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Hm. What is this resistor for?

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Nrets

New Member
So I wanted to procrastinate studying for an exam last night and whipped out some components to make a simple light sensor circuit that would output analog between 0-5v. I had an IR phototransistor laying around that, so I used that instead of a photoresistor. At first I did not have R2 in the circuit, and although I was able to measure a signal, the data was very noisy when no light was on, or if the light level was low. I tried changing some resistors, but it didn't help anything. I then realized I had nothing connected to ground, but couldn't really see where anything should be connected to ground. I finally ended up putting a 100kohm resistor on the emitter of the transistor, and that solved all my problems. I think I understand vaguely how this works, but I don't fully understand the purpose of R2. Can anyone explain?

I drew the schematic in power point, please forgive me.Picture4.png
 
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Reikitronic

New Member
Stability

Hello Guys, i think it is important to mention what this r2 resistor contributes with the stability of this circuits and dont allow noises.

Have a nice day. :)
 

crutschow

Well-Known Member
Most Helpful Member
Without the resistor, there's nowhere for the transistor emitter current to go. The transistor couldn't work and was not doing anything.
 

Nrets

New Member
THe BJT is behaving as a current source. R2 converts the output current into an output voltage. To get a proper reading without R2, you would need to measure the current (rather than voltage) between emitter and ground, then you would probably get a better reading.

Common collector - Wikipedia, the free encyclopedia
Common emitter - Wikipedia, the free encyclopedia
I see. Actually reading those links almost confused me with the usage of impedence and voltage buffers, but I think I get the basic jist. So all the resistor is doing is creating a voltage drop across the resistor so that a voltage can be measured on V_out? So if I used a smaller resistor, such as 500 ohms, the voltage drop would be small, and the maximum reading on V_out would be less than 5v?
 

dknguyen

Well-Known Member
Most Helpful Member
THat is correct. If you did not have the resistor you would have to connect a current-meter between the transistor's emitter and ground to get a reading, not a voltage meter. The current output from the circuit would then have somewhere to flow (through the low-resistance ammeter to ground) and you could measure it.

The emitter is outputting current, not voltage. By attachign a voltage meter (which has a high resistance to ground since no current is supposed to flow into a voltmeter) to the transistor's emitter, the emitter was basically a random floating voltage giving you no reliable voltage reading.
 
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