Help understanding project

[Karkas]

Hello, I made this simple circuit, and understand its basic functioning but, there is something left. I don't know and haven't been able to understand what is the zener diode doing there, I made the prohect and it didn't work, I put away the zener and it's working, maybe it was a mistake to put it there or maybe not. What is the idea of the zener in the circuit?

I would like to use it with HB LED(s) maybe I need a driver, I would like you to give me a hand there too, 'cause I have never used them.

 

[audioguru]

I think the circuit will work pretty well without the diodes or zener diode if the voltage of the LED is correct.
The LED on the right side is shown upside-down.

 

[Karkas]

Thanks for replying audioguru, do you know what would be the idea of puting the zener diode there? even when it is not necessary.

 

[audioguru]

Thanks for replying audioguru, do you know what would be the idea of puting the zener diode there? even when it is not necessary.

The diodes or the zener diode are used when the LED is a high voltage blue one and are not needed when the LED is a low voltage red one.
Chemelec (the circuit's designer) is in South America for a while so we can't ask him.

 

[Karkas]

Excuse me, but I have to ask, why is the zener diode needed if the LED is a high voltage one? can't see it.

And another thing, if I wanted to use the circuit with a HB LED, for example a yellow one, I have its datasheet and it says that it forward voltage is 2.1V at 20mA, I dont know what is the voltage of the red LED I am using right now, but maybe I need another transistor to get those 20mA it doesn't supply it? I've never used one.

 

[audioguru]

If the supply is 10.0V then pin 2 of the 555 goes from 3.33V to 6.67V.
If the LED is a 3.5V blue or white one then it turns off when its voltage is about 3.2V and it is very bright when its voltage is much higher.
The base voltage of the transistor is 0.7V higher than the LED voltage so only one diode is needed to cancel it.

 

[Karkas]

Then why are there four diodes or a 2.7V zener diode? isn't it because of the 100Ω limiting resistor?

 

[audioguru]

Then why are there four diodes or a 2.7V zener diode? isn't it because of the 100Ω limiting resistor?

If the supply is 12V then pin 2 of the 555 goes from 4.0V to 8.0V.
If the LED is a 1.8V red one then it turns off when its voltage is 1.6V and is very bright when its voltage is much higher.
The base voltage of the transistor is 0.7V higher than the LED voltage so you want it to be (1.6V + 0.7V=) 2.3V when pin 2 is 4V.
So you need a voltage drop of (4V - 2.3V=) 1.7V which is provided by 2 or 3 diodes in series.

 

[Karkas]

I can see it now, thank you very much. I appreciate it.

 

[chemelec]

Hi Audioguru, Now in Punta Aranas, Chile.
Fly to Ushuaia in Southern Argentina in Two days.
GOOD TRIP so far.

 

[Vizier87]

have a nice trip gary.

 

[audioguru]

I have never been to South America but I was in Mexico, Jamaica and Cuba which were nice and warm when it was freezing winter in Canada.
Have a nice warm vacation, Gary.