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Details, datasheet, quote on part number:QT115H-IS
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Datasheet text preview:
CHARGE-TRANSFER TOUCH SENSOR
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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 2.5 to 5V 20µA single supply operation µ 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 Active Low (QT110), Active High (QT110H) versions
QProxTM QT110 / QT110H
Vdd Out Opt1 Opt2
1
8
Vss Sns2 Sns1 Gain
Q T 110
2 3 4
7 6 5
APPLICATIONS ! !
Light switches Industrial panels
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Appliance control Security systems
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Access systems Pointing devices
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Elevator buttons Toys & games
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 sm all 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 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 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 m onitor 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.
TA
00C to +700C 00C to +700C -400C to +850C -400C to +850C
AVAILABLE OPTIONS SOIC
QT110-S QT110H-S QT110-IS QT110H-IS
8-PIN DIP
QT110-D QT110H-D Copyright © 1999 Quantum Research Group Ltd R1.02/0109
Quantum Research Group Ltd
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 m ode to transmit output state to the host controller (QT110 only).
Figure 1-1 Standard mode options
+ 2 . 5 to 5
1 2
Vd d OU T S N S2
S E NS I N G E L EC T R O D E
7 Cs
1 0nF
3
OP T 1
G A IN
5
Cx
4
O U T P U T =D C T IM EO U T = 1 0 S e c s T O GG L E = OF F GA I N= HI G H
OP T 2 Vs s
S N S1
6
1.1 BASIC OPERATION
The QT110 employs short, ultra-low duty cycle bursts of charge-transfer cycles to acquire its signal. Burst mode perm its power consumption in the low microamp range, dram atically 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 dynam ically 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).
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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 Cpiezo is in the proper range, no other external component is required. If Cpiezo 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 75m s 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 sim ultaneously, 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 com ponents associated with or in contact with SNS1 and SNS2 will becom e touch sensitive and should be
Figure 1-2 2-wire operation, self-powered (QT110 only)
+
+ 3V
CMOS G ATE
2 . 2k
Tw i s t e d pa i r
1 2 3 4
V dd O UT
2 2 µ F 1 0 V AL
S E N S IN G E LE C T R O D E
S NS 2
7 5 6 Cs
10 nF
O PT 1
G A IN
Cx
O PT 2 S NS 1
V ss
8
-2-
S ing le -S lo p e 14-bit S w i tch e d Cap a c ito r AD C
treated with caution to limit the touch area to the desired location. Multiple touch electrodes can be used, for exam ple to create a control button on both sides of an object, however it is im possible for the sensor to distinguish between the two touch areas.
Figure 1-3 Internal Switching & Timing
E LE C T R O D E
R esul t
S NS 2
B u r s t Controller
1.3 ELECTRODE DESIGN
1.3.1 ELECTRODE GEOMETRY AND SIZE
S ta r t
Cs Cx
S NS 1
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
Do n e
C ha r g e Am p
Figure 1-4 Mesh Electrode Geometry
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 VIRTUAL CAPACITIVE GROUNDS
W hen 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.
show that the electrode has more capacitance than the QT110 can tolerate, the electrode can be made into a sparse m esh (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.
Figure 1-5 Kirchoff's Current Law
1.3.2 KIRCHOFF'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 m anner 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 exam ple, powering the sensor via an isolated transformer
CX2
S e n s e E le c t r o d e
SENSO R
CX 1
S u r r o u n d in g e n v ir o n m e n t
CX3
-3-
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