Photometer

[dickcruz]

I need help in figuring out the amperage of the 5V supply.
I also want to know what the output voltage would be like and what the transistor is doing there. Thanks
URGENT

 

[Nigel Goodwin]

You would only need a low current supply, a 78L05 should be more than enough - on a quick glance the transistor looks to be a common base amplifier?. Presumably the text explains it all?.

 

[Russlk]

The transistor (Q1) allows a single supply to be used. The op amp input and output are at the same voltage level, only the pulse from the opto is amplified. If the 510 ohm resistor were directly connected to the op amp, the output DC would be very difficult to control.

 

[dickcruz]

I have a 400mAh, 4.8V . would this do?

 

[Nigel Goodwin]

Should be fine!.

 

[dickcruz]

What should the output be like. between 1-3 volts? i need something more presicise

 

[dickcruz]

If i were to measure the variation in potential difference as a function of intensity, where would i have to plug the volt meter.

 

[Nigel Goodwin]

What are you trying to do?, this circuit merely amplifys the AC signal received by the photodiode, it's not measuring the intensity.

 

[dickcruz]

I'm trying to measure the intensity of incident light on the photodiode.
Why can't i do this

 

[dickcruz]

how much of bias voltage should i apply and how will it affect my ouput

 

[Nigel Goodwin]

The application note you posted does nothing like that, you need a DC coupled circuit.

 

[dickcruz]

could you help me couple it? please. its urgent. i have a week to finish this.

 

[dickcruz]

what i meant was like this circuit will have a different vout as the current on the photodiode varies right. so i can, measure the voltage recorded under different sources of light cant i?otherwise wht youre saying is that this will always give a constant voltage indepedant of the the intensity?

 

[Russlk]

You need to turn the light off and on or put a rotating wheel between the light source and photodiode to chop the light.

 

[dickcruz]

Ok , may i know why im doing that please?

 

[dickcruz]

Finally,

just one question, the current between the photodiode and the inverting input of the opamp, would it be in the direction of the photdiode or the opamp.
and this current would be alternating right? otherwise the capacitor would not let it pass through.

thanks for all the help.

 

[Russlk]

If you chop the light, the signal can pass thru a coupling capacitor. Making a stable DC amplifier is difficult, but an AC amplifier is easy. The schematic in the appbrief is AC coupled.

 

[dickcruz]

SO the incident light causes a photocurrent that goes and gets all its DC components taken off and then goes on to the inverting input and the output is V+ + V- which then is partially fed back via the feedback circuit and gets amplified.

Correct me if i wrong.
Thanks guys, I dint know anything about this. Thats why i chose such a project.