPLICATIONS -
Light switches
Industrial panels
Appliance control
Security systems
Access systems
Pointing devices
Elevator buttons
Toys & games
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 wood. It can also turn small metal-bearing
objects into intrinsic sensors, making them respond to proximity or touch. This capability coupled with an ability to self calibrate
continuously can lead to entirely new product concepts.
The device 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.
A piezo element can also be connected to create a feedback click sound.
The IC requires only a common inexpensive capacitor in order to function. Average power consumption is under 20µA in most
applications, allowing battery operation.
The QT118H employs digital signal processing techniques pioneered by Quantum, designed to make it survive real-world
challenges, such as ‘stuck sensor’ conditions and signal drift. Sensitivity is digitally determined for the highest possible stability. No
external active components are required for operation.
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 twisted-pair interface. Quantum’s unique HeartBeat™ signal is also included, allowing a host
controller to continuously monitor the health of the device.
By using the charge transfer principle, the IC delivers a level of performance clearly superior to older technologies in a highly
cost-effective package.
0 C to +70 C
-40
0
C to +85
0
C
0
T
A
0
AVAILABLE OPTIONS
SOIC
-
QT118H-ISG
8-PIN DIP
QT118H-DG
-
lq
©1999-2004 Quantum Research Group
R1.08 / 0405
1 - OVERVIEW
Figure 1-1 Standard mode options
+2.5 ~ +5
1
2
3
4
OUTPUT = DC
TIMEOUT = 10 Secs
TOGGLE = OFF
GAIN = HIGH
Vdd
OUT
SNS2
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 few low cost, non-critical discrete external
parts are 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.
7
5
Cs
6
2nF - 500nF
R
E
SENSING
ELECTRODE
OPT1
GAIN
Rs
C
x
OPT2
Vss
SNS1
1.1 BASIC OPERATION
The QT118H employs short, low duty cycle bursts of QT
cycles to acquire capacitance. 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
single-slope switched capacitor ADC includes both the
required QT charge and transfer switches in a configuration
that provides direct ADC conversion. The sensitivity depends
on the values of Cs, Cx, and to a smaller degree, Vdd. Vdd is
used as the charge reference voltage.
Higher values of Cs increase gain; higher values of Cx load
reduce it. The value of Cs can thus be increased to allow
larger values of Cx to be tolerated (Figures 4-1 and 4-2, page
10).
Piezo sounder drive:
The QT118H can drive a piezo
sounder after a detection for feedback. The piezo sounder
replaces or augments the Cs capacitor; this works since
piezo sounders are also capacitors, albeit with a large
thermal drift coefficient. If C
piezo
is in the proper range, no
additional capacitor. If C
piezo
is too small, it can simply be
‘topped up’ with a ceramic capacitor in parallel. 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.
8
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 in theory be
connected to either SNS1 or SNS2 with no performance
difference. However, the noise immunity of the device is
improved by connecting the electrode to SNS2, preferably via
a series resistor Re (Figure 1-1) to roll off higher harmonic
frequencies, both outbound and inbound.
In order to reduce power consumption and to assist in
discharging Cs between acquisition bursts, a 470K series
resistor Rs should always be connected across Cs (Figure
1-1).
The rule Cs >> Cx must be observed for proper operation.
Normally Cx is on the order of 10pF or so, while Cs might be
10nF (10,000pF), or a ratio of about 1:1000.
It is important to minimize the amount of unnecessary stray
capacitance Cx, for example by minimizing trace lengths and
widths and backing off adjacent ground traces and planes so
as keep gain high for a given value of Cs, 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.
Figure 1-2 2-wire operation, self-powered
+
3.5 - 5.5V
CMOS
LOGIC
1K
Twisted
pair
1N4148
1
2
OUT
10µF
Vdd
SNS2
7
5
6
C
s
R
s
R
E
SENSING
ELECTRODE
n-ch Mosfet
3
4
OPT1
GAIN
C
x
OPT2
SNS1
Vss
8
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QT118H R1.08 / 0405
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.
R esult
Figure 1-3 Internal Switching & Timing
E LE C TRO DE
S NS 2
Single-Slo pe 14-bit
Switched Capacitor ADC
Burst Controller
C
s
C
x
S NS 1
S tart
Do ne
C ha rge
Amp
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.
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.
‘Ground’ as applied to capacitive fields can also mean power
wiring or signal lines. The capacitive sensor, being an AC
device, needs only an AC ground return.
1.3.5 S
ENSITIVITY
A
DJUSTMENT
1.3.5.1 Gain Pin
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
values of Cs and Cx, 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.
The Gain input should never be connected to a pullup or
pulldown resistor or tied to anything other than SNS1 or
SNS2, or left unconnected (for high gain setting).
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 1-4), 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 1-4) 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:
- A nearby piece of metal or metallized housing;
- A floating conductive ground plane;
- Another electronic device (to which its might be
connected already).
