### Author Topic: Calculating the resistor needed for photoresistors, something is wrong  (Read 1671 times)

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#### Friendlyfire

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##### Calculating the resistor needed for photoresistors, something is wrong
« on: January 01, 2009, 07:08:30 PM »
I am making the \$50 robot, and was going on to the photo sensors.  The problem is the largest sensors in bright light give me my lowest resistance of around 10 Kohm.  When using the equation, even if I use the bright light resistance twice, I am getting that I need a resistor of around 10 Kohm (Since it would be the square root of 10 squared), which is drastically more then what was suggested if you aren't testing them, and drastically more then the resistors I already bought.  Are my photo resistors wacky?  Am I doing this basic math wrong? Should I just slap on the 1.6 Kohm resistor I already have and run with it?  I cannot figure this out.

#### Friendlyfire

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##### Re: Calculating the resistor needed for photoresistors, something is wrong
« Reply #1 on: January 03, 2009, 03:10:25 PM »
I guess I will just use the 1.6Kohm resistor on it, but has no one else tested there photoresistors?  Were you getting ranges of 10-35 Kohm (The 35 was on the the medium sized resistors in a dark area).  Anyone elses experiences would be helpful.

#### Soeren

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##### Re: Calculating the resistor needed for photoresistors, something is wrong
« Reply #2 on: January 04, 2009, 10:46:48 AM »
Hi,

Are my photo resistors wacky?
Either that or you are measuring in a wacky way

Most LDR's have a dark resistance in the MOhm range or close to and a resistance of a few hundred Ohms at most when you shine a lamp or a flashlight directly on them.

How did you measure the LDR's?

Regards,
SÃ¸ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

#### Friendlyfire

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##### Re: Calculating the resistor needed for photoresistors, something is wrong
« Reply #3 on: January 04, 2009, 06:37:42 PM »
I have a multimeter with a resistance setting.  When I shined a light right at the large resistors I got about 3Kohm of resistance.

#### Admin

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##### Re: Calculating the resistor needed for photoresistors, something is wrong
« Reply #4 on: January 10, 2009, 10:14:27 PM »
Quote
The problem is the largest sensors in bright light give me my lowest resistance of around 10 Kohm.
Quote
When I shined a light right at the large resistors I got about 3Kohm of resistance.
I'm confused, which resistance is it?

And whats your dark resistance?

Anyway, other resistor values will work. But calculating it will give the optimal value.

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