I am trying to understand diodes

[Hextejas]

And my 1st self designed project involves a SFH 205, photo diode and an LM 358.
Reading the datasheet for the diode says a Vf = 1.3, I seem to remember.
So, the tutorials that I am reading say that I can measure the Vf of the diode outside the circuit.
I did that on 5 different diodes and they are all right around .421.
How come ?

Next, It seems that every schematic that i have seen seems to have  about a 10k resistor and a .01uf capacitor hanging around pins 1-3.
Why's that ?

Thanks

[sleemanj]

Datasheet states forward voltage at 100mA.  What did you measure at. 

Look at charts of forward current vs forward voltage, you should get your answer.

[alexanderbrevig]

Hi! Please show us a schematic. 
Also note that a diode and a photodiode are really not very similar.

[Dave]

Also note that a diode and a photodiode are really not very similar.

Please elaborate.

[Nerull]

Photodiodes aren't built to be operated in forward bias and typically can only handle very low currents, and the Vf will also change based on incident light, this could make them tricky to measure.



Components don't "hang around" - what do you mean? Are they connected between 1-3? What type of circuit is it? There are a million ways to use opamps and a million ways to connect them in a circuit.

[Hextejas]

Components don't "hang around" - what do you mean? Are they connected between 1-3? What type of circuit is it? There are a million ways to use opamps and a million ways to connect them in a circuit.

Sorry for being flippant, I should have phrased that better.
I just looked at a site of instructables that had, I think, a post showing 47 projects to be built with the 555 timer. It seems that there is always a small value capacitor between pin 2 and ground and 6. So there must be a general rule or consideration here.
I wanted to understand the "why" of that capacitor and the 10k resistor.

[Brumby]

That capacitor is the key of the 555 timer.  That, and the resistor above it.

It is the time taken to charge that capacitor up and discharge it that dictates the period of the timer.

What also affects the period, is the voltage on pin 5.  Normally, it is left unconnected or with a bypass cap, but here it is used to raise and lower the threshhold voltage.  This will change the period of the 555 depending on the input.

I won't spoil the mystery on how this is an amplifier ... 

[Hextejas]

Hi! Please show us a schematic. 
Also note that a diode and a photodiode are really not very similar.

I don't really have one yet. I was going to try and build a circuit that turned on a LED as the light got dimmer.
And the bit about photo diodes being different makes a lot of sense.
I covered them from the light and the voltages dropped.

[Hextejas]

That capacitor is the key of the 555 timer.  That, and the resistor above it.

It is the time taken to charge that capacitor up and discharge it that dictates the period of the timer.

What also affects the period, is the voltage on pin 5.  Normally, it is left unconnected or with a bypass cap, but here it is used to raise and lower the threshhold voltage.  This will change the period of the 555 depending on the input.

I won't spoil the mystery on how this is an amplifier ... 

Ok, they sound important,  so how do I get from a resistor capacitor combination to a frequency ?
Hey, is that all the RC discussion I have been seeing and ignoring ?

[Brumby]

Just Google "555 timer calculations".

Hey, is that all the RC discussion I have been seeing and ignoring ?

If it's in relation to the 555 - then, absolutely!

[Hextejas]

I have these components confused.
I don't need the 555 to do what I want to do. Just need the photo diode and an LED.
That was too easy so now I need to figure out how to work the 555 into it.
Maybe I will try to blink the LED faster based upon the photo diode.
Something like a normal fade based upon the light but full luminescence and blinking if totally dark.
Probably beyond me.
A speaker would work better if I had one.

[Hextejas]

Cancel this as it is a dumb 1st project. Since I am using a photo diode, the light keeps getting in the way.

[Jwillis]

Try this one.

[Zero999]

Try this one.

Why use two 5V PSUs, rather than a single 12V PSU?

One problem with using multiple switched mode power supplies, like the ones shown in the photo, is the leakage currents add together, increasing the shock risk.

[Brumby]

Why use two 5V PSUs, rather than a single 12V PSU?

My guess is whoever put this together used what was available - probably 2 phone charger plug packs.

[alsetalokin4017]

It may also be important to note here that the LDR (light dependent resistor), aka CdS "photocell", is not the same thing as a photodiode.

[Zero999]

Why use two 5V PSUs, rather than a single 12V PSU?

My guess is whoever put this together used what was available - probably 2 phone charger plug packs.

I suspected that was the case. I still think it's silly putting it on the Internet though. If it were me, I would have shown a single 12V PSU, even if I'd used two 5V, when I built it. I might write that I'd used to 5V supplies, because that's what I had available, but wouldn't recommend it.

[Brumby]

I still think it's silly putting it on the Internet though.

        (Sorry)