Designer's guide to rapid prototyping of capacitive sensors on any surface
* ~4 N$ m& E( f$ k* OBy Mark Lee, Cypress Semiconductor Corp.
; D" S O& O5 r- f+ y( X$ nPlanet Analog
* P, b: I4 |! H& V 23, 2007 (11:18 H), t" {: u! d2 s/ s
URL: http://www.planetanalog.com/showArticle?articleID=196902891 Capacitive sensing provides consumers a cool new way to interact with their
MP3 players and mobile phones. Touch sensors are hidden below the surface, so the area normally covered with the uneven features of mechanical buttons now look clean and uncluttered. An effortless finger
touch accesses product features that are usually controlled by the click of mechanical buttons. To encourage experimentation with this new technology, this article presents a designer's guide to rapid prototyping of capacitive sensors on any nonconductive surface.
, r% h; y! X* L, pAlternatives to PCBs and Flex Circuits
" R* }" a! n6 A7 f& o7 zImagine that you have a great idea for a product that includes a plastic injection-molded case or a glass panel, and you would like to explore the possibility of replacing the mechanical buttons with capacitance sensors buried under the surface. The first step in making a capacitive sensing system is to arrange a set of conductors on a nonconductive surface,
Reference 1.
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The traditional approach to implementing the sensors is to create a printed
circuit or a flex circuit, but this can be time consuming, and may not fit within your budget for a prototyping effort. How can you avoid the time and expense associated with traditional prototyping methods, and open the design process to after-the-fact nudging and tweaking?
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Silver ink pens and copper tape provide a prototyping solution for capacitive sensing that is both inexpensive and quick to implement. A cross-sectional view of the concept is shown in
Figure 1.
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Figure 1: Cross-sectional view of prototyping technique for capacitive sensors. D ~1 F5 L4 L5 B1 |
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7 }: b5 _' h6 r _. FConductive patterns are applied by hand, directly to the nonconductive overlay material. Jumper wires connecting the microcontroller to sensor pads replace PCB traces of the more traditional approach to prototyping.
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Sensors applied to acrylic sheet
1 ~2 G6 S4 T ^6 W. d0 y+ b% @# eAs a demonstration of the technique, two capacitive sensors were applied to a 3mm-thick sample of acrylic sheet. One sensor is copper tape, and the other is silver ink as shown in
Figure 2 and
Figure 3. It only took a few minutes to apply these conductive patterns by hand to the backside of the acrylic sheet.
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4 g- p& \- } EFigure 2: Two capacitive sensors applied directly to a 3mm-thick acrylic sheet. The sensor on the left is made of silver ink, and on the right is copper tape.! j, o! e/ Q/ j* p5 w1 ~% I
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2 ^( g/ m1 G# E" ^* e5 C1 s$ \/ c3 iFigure 3: Close-up view of the copper tape construction. The outer ring is ground, and the inner square is the sensor pad. The wire is a jumper back to the PSoC.9 R7 b! y) Y) N: ^
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Total project time was about one hour. Most of the time was spent soldering connections to the controller IC that was glued "dead-bug" style to the sheet. A connector was glued to the edge of the sheet to provide access to the Cypress PSoC, both for programming and for a data-logging
serial connection to a host PC.
# z/ k: i4 A4 T! Y* [! O+ f8 [$ MSilver Ink Pen2 r A1 ?+ f0 a4 T9 Y/ T3 k
The silver "ink" pen is most frequently used to do electrical repairs, such as fixing broken heating elements in a window defogger. High conductivity silver particles are suspended in acrylic epoxy system that cures into a flexible electrical connection. It forms a good bond with many surfaces, including glass and acrylic. There are a few versions of this kind of pen from different manufacturers. For this demonstration, a
CircuitWorks Micro Tip pen from
Chemtronics was used.
1 |& ^& b% e" ^With the silver ink pen, the conductive sensor pattern is defined by drawing directly onto the surface. It takes an hour or two to cure to the point were it can be used as a sensor. For this demonstration, the ink cured overnight at room temperature. The product description says the cured ink can be soldered to with a low temperature soldering iron.
( \! Q. @: x7 |Based on earlier experience with this product, it was decided to attach the jumper wire to the sensor pad using an extra dab of ink. The jumper wire became glued to the sensor pad after this ink cured. Although the conductive pattern can be completely done by drawing freehand, the pattern shown in Figure 2 received crisply defined edges by masking-off the pattern with tape before the ink was applied. The mask was removed after curing.
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Copper Tape1 ^( H8 p3 h% t5 |. t1 P( N/ O
The copper tape is made of thin copper foil, and is most frequently used for shielding. The conductive tape used for this demonstration, product number
1181 from
3M, has a special conductive adhesive that makes it easier to electrically connect overlapping tape sections. Unlike the silver ink, the copper tape can be used for sensing immediately after the tape is applied. Soldering the jumper wire to the sensor works well with this conductive tape. Copper was selected for the demonstration, but the tape is available in other metal types, like aluminum, that work equally well.
" @! d( ]) I5 ?* f2 e! e2 g2 tCircuit details$ [, h" O7 F( d* p0 Z$ n6 Z
Applying the conductive traces by hand leads to less than ideal sensor performance, compared to PCBs and flex circuits. To compensate for this, a sensing method with high signal-to-noise ratio (SNR) is required. The CSD configuration of PSoC from Cypress Semiconductor was selected for this demonstration,
Reference 2. The designation "SD" in the name "CSD" stands for Sigma-Delta, for a new sensing method that can be implemented in both the CY8C21x34 and CY8C24x94 versions of PSoC. The CSD functional blocks are shown in
Figure 4.
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Figure 4: CSD configuration of PSoC CapSense for CY8C21x34 devices. Y8 K; E( b- a9 A: H
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& p* p; C* R# J$ B+ I! s) A5 YAn equivalent circuit of CSD is shown in
Figure 5. The switches on the front end of the circuit have turned the sensor capacitance into an equivalent resistance that is modulated by the presence of a finger. The change in the resistance results in variation in the
bit stream at the
output of the sigma-delta modulator.
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# M4 c8 l# N* i" \# H9 V! dFigure 5: CSD block diagram with equivalent resistance of the switched sensing capacitor.! a, w& V# f7 n, f
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The CSD User Module is a standard functional block included in the PSoC Designer library. It contains
API routines that perform all the low-level and high-level functions required to make a practical capacitive-sensing system. The PSoC is selected by dragging-and-dropping the CSD User Module from the library. The system level operation of the sensors is defined in C.
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An additional functional block is added to the project to provide a communication link to a host computer. The TX8 user module implements a transmit-only block that sends serial data at 115.2 kbaud. This turns the system into a programmable data logger for capacitive sensing.
8 s8 B; F) F \9 U, m* K1 `The schematic for this project is shown in
Figure 6. The circuit contains only the PSoC IC for capacitive sensing and serial communication, plus another IC for RS232 level shifting. The pin assignments for the two CapSense buttons are shown in the table in the figure.
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Figure 6: Schematic for the capacitive sensing circuit configure with the CSD User Module.
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