Photocell Bonus

[Dark Sabre]

Here's what I showed the people at Built It Day for the photocell bonus:



The actual components I used:
+ batteries appropriate to the motor. I used 1.5 or 3V for this example.
The light source you use is arbitrary, but the brighter it is, the closer you can get the photoresistors to being 0 Ohms. With that light, I got them down to 8 Ohms, which is pretty good.
The motor is the kind that you would find in your vibrating pager/cellphone if you took it apart. It is tiny, runs off 1.5-3V, and uses only 10-20mA to run. Bought it at RadioShack.
The photoresistors were from RadioShack too. They sell a 5-pack and I wired the two larger cells together in parallel so I could pass more current through them.
It isn't pictured, but the circuit (minus the lightbulb) was run off of a 1.5V AA battery.

When the light shined on the photoresistors, the motor spun. When the photocells were in ambient light or dark conditions, they motor did nothing.

[gh]

DS, I don't see the point of the fixed value resistor in your circuit. The motor and photocells would just be a separate path for the current from the battery, no? Do you need the battery voltage depressed through current draw or something?

[Flavorflav]

To me, this is not a "photocell which will provide the power to operate a motor." If I understand your setup correctly, your photocell is really a switch which allows the battery to provide the power.

[Dark Sabre]

See the clarification that was posted yesterday on NSO:

Mission Possible: 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." (11/9/09)

[Flavorflav]

See the clarification that was posted yesterday on NSO:

Mission Possible: 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." (11/9/09)

Oh. Well, then, nevermind. That is quite different, and much easier I would think.

[Dark Sabre]

Had a chance to get the photoresistor circuit back out this weekend.

Photoresistor Circuit Wiki

Long story short is that, as suspected, the little resistor was not necessary since I wasn't actually making a voltage divider. It works either way though.

[JimY]

Now that was a huge change in the rules, rather than just a clarification. Our team just got a photocell circuit to work yesterday. It consists of 4 photo cells and 4 6V lights that are about 4 Watts each. Not expensive, other than it chews up batteries. It runs a small motor with a fan blade attached quite nicely. We've got two teams this season that are each competing at two competitions (invitational and regional), so we'll likely go the CdS cell route in the other device and see which one we like better for our state competition. I can only guess that the folks that wrote the rules tried running a motor on one photocell and one light source and it didn't work. Then kept adding until something finally worked and said to themselves that this is crazy. That's basically what we did at least. However, now that we got something to work, I wish we were back to the printed rules. Oh well.

[Primate]

So if I'm not mistaken, this clarification allows you to wire up a photoresistor to a battery in a dark box, then flip a switch on the flashlight and have it count?

[Paradox21]

So if I'm not mistaken, this clarification allows you to wire up a photoresistor to a battery in a dark box, then flip a switch on the flashlight and have it count?

Yes, but you don't really need to put the battery in the dark box. I got a motor turning with 2 photoresistors, 6 AA batteries and a medium sized flashlight hooked up to 2 D batteries.

[cypressfalls Robert]

could the light source of the photoresistor circuit mentioned above consist of a laser by any chance?