QPROX QT110-D, QT110-IS, QT110-S, QT110H-D, QT110H-IS Datasheet

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APPLICATIONS -

Elevator buttons

Toys & games

Access systems

Pointing devices

Appliance control

Security systems

Light switches

Industrial panels

The QT110 / QT110H 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 long distances.

Power consumption is under 20µA 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 C

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 HeartBeat™ signal is also included, allowing a host controller to monitor the health of the QT110 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

R1.01/0106

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Less expensive than many mechanical switches

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Projects a ‘touch button’ through any dielectric

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Turns small objects into intrinsic touch sensors

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100% autocal for life - no adjustments required

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Only one external part required - a 1¢ capacitor

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Piezo sounder direct drive for ‘tactile’ click feedback

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LED drive for visual feedback

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2.5 to 5V 20µµµµA single supply operation

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Toggle mode for on/off control (strap option)

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10s or 60s auto-recalibration timeout (strap option)

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Pulse output mode (strap option)

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Gain settings in 3 discrete levels

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Simple 2-wire operation possible

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HeartBeat™ health indicator on output

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Active Low (QT110), Active High (QT110H) versions

-QT110H-IS-400C to +850C

-QT110-IS-400C to +850C

QT110H-DQT110H-S00C to +700C

QT110-DQT110-S0

C to +700C

8-PIN DIPSOICT

AVAILABLE OPTIONS

QProx™™™™ QT110 / QT110H

HARGE-TRANSFER TOUCH SENSOR

Sns2

Vss

Sns1

GainOpt2

Opt1

Out

Vdd 1

QT110

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 single low cost, non-critical capacitor is required for operation.

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).

1.1 BASIC OPERATION

The QT110 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 QT110 (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 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 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.

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Figure 1-1 Standard mode options

SENSING ELECTRODE

10nF

+2.5 to 5

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

OUTPUT=D C TIMEOUT=10 Secs TOGGLE=OFF GAIN=HIGH

Figure 1-2 2-wire operation, self-powered (QT110 only)

22µF10V AL

10nF

CMOS GATE

+3V

2.2k Twisted pair

S E NSING ELECTRODE

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

1.3 ELECTRODE DESIGN

1.3.1 E

LECTRODE GEOMETRY AND SIZE

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 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 QT110 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 CURRENT LAW

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 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 CAPACITIVE GROUNDS

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 QT110 via earth. The QT110's PCB however can be physically quite small, so there may be little ‘free space’ coupling (Cx1 in Figure 1-5) 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:

(1) A nearby piece of metal or metallized housing; (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).

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Figure 1-3 Internal Switching & Timing

SNS2

SNS1

ELECTRODE

Single -S lope 14-bit

Switched Capacitor ADC

Charge

Amp

Burst Controller

Result

Done

Start

Figure 1-4 Mesh Electrode Geometry

Figure 1-5 Kirchoff's Current Law

Sense Electrode

Surrounding environment

SENSOR

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 SHAPING

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 QT110, 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 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.

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 ~ 4-3). 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 dramatically; 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 QT110 may be too sensitive, even on low gain. In this case gain can be lowered further by any of a number of strategies: making the electrode smaller, connecting a very small capacitor in series with the sense lead, or making the electrode into a sparse mesh using a high space-to-conductor ratio (Figure 1-4). A deliberately added Cx capacitor can also be used to reduce sensitivity according to the gain curves (see Section 4).

Intermediate levels of gain (e.g. between 'medium' and 'low' can be obtained by a combination of jumper settings with one or more of the above strategies.

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Figure 1-6

Shielding Against Fringe Fields

Sense

wire

Sense

wire

Unshielded

Electrode

Shielded

Electrode

Figure 2-1 Drift Compensation

Threshold

Signal

Hysteresis

Reference

Output

Pin 7

Low

Pin 6

Medium

None

High

Tie Pin 5 to:Gain

Table 1-1 Gain Setting Strap Options

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