General competition discussion

Paradox21
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Re: General competition discussion

Post by Paradox21 » March 28th, 2010, 10:11 am

maggymay wrote:kinda OT but right now :cry: ?...having serious trouble getting anything to work. we are so in over our heads it's not even funny. a perfect score :lol: ?...for me I think I'll be happy if we get any single task to work, much less first/final/timed-step/whatever.
If that is the case, I would start simple and just try to work on 1 task at a time. You may even need to completely redesign some steps if you can't think of a way to make them work. Remember, transfers do not need to be fantastic displays of engineering prowess. Usually small motions are best because they are consistent and there is less room for things to go wrong.
When it comes to the future, there are three kinds of people: those who let it happen, those who make it happen, and those who wonder what happened.

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Re: General competition discussion

Post by maggymay » March 28th, 2010, 11:29 am

Yea, that's me paradox..."simple" :mrgreen: .

Have a question. Was doing some research and came across an EBay auction that is selling parts/plans for the photocell thingy. In the auction (just do an EBay search for "Mission Possible Science Olympiad 2010") they say:

Are you struggling with Task 4b?

"Activate a photocell which will provide the power to operate a motor, which leads to the next action."

If so, don't feel bad, you are not alone! When the event organizers 'thought up' that task, they had no idea how difficult it would be for High School students to accomplish it. On November 9, 2009 they posted the following rule clarification:

Section 4.b. should read (see additions in bold): "Activate a photocell that allows current to flow to a motor, which leads to the next action".

That was all well and good, except that they also posted the following rule clarification the same day:

Section 3.d. should read (see additions in bold): "Electric components shall be limited to batteries, wires, motors, switches, resistors, capacitors, commercial photocells (i.e., cadmium sulfide (CdS) cell, photoresistors or light dependent resistors), mechanical relays, lightbulbs, and LEDs. No computers or transistors will be permitted in the device."

This caused even more confusion, which prompted the following questions from students, coaches and parents:

01/16/2010 - 22:25 The revision to Section 3.d. "commercial photocells (i.e.cadmium sulfide (CdS) cell, photoresistors or light dependent resistors" seems to only include photoconductive type photocells. Does this mean that photovoltaic type photocells (i.e. solar c...
Correct, photovoltaic cells are not allowed.

01/16/2010 - 22:28 What exactly is meant by a photocell? Is it the variable resistor (dependent on how much light hits it) or more like a photovoltaic?
A photocell is a variable resistor, dependent on how much light hits it. It is not a photovoltaic cell.

01/16/2010 - 22:47 Are photoresistors and/or light-dependent resistors (as allowed under 3.d.) considered a variety of "photocell"? Would a circuit where light shining on a photoresistor allowed/triggered current flow to a motor meet the task requirement?
Yes.

Do a quick Google search for "light activated switch", and you will find nearly 23,000 commericial products and DIY diagrams. The problem is that all of them use one or more transistors.

Why? Because CdS cells (as the organizers later specified) do not allow enough current to flow through them, to use them as switches. A transistor on the other hand, amplifies electrical signals, which is why EVERY LIGHT SENSITIVE DEVICE USES THEM. This is simply how it is done in industry (the real world). Unfortunately, the Science Olympiad organizers are in academia, not the real world.

So how do you complete this task without a transistor? Not to worry my friends. ...
Is this Seller correct?!? That all light sensitive devices use transistors?!? Is there actually no way to achieve 4b without transistors?!

Color me VERY confused...

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Re: General competition discussion

Post by cypressfalls Robert » March 28th, 2010, 11:35 am

wow wouldn't that be cheating , to buy from that seller

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Re: General competition discussion

Post by Dark Sabre » March 28th, 2010, 11:55 am

maggymay wrote:Yea, that's me paradox..."simple" :mrgreen: .

Have a question. Was doing some research and came across an EBay auction that is selling parts/plans for the photocell thingy. In the auction (just do an EBay search for "Mission Possible Science Olympiad 2010") they say:

Is this Seller correct?!? That all light sensitive devices use transistors?!? Is there actually no way to achieve 4b without transistors?!

Color me VERY confused...
Look at the circuit on the wiki: Photoresistor Circuit Wiki

That works without any transistor. If you are only driving a small motor, like the one shown, you don't even need a relay (you can have the photoresistors trigger a relay, according to the National FAQs).

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Re: General competition discussion

Post by maggymay » March 28th, 2010, 2:57 pm

Dark Sabre wrote:
maggymay wrote:Yea, that's me paradox..."simple" :mrgreen: .

Have a question. Was doing some research and came across an EBay auction that is selling parts/plans for the photocell thingy. In the auction (just do an EBay search for "Mission Possible Science Olympiad 2010") they say:

Is this Seller correct?!? That all light sensitive devices use transistors?!? Is there actually no way to achieve 4b without transistors?!

Color me VERY confused...
Look at the circuit on the wiki: Photoresistor Circuit Wiki

That works without any transistor. If you are only driving a small motor, like the one shown, you don't even need a relay (you can have the photoresistors trigger a relay, according to the National FAQs).
I'll rely on your expertise Dark Sabre, but what do you think the EBay seller actually means, that *all* light sensitive devices because of their nature have transistors inside them? I think I understand what you posted, but am soooooooo confused...

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Re: General competition discussion

Post by Sir_L_Jenkins » March 28th, 2010, 3:07 pm

Looks like he may be oversimplifying a problem so he could possibly sell something that fixes it =P
*Ahem*
Mentlegen.

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Re: General competition discussion

Post by Dark Sabre » March 28th, 2010, 9:00 pm

maggymay wrote:I'll rely on your expertise Dark Sabre, but what do you think the EBay seller actually means, that *all* light sensitive devices because of their nature have transistors inside them? I think I understand what you posted, but am soooooooo confused...
Well, what he means is that like if you took apart a night light or some other photoresistor-based device, the photoresistor would probably be wired into either a transistor or set up to provide a logic signal to an IC. The reason is just that the photoresistors can't carry that much current, so you either need to drive a very small load or use the photoresistor to drive a signal. In modern devices, they would use transistors because they are cheaper, solid-state alternatives to relays. The transistor performs the same function as a relay (a small current/voltage triggers a "switch" to be closed that carries a much larger current/voltage).

The actual photoresistors do not have transistors inside them, the devices they are in just usually use transistors in conjunction with them.

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