Photocell Bonus

[Flavorflav]

I was able to get the motor in this solar racer kit to spin with just the light from a common 2-cell mini-maglite.

Proving that solar cells could have worked. I don't know why they banned them - there is so little choice in this year's rules already.

[Uncle Fester]

I had two devices have working photocells at this past weekend's invitational. One was from my team, done the night before with one working test beforehand.

ELECTRONICS GOLDMINE sells a pack of tiny 3v cellphone vibrating motors for (I think) $2.50. We also got a dozen of the 75 cent photocells on sale. While we were at it, we got an assortment of gearhead motors; real powerful and nice and slow.

Photocells had a bright resistance of 24 ohms. This was low enough to be a problem! So, each photocell was wired in series with a 24 ohm resistor to bring the bright resistance UP to 50 ohms. This effectively increased the number of photocells needed to four, but it eliminated overheated photocells, which causes aggravating intermittent problems.

Hooked up to 9v (D cells-- we don't use those rectangular niners), there was plenty of voltage and power available to run the 3v cell pager motor, which hopped around on a precarious spot, fell about 50cm, reached the end of its wire, and tripped a really touchy snap switch.

[Flavorflav]

Photocells had a bright resistance of 24 ohms. This was low enough to be a problem! So, each photocell was wired in series with a 24 ohm resistor to bring the bright resistance UP to 50 ohms. This effectively increased the number of photocells needed to four, but it eliminated overheated photocells, which causes aggravating intermittent problems.

Aha, that might explain some of the things we have been experiencing - the CDS cells seem inconsistent, which has been driving us nuts. I'll try the resistor.

BTW, what kind of light source are you using?

[Balsa Man]

One combo that seems to be working nicely-

9.6v battery from a portable drill. 12v lamp Home depot- landscaping light, oh, 1/14" diameter; seriously bright w/ 9.6v battery; photoresistor about 1/4" in front of the lamp in a little box. Photoresistor to a Radio Shack 5v relay, relay to a 12v computer cooling fan. Fan reaction time to when the light is tripped is almost instantaneous, and plenty wind from the fan to push a paddle on the top of a lever (pivoted at the bottom); lever falls.....leads to the next action....

[Uncle Fester]

Problem with the arrangement immediately above, the fan motor gets none of its electricity from/through the photocell(s). It gets it through a switch contact (relay).

Answer to the latest question addressed to me: the photocells were lit up by a $1.00 dollar-store 3-LED tap light, and the whole setup was enclosed in a bright shiny coffee can, with the lid sprayed with black spray-on "plasti-dip" that's used on tools and such for waterproofing. Kids chose that because itw as black and thick.

[Flavorflav]

Problem with the arrangement immediately above, the fan motor gets none of its electricity from/through the photocell(s). It gets it through a switch contact (relay).

Answer to the latest question addressed to me: the photocells were lit up by a $1.00 dollar-store 3-LED tap light, and the whole setup was enclosed in a bright shiny coffee can, with the lid sprayed with black spray-on "plasti-dip" that's used on tools and such for waterproofing. Kids chose that because itw as black and thick.

Thanks for the info on the lights - I'm not sure why we are having trouble, then. As for the relay issue, it isn't a problem - the photocell is still "allowing current to flow," and the clarification of 1/16 makes it explicit that this is legal.

[Balsa Man]

[quote="Uncle Fester"]Problem with the arrangement immediately above, the fan motor gets none of its electricity from/through the photocell(s). It gets it through a switch contact (relay).
Oh, interesting.... very interesting, and from a very respected source; thanks Uncle Fester. Your insights into the intentions of the rules writers is much appreciated. Would certainly appreciate your thoughts on the analysis below, which is how our guys got to this approach.
Glad we have this board, and glad I decided to post this approach.

