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ASSEMBLING
the
ECEbot
Bumper Switch Assembly
The bumper switch assembly steps must be completed prior to lab session #7:
Due Date:
Prepared by R.C. Maher
October 2005
Copyright © Department of Electrical and Computer Engineering, Montana State University, 2005
All Rights Reserved
20051017RCM
Bumper Assembly
2
Small PCB Preliminaries
One way your robot can move autonomously from place to place is to have bumper switches on
the front: if the robot hits something, the switch gets closed, and the robot's microcontroller can
command the motors to back up and turn. Your robot will have two bumper switches mounted
on the forward "L" brackets.
The bumper switch circuit boards you will assemble contain just a few components. The simple
circuit diagram is shown below.
Vcc
=5V
10 k
S2
GND
=0V
The switch is normally open. In this configuration the signal S2 is 'high' because the node is
connected to +5 V (Vcc) through the 10 k resistor: since no current flows in the resistor Ohm's
Law says that the voltage across the resistor must be zero, so S2 must be at the same voltage as
Vcc. If the switch gets bumped or pushed, the switch closes and S2 is pulled 'low' because that
node is connected to 0 volts (ground) by the switch, and current flows through the 10 k resistor
from Vcc to ground.
The S2 signals from the left and right bumper switch sensors are connected to one of the digital
input pins on the microcontroller. The program running on the microcontroller can test the state
of the signal to see if it is high or low, and then take some action in response.
The bumper switch circuits consist of:
2
2
2
2
2
2
Small printed circuit boards (snap-apart squares)
10 k ¼ watt resistors (Brown Black Orange Gold)
Large pushbutton switches
Four-wire harness assemblies
¼" 6-32 screws
Hub flanges (from the servo motor spare parts)
Bumper Assembly
3
Step 1: PCB Orientation
The two square PCBs come attached together. Gently bend the connections between the boards
until they snap apart.
Look closely at the two square printed circuit boards. The copper traces on the board may have
an oily oxide on them: clean the copper by lightly rubbing with a pencil eraser and/or wiping
with a paper towel dipped in rubbing alcohol.
 Set the boards on the bench so that the copper side is facing up, the printed copper numbers
are right side up, and the remnants of the board connection joints are toward the outside. This is
shown in Figure 1 below.
CIC
Datak
21-117
12-607B
21-117
CIC
12-607B
Datak
Edge holes line up
2nd holes line up
Figure 1: PCB orientation—COPPER side facing up
 Label each board so that you can keep track of which is which.
The switches will be mounted on the copper side, while the resistors and connection wires will
be inserted from the front (silk screen) side and then soldered into place on the back (copper)
side of the board. The electrical circuitry on each board will be identical, but the mounting will
be slightly different to accommodate the mechanical mounting hole locations.
Bumper Assembly
4
Step 2: Switch Mounting
 WITH THE COPPER SIDE OF EACH PCB FACING UP, place each switch into position
(wide dimension oriented left-to-right), as shown in Figure 2. Solder each leg to the adjacent
copper on the board.
Copper side face up
CIC
Datak
21-117
12-607B
21-117
CIC
12-607B
Datak
Figure 2: Mounting position for the switches—mount on the copper side of the board!
Bumper Assembly
5
Step 3: Resistor Placement
With the switches in place, now install the 10 k resistors, one on each board. The resistors
should be inserted from the silk screen side of the board at the positions shown in Figure 3.
 Make sure the bottom lead of each resistor is in the same row as the upper leg of the switch!
Boards flipped over with
silk screen side facing up
Switches mounted behind
(on copper side)
Figure 3: Resistor locations—insert from silk screen side!
Slide the legs through the holes on the silk screen side of the board, solder them to the back,
and gently clip off the excess wire.
Bumper Assembly
6
Step 4: Wiring Harness
Each switch board will connect to the ECEbot main PCB using a four-wire connection.
 Locate the two 4-wire connectors and verify that the wire colors are
Yellow:Orange:Green:Blue, in that order:
Yellow
Orange
Green
Blue
 Remove about 2 millimeters of insulation from the free end of each wire using a wire stripper,
then insert the stripped end of each wire through the proper hole in the silk screen side of the
board and solder it in place to the copper pad on the back side.
 Make sure the wires are in the proper holes relative to the other components!
BLUE
GREEN
ORANGE
YELLOW
YELLOW
ORANGE
GREEN
BLUE
Silk screen side facing up
Figure 4: Wiring connections to each board—insert from silk screen side!
NOTE that the Blue wire is not actually connected to the bumper switch circuit. Solder it in
place anyway: it allows for additional expansion if you work on your own projects later.
Bumper Assembly
7
Step 5: "L" Bracket Attachment
The bumper switch boards connect to the "L" brackets on the front of the robots.
 Hold each board so that the switch (copper side) is facing forward, the switch is at the bottom,
and the wires are running off to each side. Attach the boards to each bracket as shown in
Figure 5.
Copper side facing forward
CIC
Datak
21-117
12-607B
21-117
Base Plate
12-607B
CIC
Datak
Figure 5: Attaching the switch boards to the front of the robot— copper side forward
Bumper Assembly
8
Step 6: Bumper Completion
The two bumper switches act like "feelers" for the robot: the robot can move forward until it
runs into something.
In order to make these bumper "targets" larger and more reliable, attach the 4-prong hub
flanges from the motor kit onto the pushbutton switch shafts. You can gently push the flange to
seat it in place. The flange should stay in place by friction, or you can use a small screw to
secure it.
Pushbutton on
front PCB
You will also want to attach a stick, wire, or some other lightweight structure to each flange so
that your robot doesn't have a "blind spot" between the switches, nor on either side. Talk to your
instructor or lab TA if you need some ideas.
The wire harness from each bumper switch will be connected to the robot's main PCB during
Lab #7.
This completes the bumper switch PCB assembly. Good job!