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### Author Topic: phototransistor  (Read 5793 times)

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#### andreahmed

• Jr. Member
• Posts: 37
##### phototransistor
« on: May 15, 2007, 12:24:01 PM »
Hi, i read the infrared schematics sensor , and wondered of the calculations of R2 for the phototransistor , i once used the usual 10K and the transistor didnt work , i tried ur method i found the resistance in a dark are were approx 45K i chosed 50K resistor and it worked !! how did you come up with that soultion ??
here's the datasheet of mine phototransistor
http://www.ram.com.eg/RTE/my_documents/my_files/PT204_6C.pdf
, could you tell me how can i calculated the R2 of it ?
i suppose R2 = VC/ICon ~=2.5 K but that value didnt work ..

also i used ultrabright led with it it worked nice with whilte and blue surfaces is that good or should i use red leds ??

thanks !!
« Last Edit: May 15, 2007, 12:31:40 PM by andreahmed »

#### andreahmed

• Jr. Member
• Posts: 37
##### Re: phototransistor
« Reply #1 on: May 15, 2007, 06:17:24 PM »
any suggestions

also i've a question reagarding the arrangment of the LED and the phototransistor
it should be like LED O X Phototransistor  or   its better to be    LED O
X  Phototransistor

• Supreme Robot
• Posts: 11,632
##### Re: phototransistor
« Reply #2 on: May 15, 2007, 06:22:05 PM »
Quote
i once used the usual 10K and the transistor didnt work , i tried ur method i found the resistance in a dark are were approx 45K i chosed 50K resistor and it worked !! how did you come up with that soultion ??

5 years experience combined with analytical thought

Quote
here's the datasheet of mine phototransistor
http://www.ram.com.eg/RTE/my_documents/my_files/PT204_6C.pdf
, could you tell me how can i calculated the R2 of it ?
i suppose R2 = VC/ICon ~=2.5 K but that value didnt work ..

All sensors basically work on the same concept - you calibrate them using a realistic setting first. Check out how I choose the resistor for the photoresistor circuit (its done the same way as with IR). I even have an excel sheet done for you to calculate it:
http://www.societyofrobots.com/schematics_photoresistor.shtml

Quote
also i've a question reagarding the arrangment of the LED and the phototransistor
it should be like LED O X Phototransistor  or   its better to be    LED O
X  Phototransistor

I dont really understand what you mean. The best way to arrange them is to just experiment and see what works best for you.

#### andreahmed

• Jr. Member
• Posts: 37
##### Re: phototransistor
« Reply #3 on: May 15, 2007, 06:34:48 PM »
but its really should be  R2 = VCC/Icon or isnt it ?!

ah i mean , the way you put the LED and the phototransistor , they should be in parrael horizontally or verticaly ?

also what's your opinion about white led in differentiating between white and blue strips ?

thanks again

• Supreme Robot
• Posts: 11,632
##### Re: phototransistor
« Reply #4 on: May 15, 2007, 06:40:23 PM »
Quote
but its really should be  R2 = VCC/Icon or isnt it ?!
Whats Icon mean? The problem is that there is really no way to calculate it until you know how much light your robot will sense. The only way to know this is to test it in a realistic setting before plugging values into the equations.

Quote
ah i mean , the way you put the LED and the phototransistor , they should be in parrael horizontally or verticaly ?
isnt that the same? they should be pointing at what you need to sense.

Quote
also what's your opinion about white led in differentiating between white and blue strips ?
for infrared or for visible light? for visible light i'd say a blue or a white led would be best. for infrared, you will have to test.

#### andreahmed

• Jr. Member
• Posts: 37
##### Re: phototransistor
« Reply #5 on: May 15, 2007, 06:45:38 PM »
yea i mean  IC on , collector resistanc on ,  if you downloaded the datasheet of mine sensor you will see it , so there'r no calculations to at least determine what's the minimum resistor to put ?

• Supreme Robot
• Posts: 11,632
##### Re: phototransistor
« Reply #6 on: May 15, 2007, 06:49:18 PM »
Quote
so there'r no calculations to at least determine what's the minimum resistor to put ?
if you dont know the input (light your robot will see), you cant calculate the output. therefore you cant select a resistor to optimize this until you test it.

#### andreahmed

• Jr. Member
• Posts: 37
##### Re: phototransistor
« Reply #7 on: May 15, 2007, 07:00:50 PM »
yea but the transistor should work in either active mode or stauration ( a switch ) so there must be a calculation for it work in active mode or switch ,i googled and didnt find any helpfull thing

#### zabuza™

• Jr. Member
• Posts: 25
##### Re: phototransistor
« Reply #8 on: May 29, 2010, 12:17:03 PM »
Quote
so there'r no calculations to at least determine what's the minimum resistor to put ?
if you dont know the input (light your robot will see), you cant calculate the output. therefore you cant select a resistor to optimize this until you test it.

how would you go about calculating/ testing this

• Supreme Robot
• Posts: 11,632
##### Re: phototransistor
« Reply #9 on: May 29, 2010, 08:41:05 PM »

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: phototransistor
« Reply #10 on: May 29, 2010, 08:42:14 PM »
Hi,

Quote
so there'r no calculations to at least determine what's the minimum resistor to put ?
if you dont know the input (light your robot will see), you cant calculate the output. therefore you cant select a resistor to optimize this until you test it.

how would you go about calculating/ testing this
Place the phototransistor in the approximate amount of light it will see during active duty and measure.
Or better yet, measure at the highest amount and the lowest amount of light it will see and use a resistor in series with a trimmer (or potentiometer). That way you can dial in the needed resistance in situ.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?