|
|
Part: QT118H-D
Category:
Sensors
-> Touch Screen
Description: Single Channel Charge-transfer Touch & Proximity Sensor
Company: Quantum Research Group Ltd.
Datasheet: Download QT118H-D datasheet File size : 98 kB
Request For quote: Find where to buy QT118H-D
Datasheet text preview:
lQ
CHARGE-TRANSFER TOUCH SENSOR
QProxTM QT118H
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 3V 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
Vdd O ut O p t1 O p t2
1
8
Vss S n s2 S n s1 G ain
Q T1 18H
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 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 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 monitor the health of the QT118H 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.
00C to +700C -400C to +850C
TA
AVAILABLE OPTIONS SOIC
QT118H-S QT118H-IS
8-PIN DIP
QT118H-D -
lq
©1999-2000 Quantum Research Group R1.03 / 0302
1 - OVERVIEW
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 single low cost, non-critical capacitor is 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.
Figure 1-1 Standard mode options
+2.5 to 5
SENSING ELECT RO DE
1 2 3 4
OUT PUT = DC T IMEOUT = 10 Secs T OGG LE = OFF GAI N = HIGH Vdd OUT SNS2
7 5 6 Cs
2nF - 500nF
OPT 1
G AI N
Cx
OPT 2 Vss
SNS1
1.1 BASIC OPERATION
The QT118H 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 QT118H (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.
8
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 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. 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.
Figure 1-2 2-wire operation,
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 self-powered 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
lq
1
object, however it is impossible for the sensor to distinguish between the two touch areas.
Figure 1-3 Internal Switching & Timing
R esu l t S in g le -S lo p e 14-bit S w i t ch e d Capacitor ADC
E LE C T R O D E
S NS 2
1.3 ELECTRODE DESIGN
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.
B u r s t Controller
1.3.1 ELECTRODE GEOMETRY AND SIZE
Cs Cx
S NS 1
S ta rt
Do n e
C ha r g e Amp
If a relatively large electrode surface is desired, and if tests show that the electrode has more capacitance than the QT118H can tolerate, the electrode can be made into a
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
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 QT118H via earth. The QT118H's PCB however can be physically quite small, so there may be little `free space' coupling (Cx1 in Figure 1-5) 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: 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) A nearby piece of metal or metallized housing;
1.3.2 KIRCHOFF'S CURRENT LAW
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'.
Figure 1-5 Kirchoff's Current Law
CX2
S e n s e E le c t r o d e
SENSO R
CX 1 CX3
Su rro u n d in g e n v iro n m e n t
lq
2
Others parts begin by qt
QT-1 QT-2
|
|
|