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 1-4 Kirchoff's Current Law
C
X2
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
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QT118H R1.08 / 0405
Table 1-1 Gain Strap Options
Gain
High
Medium
Low
Tie Pin 5 to:
Leave open
Pin 6
Pin 7
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.
1.3.5.2 Changing Cs, Cx
The values of Cs and Cx have a dramatic effect on
sensitivity, and Cs can be easily increased in value to
improve gain. Sensitivity is directly proportional to Cs and
inversely proportional to Cx:
S
=
k$C
S
C
X
Where ‘k’ depends on a variety of factors including the gain
pin setting (see prior section), Vdd, etc.
Sensitivity plots are shown in Figures 4-1 and 4-2, page 10.
1.3.5.3 Electrode / Panel Adjustments
Sensitivity can often be increased by using a bigger
electrode, or reducing overlying panel thickness. Increasing
electrode size can have a diminishing effect on gain, as the
attendant higher values of Cx will start to reduce sensor gain.
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.
1.3.5.3 Ground Planes
Grounds around and under the electrode and its SNS trace
will cause high Cx loading and destroy gain. The possible
signal-to-noise ratio benefits of ground area are more than
negated by the decreased gain from the circuit, and so
ground areas around electrodes are discouraged. Keep
ground, power, and other signals traces away from the
electrodes and SNS wiring
2.1.2 T
HRESHOLD AND
H
YSTERESIS
The internal signal threshold level can be set to one of three
settings (Table 1-1). These are fixed with respect to the
internal reference level, which in turn moves 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.
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
2 - QT118H SPECIFICS
2.1 SIGNAL PROCESSING
The QT118H digitally processes all signals 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.
Figure 2-1 Drift Compensation
S ig na l
H ys te res is
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
T hre sh old
R efer ence
Ou tpu t
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QT118H R1.08 / 0405
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 95ms, if this
spacing was maintained through 4 consecutive bursts the
sensor would be very slow to respond. In the QT118H, after
an initial detection is sensed, the remaining three bursts are
spaced only about 2ms apart, so that the slowest reaction
time possible is the fastest possible.
Max On-Duration expires, whichever occurs first. If the latter
occurs first, the sensor performs a full recalibration and the
output becomes inactive until the next detection.
In this mode, two nominal Max On-Duration timeouts are
available: 10 and 60 seconds.
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 state unchanged.
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. Almost any CMOS logic gate can
directly power the QT118H.
A 0.01uF minimum bypass capacitor close to the device is
essential; without it the device can break into high frequency
oscillation.
Option strap configurations are read by the QT118H only on
powerup. Configurations can only be changed by powering
the QT118H down and back up again; a microcontroller can
directly alter most of the configurations and cycle power to
put them in effect.
Table 2-1 Output Mode Strap Options
Tie
Pin 3 to:
DC Out
DC Out
Toggle
Pulse
Vdd
Vdd
Gnd
Gnd
Tie
Pin 4 to:
Vdd
Gnd
Gnd
Vdd
Max On-
Duration
10s
60s
10s
10s
2.2.3 P
ULSE
M
ODE
O
UTPUT
This generates a positive pulse of 95ms duration with every
new detection. It is most useful for 2-wire operation (see
Figure 1-2), but can also be used when bussing together
several devices onto a common output line with the help of
steering diodes or logic gates, in order to control a common
load from several places.
Max On-Duration is fixed at 10 seconds if in Pulse output
mode.
The piezo beeper drive does not operate in Pulse mode.
2.2 OUTPUT FEATURES
The QT118H 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.
OPT1 and OPT2 should never be left floating. If they are
floated, the device will draw excess power and the options
will not be properly read on powerup. Intentionally, there are
no pullup resistors on these lines, since pullup resistors add
to power drain if the pin(s) are tied low.
2.2.4 H
EART
B
EAT
™ O
UTPUT
The output has a full-time HeartBeat™ ‘health’ indicator
superimposed on it. This operates by taking 'Out' into a
tri-state mode for 350µs once before 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.
Since Out is normally low, a pullup resistor will create positive
HeartBeat pulses (Figure 2-3) when the sensor is not
detecting an object; when detecting an object, the output will
remain active 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 controller can reconfigure the load resistor to either
ground or Vcc depending on the output state of the device,
so that the pulses are evident in either state.
Electromechanical devices will usually ignore this short
pulse. The pulse also has too low a duty cycle to visibly
activate 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).
2.2.1 DC M
ODE
O
UTPUT
The output of the device can respond in a ‘DC mode’, where
the output is active-high upon detection. The output will
remain active for the duration of the detection, or until the
Figure 2-2 Powering From a CMOS Port Pin
P ORT X. m
0.01µ F
CMOS
m ic ro c o n tr o lle r
Vdd
P ORT X. n
OUT
Q T11 8
V ss
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QT118H R1.08 / 0405
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