Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

Touch Sensor

• This lesson will explain how to hook a standard micro switch into the Vex system to function as a custom digital sensor

- Vex offers a limited selection of sensors that are optimized to work with the system

- Don’t let this limit your creativity- Off-the-shelf sensors and electrical

components can be adapted to work with the Vex system

- Adapting off-the-shelf components is different in every case; you should take all the necessary precautions and be sure to understand the electrical behavior and requirements of each device before attempting to use it

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

Materials Needed

• Basic components– Limit switch– 3-wire cable (12") OR two

single wires– Wire stripper

• Soldering– Soldering pen– Solder– Vice

• Crimping– Male Molex pins (3)– Molex connector (1)– Crimping tool

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

The Limit Switch as an Electrical Switch

• The Limit Switch either connects or disconnects two terminals electrically– The switch has 3 terminals,

but only uses two here– COM (sometimes marked

GND or ) establishes the electrical ground voltage

– The NO ("Normally Open") terminal is electrically disconnected from the COM terminal when the switch is open, and connected when the switch is closed

COMNO

Switch

COMNO

Switch

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

Limit Switch as a Digital Sensor

• The voltage at the NO terminal can be used as a digital signal to determine whether the switch is closed or not– NO terminal voltage will

default to HIGH (+5V) if the switch is not closed

– Closing the switch changes the NO terminal voltage to LOW (0V = ground)

– HIGH is a digital 1,LOW is a digital 0

LOWHIGH

Switch

LOWLOW

Switch

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

Solder Limit Switch Terminals

• Set LOW voltage:Black wire toCOM terminal

• Set output voltage: White wire toNO terminal

• NC terminal andRed wire are unused

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

Crimp 3-Wire Connector

• This end of the cable will be plugged into the Micro Controller

• Create a standard 3-pin Molex connector– Wire order must be

Black-(Red)-White– Red cable may be

omitted because it is not used

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

Plug In to Micro Controller

• Plug the crimped connector into the Micro Controller – Analog/Digital port

bank in an Digital Sensor Port

– Port 5 is set this way by default

– Black wire must go on the same side as black wire on regular Vex connectors

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

Load Online Code to Test

• Connect the micro controller to the PC and turn it on

• In easyC:– Build & Download menu– On-Line Window– Download On-Line Code

• Push the switch arm and watch “Digital Input 5” change