Touch-1 Thing Overview:

Transcription

1 Touch-1 Thing Overview: Single capacitive touch button interface with relay output, for use where mechanical switches are either unsuitable or not desired. The Touch-1 is capable of detecting touches through a variety of materials, such as glass, stone, plastic, ceramic and even most kinds of wood. It can also turn metal-bearing objects into intrinsic sensors. The output is active as long as the touch is detected. Feedback LED support either directly from touch sensor or from external controller. Installation: The rear of the sensing electrode is supplied with double sided acrylic adhesive tape, remove the backing paper and press firmly on the rear of the surface you want to sense through. Ensure that the surface is dust and oil free prior to attaching the Touch-1. Electrical connections are made via 8 screw terminal on the printed circuit board. Two terminals are provided for power and ground connections to allow looping to additional units in an installation. The terminal marked 'L' provides external control of the optional LED indicator. Connecting 'L' to Gnd will turn the LED on. Power for the LED is provided by the Touch-1 12v supply. A jumper switch configures the control of the optional LED, it can be controlled from the 'L' terminal if set to 'EXT' or from the capacitive touch if set to 'INT'. The relay output from the Touch-1 provides isolated connection to a control system with both normally open and normally closed terminals available. Connections: PSU Gnd PSU Gnd +12v +12v External LED trigger input Output Relay Normally Open Output Relay Common Output Relay Normally Closed

2 Power Requirements: The Touch-1 Thing requires a 12 volt power supply (not supplied) and draws around 50mA. If the optional LED is included current draw rises to around 120mA depending LED option chosen. LEDs: Status LED: An on board status LED is provided (D3) on the board to provide an indication of when a touch has been detected. Optional LED ring: The optional feedback LED ring allows for backlit indication around the sensing element. Current there are 3 feedback LED ring option available to special order: 53mm LED ring fitted to a 40mm sensor electrode. 43mm LED ring fitted to a 40mm sensor electrode. 25mm LED ring fitted to a 20mm sensor electrode. The LED CTRL jumper switch allows selecting of the driving source for the LED ring, 'INT' position drivers the LED from the touch sensor while the 'EXT' position allows the led to be driven from an external device. Driving terminal 'L' to gnd to turn the LED ring on. 53mm LED ring with 40mm sensor 43mm LED ring with 40mm sensor 25mm LED ring with 20mm sensor

3 ELECTRODE 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 Touch-1 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 Touch-1 can tolerate, the electrode can be made into a sparse mesh having lower capacitance to ground than a solid plane. Sensitivity may even remain the same, as the sensor will be operating in a lower region of the gain curves. It is a common mistake to assume that the electrode shape and the graphic key symbol on the panel should be the same size. In fact, it is often better to make the electrodes larger than the graphic especially with small key sizes since key sensitivity falls off at the edges; an oversize electrode not only compensates for this but also allows for off centre touch with good response. Generally, it is a good idea to make the electrode shape extend 2-3mm beyond the graphic symbol. As a rule, the electrode shape should have a minimum dimension of at least 4 times the panel thickness for reliable operation. It should at least match the diameter of a small finger about 6~7mm. Bigger is better: bigger electrodes get more signal swing from a touch and decreases the effect that noise will have on the signal. Other than that there are no hard and fast rules for size.

4 SENSITIVITY: Sensitivity a function of things: like the value of C5, electrode size and capacitance, electrode 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. 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 (capacitive coupling to ground) will reduce sensor gain. The value of C5 (on the Touch-1) also has a dramatic effect on sensitivity, and this can be increased in value with the trade-off of reduced response time. 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. Panel material can also be changed to one having a higher dielectric constant, which will help propagate the field. In the case of proximity detection, usually the object being detected is on an approaching hand, so a larger surface area can be effective. Metal areas near the electrode will reduce the field strength and increase Cx (capacitive coupling to ground) loading. Ground planes around and under the electrode and its SNS trace will cause high Cx (capacitive coupling to ground) 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 away from the electrodes and traces. Decreasing Sensitivity In some cases the Touch-1 may be too sensitive. In this case gain can be lowered further by decreasing C5. Proximity Sensing By increasing the sensitivity, the Touch-1 can be used as a very effective proximity sensor, allowing the presence of a nearby object (typically a hand) to be detected. In this scenario, as the object being sensed is typically a hand, very large electrodes sizes can be used, which is extremely effective in increasing the sensitivity of the detector. Note that although this affects the responsiveness of the sensor, it is less of an issue in proximity sensing applications; in such applications it is necessary to detect simply the presence of a large object, rather than a small, precise touch. OTHER PROBLEM AREAS: Electrode Wire Length Longer electrode wires will have higher Cx (capacitive coupling to ground) capacitive loads than shorter ones, resulting in reduced sensitivity. Multiple devices with varying electrode wire lengths will have unbalanced sensitivities from electrode to electrode. This can be avoided by ensuring that electrode wires are the same length. Electrode Size Similar problems can arise if electrodes are of varying sizes; try to ensure that electrodes are of the same size, if that is not possible then it may be possible to balance sensitivity by varying wire length or the value of C5 on the Touch-1 PCB but this will require some experimentation and is best avoided if at all possible. Floating Metal Floating electrical conductors near sense traces and electrodes will pickup the sense fields and reradiate them. Usually this is highly undesirable as it can cause strange behaviour in key detection depending on what the metal is contacting. Touching such nearby floating metal can also cause false key detection. Floating metal should be connected to AC or DC circuit ground. This can be accomplished by a direct wire connection to the power supply common, or by means of a 47nF capacitor back to supply common. A second ground connection is provided on the Touch-1 for the purpose of providing such a direct connection.

5 THE SCIENCE BIT KIRCHOFF S CURRENT LAW Like all capacitance sensors, the Touch-1 relies on Kirchhoff s Current Law (Figure 1) 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 Kirchhoff 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 Pico farads back to local earth. Figure 1 Kirchhoff s Current Law 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 Pico farads of free space capacitance to the local environment (Cx3 in Figure 1), which is more than two orders of magnitude greater than that required to create a return path to the Touch-1 via earth. The Touch-1's PCB however can be physically quite small, so there may be little free space coupling (Cx1 in Figure 1) between it and the environment to complete the return path. If the Touch-1 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 Touch-1 s own circuit ground to: - A nearby piece of metal or metalized housing; - A floating conductive ground plane; - Another electronic device (to which its output might be connected capacitive coupling to ground 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. FIELD SHAPING The electrode can be prevented from sensing in undesired directions with the assistance of metal shielding connected to circuit ground (Figure 2). 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. 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 (capacitive coupling to ground)lower to reduce loading and keep gain high. Figure 2 - Shielding Against Fringe Fields iztech ltd. unit 75 joseph wilson ind est, millstrood road, whitstable, kent. ct5 3ps t: f: sales@iztech.co.uk