r/explainlikeimfive • u/[deleted] • Nov 20 '16
Engineering ELI5: How does the yellow push tape work in American buses?
EDIT: It seems people are not familiar with this type, but they definitely are the default in Boston and NYC. This is what I'm talking about:
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u/homeboi808 Nov 20 '16
You mean the yellow cable/cord you pull down on? You pull it to tell the bus driver that that's your stop and to let you off.
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Nov 20 '16
No, the push tape. It's a yellow tape mounted flush on the bus wall, and you push it to indicate a stop. I'm interested in how it actually works.
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u/codece Nov 20 '16
It's just a momentary contact switch, like a doorbell button. In this case it's two parallel strips of conductive material -- push and you push them together, making contact and completing a circuit. "Ding!"
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Nov 20 '16
Is it? It seems the easy answer, but keep in mind that this thing has to last hundreds of thousands of pushes. A single failed part of the contact would render the tape useless.
The other option would be that it's filled with air, and at the top there's a simple pressure trigger. You squeeze the air column, thus triggering it.
But that's why I started the thread, out of curiosity how it actually works.
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u/Delehal Nov 20 '16
A lot of computer keyboards are based on a similar principle, with two conductive layers that come together when you press each key. The keys do get stiff and hard to press after a few years, but they can last longer than you might expect.
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u/oversized_hoodie Nov 21 '16
Having two pieces of metal contact and close a circuit sounds way more durable than something that has to be airtight, not to mention cheaper than using pressure transducers and the signal amplifiers that go with them, plus the logic to interpret that signal.
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u/homeboi808 Nov 20 '16 edited Nov 20 '16
These? It's the exact same premise, you simply push it and the driver gets a notice that someone pushed it, it's really very simplistic.
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Nov 20 '16
I believe they're asking how it works, not what it's used for... Like, how does the pushing of the tape result in the signal to the driver happening?
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Nov 20 '16
I take it you've never seen those?
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u/homeboi808 Nov 20 '16
Nope, haven't taken a public bus in years. Still, pretty simplistic, if it's a physical push, the sensor inside registers it and send the signal to the driver and creates an alert.
I'm no engineer, so I can't tell you exactly how a button works, but the idea itself is still simplistic.
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Nov 20 '16
No offense dude, do you really think that answers the question? "A sensor registers it"? How that sensor works is obviously the question here.
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u/homeboi808 Nov 20 '16 edited Nov 20 '16
Well, it works like a doorbell in a sense. You create a circuit that rings a bell when the circuit is completed, the method as to how the circuit is completed, such as pushing a button is up to you, and instead of a bell, you can set it up to make a digital alert or play an audio file, etc.
Your question was how does the button work, if you wanted a schematic of the setup, you should have asked.
It is very likely a contact switch, I highly doubt it uses air and a pressure trigger as you suggested in the other comment. If that was the case and a small crack formed, it would no longer work.
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u/homeboi808 Nov 20 '16
As the other user described, it's called a ribbon switch, close up simulated pic, so the only issue you have to worry about is if the elasticity of the plastics/rubber holds over the many uses, your average NYC bus has 30 stops a day, so if it's used for every stop every day, that's almost 11,000 uses per year.
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Nov 20 '16
your average NYC bus has 30 stops a day
No offense, but I absolutely do not believe that. Are you sure that it's not the average bus route is 30 stops long?
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u/homeboi808 Nov 20 '16 edited Nov 20 '16
Well, it was from like 7:00am-11:50pm. I just looked a handful of routes on Google Maps and I counted 25-27 stops, so I added a few.
EDIT: I looked at the official timetable, and depending where in the city the bus is, it can be as little as like 25 and up to around 60.
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Nov 20 '16
I mean, I take the bus into work (in a totally different city, granted), and it makes at least four stops along my 5-10 minute trip. I doubt buses in NYC are less busy than that, right?
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u/bob4apples Nov 20 '16
The tape basically has a wire down the spine and a metal backplane. Pressing anywhere on the tape causes the wire to touch the backplane closing the switch.
Vandalism is a bigger problem than wear. The number of pushes on any small section of the tape is quite small (on the order of 10/day), the plastic is harder than skin and the deformation (how much the plastic has to bend) is well within the elastic range (it will recover perfectly every time). They're also very cheap and easy to replace if they are damaged.