Photocell Bonus

[pjsetnicker]

Just to help everyone. I recently discovered this discussion. It helped my team alot.

What I did was take the solar cells and attach three of them to a C-battery pack (2 batteries)... along with a standard motor you would find in a remote controlled car persay. We shined the light used in a Eveready Heavy Duty flashlight (powered by one of those square lantern batteries) - worked like a charm.... until I burnt the light bulb out ... hahaha

[Balsa Man]

Re: photoresistor reliability and failure modes

Anybody have any experience/info/insights on the reliability of photoresistors, and factors that affect the reliability?

We're not using the Radio Shack ones- got some from our local electronics supply place. Using a single one- 9.6v power, tripping a relay/powering a fan motor. Light source is a 12v/20w landscaping bulb; photoresistor mounted about a quarter inch away. Light-on for ....a second or less is part of set-up testing process.

Pre-Regionals testing and calibration put the unit through....25-30 duty cycles. It was working fine Friday night, and on a check Saturday morning. Then when the guys were doing setup to run at Regionals, the circuit test for fan failed. They correctly figured out they'd had a photoresistor failure, and scrambled to cut it out and do an alligator clip replacement. Replacement worked.

The problem was, their setup time was down to 3 min, hurried through, missed a check on flag raising mechanism, which led to one touch - and 4th, while our other team's box (same component set and sequence) ran flawlessly, and hit 60.04 sec....

Plan is to put a fresh (but tested a couple of times) photoresistor in (on both boxes) for State, which should prevent the problem, but just wondering how much of the problem is the nature of photoresistors, and how much might be from heating from the lamp
?


Problem was

[Flavorflav]

Uncle Fester posted this a while back:

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.

We haven't run into a problem yet, but honestly we haven't really finished building yet. It ran at regionals half-built.

[Balsa Man]

Thanks.
That does suggest heating could be the source of the problem - from a solid source.
Thanks.
I find myself often telling others to "read the previous posts" before asking a question...
Mea culpa...