Photocells (also called photoresistors or photoconductors) are sensors that vary resistance depending on the amount of light they receive. Photocells are commonly used in line-following and light-seeking robots.
All of the different types of photoresistors from the first grab bag.
And, yes, they’re photographed on my wife’s “good” pillowcase.
In September 2000, I purchased “100 asst. sizes and values photocells", part number 169578 or 169578CE, cross reference number GB125, from Jameco Electronics for $6.95 for use as robot sensors. The grab bag consists of a variety of cadmium-sulfide photoresistors.
In May 2002, I purchased two more of the same bag, but the price had increased to $7.95. In April 2004, the price had gone up to $12.95. In August 2008, Tuomo Tanskanen alerted me that the price is now $19.95. In August 2009, I noticed that the price is now $21.95. (Goodness! What’s with that???)
Unfortunately, although the current bags have the same cross reference number and base part number as the previous bags, these new bags only contain the smallest-size photoresistors. So, I no longer recommend the Jameco grab bag if you want medium, large, or hermetic (sealed in a metal case) photoresistors.
In March/April 2006, I purchased five bags of “Giant CDS Cell Assortment”, part number G14025, from Electronic Goldmine for $3.00 each. Each bag contained 20 photocells of assorted sizes and values. The total cost was $15.00 for 100 photoresistors. That is less expensive than the current Jameco bag of 100, and the Electronic Goldmine assortment has a variety of sizes and styles, and can be purchased in lots of 20 photoresistors for only $3.
My expectations are pretty low for grab bags and I could have ordered specific individual CdS photoresistors if I had really wanted. But, I wanted as many varieties and as much experience as I could get for robotic experiments, such a line-following and floor reflectivity sensing.
Here’s what I received; your bag may vary:
(Minus sign in quantity columns indicates number of defective)
|Image||Description||Dimensions||Quantity in Jameco 2000 Bag||Quantity in Jameco 2002a Bag||Quantity in Jameco 2002b Bag||Quantity in five Goldmine 2006 Bags||Image of Defective|
|Huge photoresistor, 17 curves||22 mm x 25.5 mm||20 - 0||20 - 0||20 - 1||9 - 0|
|Large hermetic photoresistor, 12 curves||12.34 mm||0 - 0||0 - 0||0 - 0||16 - 0|
|Large hermetic photoresistor, very curvy||12.34 mm||0 - 0||0 - 0||0 - 0||1 - 0|
|Large photoresistor, 27 curves||10 mm x 11 mm||20 - 7||20 - 2||20 - 3||0 - 0|
|Large photoresistor, 16 curves||10 mm x 11 mm||0 - 0||0 - 0||0 - 0||24 - 0|
|Medium photoresistor, 12 curves||6.3 mm x 7.3 mm||0 - 0||19 - 0||15 - 0||4 - 0|
|Medium thick photoresistor, 9 curves||6.3 mm x 7.3 mm||16 - 0||1 - 0||5 - 0||7 - 0|
|Medium photoresistor, 9 curves||6.3 mm x 7.3 mm||16 - 0||1 - 0||5 - 0||6 - 0|
|Medium photoresistor, straight line||6.3 mm x 7.3 mm||0 - 0||0 - 0||0 - 0||1 - 0|
|Small photoresistor, hermetic, 15 curves||5 mm||0 - 0||0 - 0||1 - 0||0 - 0|
|Small photoresistor, hermetic, 11 curves||5 mm||0 - 0||11 - 2||7 - 3||1 - 0|
|Small photoresistor, orange, 11 curves||3.9 mm x 4.2 mm||22 - 1||12 - 0||11 - 4||11 - 2|
|Small photoresistor, brown, 11 curves||3.9 mm x 4.2 mm||5 - 2||2 - 0||0 - 0||6 - 1|
|Small photoresistor, 9 curves||3.9 mm x 4.2 mm||2 - 0||0 - 0||0 - 0||3 - 0|
|Small photoresistor, hermetic, 8 curves||5 mm||0 - 0||8 - 0||8 - 0||6 - 0|
|Small photoresistor, orange, 8 curves||3.9 mm x 4.2 mm||3 - 0||5 - 1||8 - 0||0 - 0|
|Small photoresistor, black, 8 curves||3.9 mm x 4.2 mm||7 - 0||1 - 0||1 - 0||0 - 0|
|Small photoresistor, hermetic, cross||5 mm||0 - 0||0 - 0||0 - 0||3 - 0|
|Small photoresistor, hermetic, straight line||5 mm||0 - 0||1 - 0||5 - 0||2 - 0|
|Small photoresistor, straight line||3.9 mm x 4.2 mm||1 - 0||0 - 0||0 - 0||0 - 0|
|Total:||96 - 10||100 - 5||101 - 11||100 - 3|
Checking the Total column shows that I received only 96 photoresistors in the Jameco Electronics 2000 Grab Bag, not the 100 advertised. The newer grab bags were fine.
