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Lab 2aUseless box
ENGR 40M
Chuan-Zheng Lee
Stanford University
21 April 2017
Switches
• A switch is a component that makes or breaks a circuit, depending on external input
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 3
open switch
(connection broken)
closed switch
(connection made)
• There are lots of different types of switches
Poles and throws (1)
A pole chooses between one or more throws, and is mechanically joined to but electrically distinct from other poles
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 4
throw
pole
Poles and throws (2)
We categorize switches by how many poles and throws they have, e.g. “single pole, double throw”
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 5
Look at your toggle switch. How many poles and throws do you think it has?
Prelab question P1
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 6
Use the resistance mode of your multimeter to figure out which pin of your toggle switch corresponds to which number in the symbol
1
Momentary switches
• A momentary switch reverts back to its “normal” position when it is not actively being pressed
• We label its contacts accordingly: common, normally closed and normally open
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 7
Prelab question P2
• Your limit switch is a micro switch, typically used for detecting things like whether a door is closed
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 8
Use the resistance mode of your multimeter to label the pins of your limit switch
• C (common)
• NC (normally closed)
• NO (normally open)
Motors
• A motor uses electromagnetism to convert electrical energy into rotational movement
• You have a DC brush motor (most common type)
• The input polarity determines the direction
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 9
motor spins in
one direction
motor spins in
the other direction
Prelab questions P3–P5
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 10
• Measure your battery pack’s voltage.
• Connect the batteries to the motor. Measure the motor’s voltage and current.
• What happens if you grab the output shaft with your fingers?
• What happens if you reverse the voltage polarity?
What’s in your useless box?
• Battery
• Motor
• DPDT toggle switch
• SPDT momentary limit switch
• pressed when arm is fully retracted
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 11
Designing the useless box circuit
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 12
We want a circuit that does the following:
Toggle switch Limit switch Motor
“on”
(closer to arm)
[either] “forward”
(coming out)
“off”
(away from arm)
normal
(arm not retracted)
“reverse”
(retracting)
“off”
(away from arm)
pressed
(arm retracted)
stopped
Prelab question P6
Start by drawing these components:
(Avoid short-circuiting the battery!)
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 13
Feedback
• Feedback is when you measure the output of a system in order to inform the input.
• It comes in lots of forms, and is one of the most pervasive ideas in engineering design.
• How does the useless box use feedback?
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 14
Practical matters: Parts
• Each person has enough parts for one box
• You are only required to make one per pair, but you can make two if you like
• We’ll be adding an Arduino next week, so one strategy is to make a basic box this week, and then a new box for the Arduino next week
• If you use the same box both weeks, you’ll need to rewire some things (which isn’t hard)
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 15
Practical matters: Wiring
• If you want to rewire the same box next week, you can solder just one end of wires to switch contacts, and plug the other end into a breadboard.
• Breadboard connections in yellow:
Apr 21, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 16
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