APPLICATIONS -

Elevator buttons

Toys & games

Access systems

Pointing devices

Appliance control

Security systems

Light switches

Prox sensors

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 600

A 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

and electrode C

. No external switches, opamps, or other

analog components aside from C

are usually required.

The option-selectable toggle mode permits on/off touch control, for example for light switch replacement. The Quantum-pioneered
HeartBeatTM 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.

Quantum Research Group Ltd

R1.10/0104

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, 600

ľ
ľ
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A single supply operation

Toggle mode for on/off control (strap option)

10s, 60s, infinite auto-recal timeout (strap options)

Gain settings in 2 discrete levels

HeartBeatTM health indicator on output

Active-low (QT113) or active-high outputs (QT113H)

Only one external part required - a 1˘ capacitor

QProxTM

QT113 / QT113H

HARGE

-T

RANSFER

OUCH

ENSOR

Sn s2

Vss

Sn s1

Gain

O pt2

O pt1

O ut

V d d

113

QT113H-IS

-40

C to +85

QT113-IS

-40

C to +85

QT113H-D

QT113H-S

C to +70

QT113-D

QT113-S

C to +70

8-PIN DIP

SOIC

AVAILABLE OPTIONS

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.

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.

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 object, however it is impossible for the sensor to

distinguish between the two touch areas.

1.3 ELECTRODE DESIGN

1.3.1 E

LECTRODE

EOMETRY

AND

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

- 2 -

Figure 1-1 Standard mode options

SENSING

ELECTRODE

10nF

+2.5 to 5

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

OUTPUT=DC

TIMEOUT=10 Secs

TOGGLE=OFF
GAIN=HIGH

Figure 1-2 Internal Switching & Timing

SNS2

SNS1

ELE C TRO DE

i
n

l
e

-
S

l
o

pe 14-

i
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c
hed

Cap

c
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A

Charge

Am p

r
s
t
C

ont

r
o

l
l
er

Result

Do ne

Start

can be made into a sparse mesh (Figure 1-3) 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

URRENT

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.

1.3.3 V

IRTUAL

APACITIVE

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 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

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

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

HAPING

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.

1.3.5 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.

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

- 3 -

Figure 1-3 Mesh Electrode Geometry

Figure 1-4 Kirchoff's Current Law

S e n se E le ctro de

Su rro und ing e nv iro nm ent

S ENSO R

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.

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.

2.1.1 D

RIFT

OMPENSATION

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 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

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

-D

URATION

If an object or material obstructs the sense pad the signal
may rise enough to create a detection, preventing further

- 4 -

Figure 1-5

Shielding Against Fringe Fields

Sense

wire

Sense

wire

U nshielded

Electrode

S hielded

E lec trode

Figure 2-1 Drift Compensation

T hr eshold

S ignal

H ysteresis

R eference

Output

Vss (Gnd)

Low - 12 counts

Vdd

High - 6 counts

Tie Pin 5 to:

Gain

Table 1-1 Gain Setting Strap Options

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.

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

finite timeout durations available via strap option: 10 and 60
seconds (Table 2-1).

2.1.4 D

ETECTION

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 detections in three successive bursts
to create an output.

2.1.5 F

ORCED

ENSOR

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.

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.

2.1.6 R

ESPONSE

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.

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.

2.2.1 DC M

ODE

UTPUT

The output of the QT113 can respond in a DC mode, where
the output is active-low upon detection. The output will

remain active-low for the duration of the detection, or until the
Max On-Duration expires (if not infinite), whichever occurs

first. If a max on-duration timeout occurs first, the sensor
performs a full recalibration and the output becomes inactive

until the next detection.

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
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

ODE

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.

2.2.3 H

EART

EAT

TM O

UTPUT

The QT113 output has a full-time HeartBeatTM `health'
indicator superimposed on it. This operates by taking 'Out'

into a 3-state mode for 300ľs 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.

- 5 -

infinite

Vdd

Gnd

DC Out

10s

Gnd

Toggle

60s

Gnd

Vdd

DC Out

10s

Vdd

DC Out

Max On-

Duration

Tie

Pin 4 to:

Tie

Pin 3 to:

Table 2-1 Output Mode Strap Options

Figure 2-2 Powering From a CMOS Port Pin

0 . 0 1 ľF

C MO S

m icro controller

O U T

P O RT X .m

P O RT X .n

V d d

V ss

Q T110

Part Number QT113