Defective Photoresistors

jeffrey morris

I found out that one photoresistor is defective because I couldn't get readings from TestPhotoresistorDividers PBASIC program and another may be defective because the readings change from 0 to 1 to 0 all of the times. I have three photoresistors and when I switched them around, I found out that one is defective and other may be defective. How can I get new photoresistors?

Jon Williams

You can either send it back to us (and go through the warranty process), or save yourself a bit of trouble and run down to your local RadioShack store -- they sell a whole pack of them for a couple bucks.

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Jon Williams
Applications Engineer, Parallax

jeffrey morris

If I buy photoresistors at Radio Shack, what are the recommended specs?

Jon Williams

CdS photocells tend to have a ~500K - 1M dark resistance; within the pack you'll find a few that match (go for size first) the units we use.

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Jon Williams
Applications Engineer, Parallax

Andy Lindsay (Parallax)

Here are a couple more things to try:

Double-check and make sure your 220 ohm resistors are correctly plugged into the P6 and P3 sockets. I've seen that kind of flickering before when the 220 ohm resistor is plugged into an adjacent socket (P2, P5, P7). Also, check the other connections, and make sure each lead in your circuit is firmly plugged into the breadboard sockets. Same with the leads that go into the headers.

Another thing to try is skipping ahead to Robotics with the Boe-Bot, Chapter 6, Activity #4. This activity will give you a better test for whether or not the photoresistors are defective. Please let us know what the range of measured values was for each photoresistor. If the values on one of the photoresistors is seriously erratic, it probably is defective.

If you do have defective photoresistors, contact our sales department, and they will send you replacements. For contact information, go to www.parallax.com -> Company -> Contact Parallax.

BTW, I tested the photoresistors in the Robotics with the Boe-Bot kit, and they tend to measure around 2 k-ohm in bright light, and 7 or 8 M-ohm in darkness.

Appended:

I ran the example program from·Chapter 6, Activity #4, and·got values under 20 for indoor lighting, up to values in the 400 to 1000 range when I cupped my hand over the photoresistor.·

Post Edited (Andy Lindsay (Parallax)) : 9/6/2005 2:21:00 AM GMT

jeffrey morris

I went to Chapter 6, Activity 4. I tested three photoresistors and found out that they're not defective. I got values of 1 under a bright flashlight, about 30 under indoor light, and 500 and over when I cupped my hand over the photoresistors. I must have made mistakes when creating circuits and when writing programs.