I tested all of the photoresistors using multimeter in ohmmeter mode. Bright resistance was measured with the cell pressed against a fluorescent bulb. Dark resistance was measured with the cell hidden underneath the desk. (Okay, not exactly an international standard of measurement.)
Jameco Electronics 2000 Bag: Five cadmium-sulfide photoresistors turned out to be internally shorted such that their values were less than 100 ohms for both brightness and darkness. Another five photoresistors were partially shorted (or otherwise defective) such that their values were less than 4 kilohms for both brightness and darkness. All the other photoresistors (good) showed a reasonable range of less than 500 ohms for brightness and greater than 30 kilohms for darkness. These results represent a defect rate of a little over 10%. Ouch!
Jameco Electronics 2002a Bag: Two photoresistors were internally shorted to less than 5 ohms. Another three photoresistors had a dark resistance less than 12, 6, and 2 kilohms. These results represent a defect rate of 5%.
Jameco Electronics 2002b Bag: Three photoresistors turned out to be internally shorted such that their values were less than 400 ohms for both brightness and darkness. Another three photoresistors had dark resistances less than 100 kilohm, 10 kilohm, and 5 kilohm. Another four changed to their high dark resistance very, very slowly. One of the huge photoresistors was missing its protective coating. These results represent a defect rate of a little over 10%.
Although not counted as defective, of all of the Jameco huge photoresistors, a few were stained, many had bent leads (the leads are very thick), and many leads had some residual gunk on them indicating they might have been used.
Electronic Goldmine Bags: One photoresistor had only one lead (the other lead was completely broken off). One photoresistor was permanently shorted (0 ohm) and another was permanently open (infinite resistance). These results represent a defect rate of 3%.
I originally paid 7.2 cents per non-defective Jameco photoresistor ($6.95 / 96), compared with a cost of 25 cents for individual CdS photoresistors if ordered in quantities of 100. Since then, with price increases, the Jameco Electronics prices would be more like 22.8 cents ($21.95 / 96) in grab bags versus 70 cents individually.
I paid 15.4 cents per non-defective Electronic Goldmine. In my case, the slightly higher bag price (per 100) was offset by a lower defect rate.
I’m a little surprised at the less-than-full bag (Jameco 2000) and the generally high defect rate. I’m accustomed to other electronic components that are usually defect free. The key lesson learned is that each photoresistor must be tested before using in a project.
Still, I didn’t expect much from a grab bag. And, I got the variety of photoresistors I hoped for at a relatively cheap price.
Since starting this web page in September 2000, I’ve learned a lot about photoresistors. I’ve learned the purpose of the squiggles on their faces, the formulas for calculating photocell resistance for a given amount of light (illuminance), photocell reaction time (and therefore maybe why they shouldn’t be used for line following), and industry standard expectations for variance and defects. Check out pages 370 to 378 in Intermediate Robot Building.