lQ
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
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
Active low (QT110), active high (QT110H) versions
QT110 / QT110H
QT
OUCH
™ S
ENSOR
IC
s
Vdd
Out
Opt1
Opt2
1
8
Vss
Sns2
Sns1
Gain
QT110
2
3
4
7
6
5
APPLICATIONS -
Light switches
Industrial panels
Appliance control
Security systems
Access systems
Pointing devices
Elevator buttons
Consumer electronics
The QT110 / QT110H charge-transfer (“QT’”) sensor chips are self-contained digital ICs used to implement near-proximity or touch
sensors. They project sense fields through almost any dielectric, like glass, plastic, stone, ceramic, and wood. They 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 leads to entirely new product concepts.
These devices are designed specifically for human interfaces, like control panels, appliances, toys, lighting controls, or anywhere a
mechanical switch or button may be found; they 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.
These ICs require only a common inexpensive capacitor in order to function. Average power consumption is under 20µA in most
applications, allowing battery operation.
The devices employ digital signal processing techniques pioneered by Quantum, designed to make them 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 devices include 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 sensor health.
By using the charge transfer principle, these ICs deliver a level of performance clearly superior to older technologies in a highly
cost-effective package.
T
A
0 C to +70 C
0
0
C to +70
0
C
-40
0
C to +85
0
C
-40
0
C to +85
0
C
0
0
AVAILABLE OPTIONS
SOIC
-
-
QT110-IS
QT110H-IS
8-PIN DIP
QT110-D
QT110H-D
-
-
lq
©1999-2004 Quantum Research Group
QT110/110H R1.03/0604
1 - OVERVIEW
The QT110 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 Standard mode options
+2.5 ~ +5
1
2
3
4
OUTPUT = DC
TIMEOUT = 10 Secs
TOGGLE = OFF
GAIN = HIGH
Vdd
OUT
SNS2
Figure 1-1 shows the basic QT110 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 output state to the host controller (QT110 only).
7
5
Cs
6
2nF - 500nF
R
E
SENSING
ELECTRODE
OPT1
GAIN
Rs
C
x
OPT2
Vss
SNS1
1.1 BASIC OPERATION
The QT110 employs low duty cycle bursts of charge-transfer
cycles to acquire its signal. Burst mode permits power
consumption in the low microamp range, dramatically reduces
EMC problems, 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 QT110 (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. 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.
The IC is highly tolerant of changes in Cs since it computes the
signal threshold level ratiometrically. Cs is thus non-critical and
can be an X7R type. As Cs changes with temperature, the
internal drift compensation mechanism also adjusts for the drift
automatically.
Piezo sounder drive:
The QT110 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 is
required. If C
piezo
is too small, it can simply be ‘topped up’ with a
ceramic capacitor in parallel. The QT110 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 other
special features and sensitivity.
8
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 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.
1.3 ELECTRODE DESIGN
1.3.1 E
LECTRODE
G
EOMETRY AND
S
IZE
Figure 1-2 2-wire operation, self-powered
+
3.5 - 5.5V
CMOS
LOGIC
1K
Twisted
pair
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 QT110 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
(not QT110H)
sensor circuit and the sensed object.
1.3.2 K
IRCHOFF
’
S
C
URRENT
L
AW
10µF
1N4148
1
2
OUT
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
Like all capacitance sensors, the QT110
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
LQ
2
QT110/110H R1.03/0604
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.
Figure 1-3 Internal Switching & Timing
Result
Single -Slo pe 14-bit
Switched Capacitor ADC
E LEC TRO DE
S NS2
Bu rst Controller
C
s
C
x
S NS1
Start
Do ne
C ha rg e
Amp
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 QT110 via earth. The QT110'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 QT110 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
QT110’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
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.
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 to one
having a higher dielectric constant. Increasing electrode size
can have diminishing returns, as high values of Cx will reduce
sensor gain.
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. Metal areas
near the electrode will reduce the field strength and increase
Cx loading and are to be avoided for maximal gain.
Ground planes 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.
The value of Cs has a minimal effect on sensitivity with these
devices, but if the Cs value is too low there can be a sharp
drop-off in sensitivity.
1.3.4 S
ENSITIVITY
The QT110 can be set for one of 3 gain levels using option pin
5 (Table 1-1). If left open, the gain setting is high. The
sensitivity change is made by altering the numerical threshold
level required for a detection. It is also a function of other
things: 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 are all influences.
Gain plots of the device are shown on page 9.
The Gain input should never be tied to anything other than
SNS1 or SNS2, or left unconnected (for high gain setting).
Se nse E le ctro de
Figure 1-5 Kirchoff's Current Law
C
X2
SENSO R
Table 1-1 Gain Strap Options
Gain
High
Medium
Low
Tie Pin 5 to:
Leave open
Pin 6
Pin 7
C
X 1
C
X 3
Su rro und ing e nv iro nm en t
LQ
3
QT110/110H R1.03/0604
2 - QT110 SPECIFICS
2.1 SIGNAL PROCESSING
The QT110 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.
