{"id":51,"date":"2012-07-04T14:04:01","date_gmt":"2012-07-04T19:04:01","guid":{"rendered":"http:\/\/www.hackingsma.com\/?page_id=51"},"modified":"2013-08-28T17:29:52","modified_gmt":"2013-08-28T22:29:52","slug":"touch-switch","status":"publish","type":"post","link":"http:\/\/www.hackingsma.com\/?p=51","title":{"rendered":"Touch Switch"},"content":{"rendered":"

I wanted to find a switch that Alejandro could operate. We tried all of the most sensitive mechanical switches available to us but he wasn’t able to operate any of them. Alejandro can move his fingers but he isn’t able to apply any force to speak of. I knew that he would be able to operate a capacitive proximity sensor, since it only requires that he touch the sensor – no pressure is needed. I did some searching and did find a few assistive products, but they were all rather expensive. Furthermore they were all designed as remote switches, \u00a0ie. they would operate a device (eg. electronic toy) that was connected remotely to the switch via wires. At Alejandro’s young age we felt that it would be better for him cognitively if the device that he operates is the same as the one that he touches, in order to better develop the cause\/effect wiring in his little brain. Hence the birth of the Touch Switch.<\/p>\n

The Touch Switch is an extremely sensitive capacitive proximity sensor switch. It is a small two-sided circuit board, designed to be as small and cheap as possible so that it can easily be embedded in just about anything for modification (eg. electronic toy).<\/p>\n

\"\"<\/a>\u00a0\"\"<\/a><\/em><\/p>\n

The Touch Switch works by detecting a change in capacitance between a sensing plate<\/em> and ground. The sensing plate is just a conductor with a large surface area (eg. piece of metal foil). When the sensing plate is approached by a finger the capacitance rises and the switch is closed. When the finger is removed the capacitance drops and the switch is opened.<\/p>\n

\"\"<\/a><\/p>\n

At the heart of the Touch Switch is the NXP PCF8883<\/a>\u00a0integrated circuit. The circuit board measures 0.385″ x 0.555″ and operates from a 3-9 volt DC power source which makes it perfect for modifying most toys.\u00a0Connected to the circuit board are five wires:<\/p>\n\n\n\n\n\n\n\n\n
Wire<\/strong><\/td>\nTerminal<\/strong><\/td>\nDescription<\/strong><\/td>\n<\/tr>\n
RED<\/td>\nV+<\/td>\n3-9 volts DC<\/td>\n<\/tr>\n
BLACK<\/td>\nGND<\/td>\nGND<\/td>\n<\/tr>\n
VIOLET<\/td>\nSW<\/td>\nnormally open switch contact<\/td>\n<\/tr>\n
VIOLET<\/td>\nSW<\/td>\nnormally open switch contact<\/td>\n<\/tr>\n
ORANGE<\/td>\nTOUCH<\/td>\ntouch sensor input<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The orange wire is connected to a sensing plate.\u00a0In most circumstances a 1″ diameter aluminum foil disc covered by clear cellophane tape works well as a sensor.\u00a0Sensitivity is increased by increasing the area of the sensing plate. Placing a thin insulator (piece of tape) over the sensing plate also improves functionality by causing a more dramatic increase in capacitance when the sensor is actually touched. \u00a0It is important to keep the length of the orange wire as short as possible. The range of capacitance that the touch sensor can measure is limited. If the orange wire is too long then the baseline capacitance will be too high and the Touch Switch will not function properly. A sensing plate that is too large will have the same effect. The length of the orange wire should be less than a few inches.<\/p>\n

When the Touch Switch is triggered the two violet wires are connected together by the circuit using a solid state relay. \u00a0The violet wires should be connected across the terminals of the device’s switch\/pushbutton whose function is being made accessible. The maximum current handling of the relay is 200mA, which is plenty for most electronic toys that play music, make sounds and flash lights. It may be not suitable for toys that use large batteries (eg. C or D cells) and have motors. If the relay chip is switching too much current it will get hot and eventually burn out.<\/p>\n

The red and black wires are connected so that the Touch Switch receives power when the device it is being used to modify is powered on. This should be between 3 and 9 volts DC. The Touch Switch circuit constantly auto-calibrates its baseline capacitance measurement. When power is first applied it may take a few seconds for it to establish proper calibration.<\/p>\n

NOTE: I have found that toys that use a pair of 1.5V batteries (eg. AA) may not work reliably as the batteries begin to drain. For reliable operation the Touch Switch power supply must remain above 3V. Toys that use 3-6 1.5V batteries work best.<\/p>\n

Circuit Documentation<\/h2>\n

Here is the schematic:<\/p>\n

\"\"<\/a><\/p>\n

Here is the Bill of Materials with part numbers from\u00a0Mouser Electronics<\/a>.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
Designator<\/strong><\/td>\nPart Type<\/strong><\/td>\nManufacturer Part#<\/strong><\/td>\nMouser Part#<\/strong><\/td>\nMouser USD<\/strong><\/td>\n<\/tr>\n
C1<\/td>\n0.47u<\/td>\nCGJ4J2X7R0J474K<\/td>\n810-CGJ4J2X7R0J474K<\/td>\n$0.15<\/td>\n<\/tr>\n
C2<\/td>\n1u<\/td>\nCGJ4J2X7R1C105K<\/td>\n810-CGJ4J2X7R1C105K<\/td>\n$0.16<\/td>\n<\/tr>\n
R2<\/td>\n6.81K<\/td>\nCRCW08056K81FKEA<\/td>\n71-CRCW0805-6.81K-E3<\/td>\n$0.08<\/td>\n<\/tr>\n
C4<\/td>\n22p<\/td>\nGRM21A5C2E220JW01D<\/td>\n81-GRM21A5C2E220JW1D<\/td>\n$0.42<\/td>\n<\/tr>\n
C3<\/td>\n22p<\/td>\nGRM21A5C2E220JW01D<\/td>\n81-GRM21A5C2E220JW1D<\/td>\n$0.42<\/td>\n<\/tr>\n
K1<\/td>\n200mA 60V<\/td>\nASSR-1218-003E<\/td>\n630-ASSR-1218-003E<\/td>\n$1.78<\/td>\n<\/tr>\n
R1<\/td>\n348<\/td>\nCRCW0805348RFKEA<\/td>\n71-CRCW0805-348-E3<\/td>\n$0.08<\/td>\n<\/tr>\n
U1<\/td>\nPCF8883<\/td>\nPCF8883T\/1,118<\/td>\n771-PCF8883T1,118<\/td>\n$2.48<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Here are the Gerber files\u00a0and NC drill for the circuit board:<\/p>\n

Touch Switch R2 CAM files<\/a><\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

I wanted to find a switch that Alejandro could operate. We tried all of the most sensitive mechanical switches available to us but he wasn’t able to operate any of them. Alejandro can move his fingers but he isn’t able to apply any force to speak of. I knew that he would be […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[],"tags":[],"class_list":["post-51","post","type-post","status-publish","format-standard","hentry","odd"],"_links":{"self":[{"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=\/wp\/v2\/posts\/51","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=51"}],"version-history":[{"count":10,"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=\/wp\/v2\/posts\/51\/revisions"}],"predecessor-version":[{"id":406,"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=\/wp\/v2\/posts\/51\/revisions\/406"}],"wp:attachment":[{"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=51"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=51"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.hackingsma.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=51"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}