We understood that the original (pre- Nats Rule clarification) required, or at least a direct hookup, "cell" to motor: "photocell which will provide the power to operate a motor...", but the key operative word in the clarification seems to be "allows"; "Activate a photocell that allows current to flow to a motor, which leads to the next action." (11/9/09). 3.d. specifically includes "mechanical relays" as electric components. "Provides the power", and "allows current to flow to" are very, and critically different. In the arrangement described, "activating" the photocell (by shining light), IS what "allows" current to flow;its "allowed" by the "activated" photocell "allowing" current flow through a relay (a legal electronic component) - to the motor. Had the rule been written, "Activate a photocell, i.e., photoresistor(s), the current from which operates a motor...", a relay would clearly be precluded. With the explicit inclusion of mechanical relays as legal, and the "allows current to flow" language (and regardless of the thoughts/intentions of the rule writers), having a relay "allow" current (and it just says "current to flow"; doesn't speak to current from where), I would have to say is within the rule as written - or rather, as clarified. The rule doesn't say "gets its electricity from." Would the inclusion in the circuitry of any other legal component - resistor, capacitor, etc, be a problem? I'm certainly not trying to argue;just trying to.....get things nailed down.

Of course, I understand nothing is official except official rules or clarifications, but we did discuss the rule, as clarified, with a couple EEs before deciding on approach, who came to the same conclusion we did- that a relay was one way to "allow" within the rules.....

I just carefully re-read the full rules, and I can't see anything else that impacts/modifies the key operative language "allows current to flow", or that states, or indicates current to motor must be....that same flock of electrons flowing from the photoresistor...... Am I missing something?

Thanks, again

[Balsa Man]

Problem with the arrangement immediately above, the fan motor gets none of its electricity from/through the photocell(s). It gets it through a switch contact (relay).

..... As for the relay issue, it isn't a problem - the photocell is still "allowing current to flow," and the clarification of 1/16 makes it explicit that this is legal.

Thanks, Flavorfav!
Whew. That’s a relief. I knew I remembered I'd seen something that made me sure we were on safe ground, but at 4:30 in the morning, I couldn't remember. Especially considering the source, the take we were off in illegal territory had me really worried. Onward.

Its been a few years, - memory has faded a little – but I’m deeply familiar with the legacy (and intracacies) of M-P rules and clarifications. Back in ’05 my older son took a full-points box to Nats (ah, but for one touch….). This year is SO much simpler. The endless hours that went into the problem of getting the bonus for using a single power source (a 12v lead-acid auto battery), trying to provide both nichrome-melting oomph, and low, stable, digital logic voltages was a major challenge (as was the “tie a knot in a shoelace”- actually, that was way fun).

Regarding that box, and this year’s rules, let me throw a couple other things out for opinion/thoughts.
My younger son’s a senior this year, and is pumped to be able to carry on the family Mission-P tradition. The box his brother took to Nats has some really cool art on the outside. He wanted to re-cycle the box because of that; stripped all the guts out (that was a walk down memory lane….)- new components/things going in, to this year’s rules. I’ve already checked with our Regionals Director, and cleared that choosing to use/finding/having a re-cycled box that fits the dimensional specs is fine – as in all the pieces/components are obtained from somewhere, and a box that meets specs from the S-O storeroom is ok. Will be clearing w/ State, and don’t expect the team to make Nats. The question, is, of course the General Rules, “must have constructed all pre-built devices” language. He could go thru the step of de-constructing the box, and then “constructing” the pieces back together (and I guess having photos of that with him, but is that really necessary? Along with the box question, how ‘bout using (a really nice/pretty Brazilian rosewood) lamp holder-that put light into a fiber-optics bundle in the ’05 box.
Thanks,

[Uncle Fester]

Straight off the clarifications page, which now lists ONE photocell clarification; its poorly-written predecessor was removed:

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)

So, it DOES clearly allow the photocell-relay arrangement. Being one to err on the side of caution, I suggest bringing along a full printout of the page in case a not-up-to-date sup combines both clarifications erroneously (like I did).

BTW, unlike a certain previous President, I do listen to questions about my opinions. I do nobody any favors by erroneously spreading bullsquish. THanks for being polite, too-- I get enough yelling and name-calling at work.

[Uncle Fester]

I doubt any sup cares how old a box is. After all, it's supposed to be imaginary. I saw one back in 06 (indiana) that had at least two dozen team numbers crossed out on it, and after I pointed that one out, it quickly became the most photographed Mission there.