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 QT110 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
QT110, 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 QT110, after an initial detection
is sensed, the remaining three bursts are spaced about 20ms
apart, so that the slowest reaction time possible is
75+20+20+20 or 135ms and the fastest possible is 60ms,
depending on where in the initial burst interval the contact first
occurred. The response time will thus average about 95ms.
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. Cs
drift has almost no effect on gain since the threshold method
used is ratiometric. However Cs drift can still cause false
detections if the drift occurs rapidly.
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 QT110 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 QT110'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.
2.1.5 F
ORCED
S
ENSOR
R
ECALIBRATION
The QT110 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 QT110'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 QT110.
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 QT110 only on
powerup. Configurations can only be changed by powering the
QT110 down and back up again; again, a microcontroller can
directly alter most of the configurations and cycle power to put
them in effect.
2.1.2 T
HRESHOLD
C
ALCULATION
Sensitivity is dependent on the threshold level as well as ADC
gain; threshold in turn is based on the internal signal reference
level plus a small differential value. The threshold value is
established as a percentage of the absolute signal level. Thus,
sensitivity remains constant even if Cs is altered dramatically,
so long as electrode coupling to the user remains constant.
Furthermore, as Cx and Cs drift, the threshold level is
automatically recomputed in real time so that it is never in error.
The QT110 employs a hysteresis dropout below the threshold
level of 50% of the delta between the reference and threshold
levels.
The threshold setting is determined by option jumper; see
Section 1.3.4.
2.2 OUTPUT FEATURES
The devices are 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.
2.2.1 DC M
ODE
O
UTPUT
The output of the device can respond in a DC mode, where the
output is active-low (QT110) or active-high (QT110H) upon
detection. The output will remain active for the duration of the
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 nominal
timeout durations available via strap option: 10 and
60 seconds. The accuracy of these timeouts is
approximate.
Figure 2-1 Drift Compensation
Signal
H ysteresis
Threshold
R eference
Output
LQ
4
QT110/110H R1.03/0604
Figure 2-2 Powering From a CMOS Port Pin
P O RT X .m
0.01µF
Figure 2-3 Damping Piezo Clicks with R
s
+2.5 ~ +5
1
2
Vdd
OUT
SNS1
C MO S
m icro controller
V dd
P O RT X .n
O UT
7
5
6
Piezo Sounder
10-30nF
R
E
SENSING
ELECTRODE
Q T110
V ss
3
4
OPT1
GAIN
Rs
C
x
OPT2
Vss
SNS2
8
detection, or until the 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 Max On-Duration timeouts are available: 10
and 60 seconds.
should be added to bring the total Cs across SNS1 and SNS2
to at least 10nF, or possibly more if Cx is above 5pF
Piezo sounders have very high, uncharacterized thermal
coefficients and should not be used if fast temperature swings
are anticipated, especially at high gains. They are also
generally unstable at high gains; even if the total value of Cs is
largely from an added capacitor the piezo can cause periodic
false detections.
The burst acquisition process induces a small but audible
voltage step across the piezo resonator, which occurs when
SNS1 and SNS2 rapidly discharge residual voltage stored on
the resonator. The resulting slight clicking sound can be greatly
reduced by placing a 470K resistor Rs in parallel with the
resonator; this acts to slowly discharge the resonator,
attenuating of the harmonic-rich audible step (Figure 2-3).
Note that the piezo drive does not operate in Pulse mode.
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.
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
2.2.5 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 3-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.
QT110:
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 2-4) 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.
QT110H:
Same as QT110 but inverted logic (use a pull-up
resistor instead of a pull-down).
If the sensor is wired to a microcontroller as shown in Figure
2-5, 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 2-6).
2.2.3 P
ULSE
M
ODE
O
UTPUT
This mode generates a pulse of 75ms duration (QT110 -
negative-going; QT110H - positive-going) with every new
detection. It is most useful for 2-wire operation, 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.
Note that the beeper drive does not operate in Pulse mode.
2.2.4 P
IEZO
A
COUSTIC
D
RIVE
A piezo drive signal is generated for use with a piezo sounder
immediately after a detection is made; the tone lasts for a
nominal 95ms to create a ‘tactile feedback’ sound.
The sensor drives the piezo using an H-bridge configuration for
the highest possible sound level. The piezo is connected
across pins SNS1 and SNS2 in place of Cs or in addition to a
parallel Cs capacitor. The piezo sounder should be selected to
have a peak acoustic output in the 3.5kHz to 4.5kHz region.
Since piezo sounders are merely high-K ceramic capacitors,
the sounder will double as the Cs capacitor, and the piezo's
metal disc can even act as the sensing electrode. Piezo
transducer capacitances typically range from 6nF to 30nF in
value; at the lower end of this range an additional capacitor
2.2.6 O
UTPUT
D
RIVE
The QT110’s output is active low (QT110) or active high
(QT110H) and can source 1mA or sink 5mA of non-inductive
current.
LQ
5
QT110/110H R1.03/0604
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