# How to increase 555 timer output voltage

#### peja5081

Jul 21, 2012
2
Hi,how to increase output voltage to12v from this circuit

http://www.555-timer-circuits.com/traffic-lights.html
bcoz my LED need 12v input for each red,yellow and green.

Last edited:

#### duke37

Jan 9, 2011
5,364
How can you increase to 12V when you already have 12V?

5V should be plenty to drive LEDs, just adjust the series resistors.

The green LED has a lower series resistance than the red LED. Will the current be too high?

#### CocaCola

Apr 7, 2012
3,635
The green LED has a lower series resistance than the red LED. Will the current be too high?

If you read the circuit description on that page they say there is a 2 volt drop going into the 2nd 555, thus the likely reason for the adjusted resistor value...

#### duke37

Jan 9, 2011
5,364
I should have read the link. Pin 8 is the power input to the 555 and this is derived from the output of the first 555. Pin 4 is not power input as I assumed, it is reset.

#### Merlin3189

Aug 4, 2011
250
I'm not sure that helps. It still doesn't explain why the LED series resistors have a 2:1 ratio.
A 2V drop from 12 to 10 or even 9 to 7 would not require a 2:1 ratio.
I can only presume there is an unmentioned difference in the current required for each LED.

Anyhow, if Peja requires 12V to drive each LED, he can use a buffer transistor with the LED (and series resistor) on the collector, connected between 12V and 0V, with a base resistor to give safe saturation drive.

#### gorgon

Jun 6, 2011
603
The red LED is driven by 12V. The green LED is driven with only 8V(approx 2V drop in each 555). Given that red LEDs drop 1,7V and green LEDs drop 2V+, the current in the red LED is 22mA, and the green LED is around 27mA. To get the same current in both, the 220 ohm resistor should be 270 ohm.

TOK

#### CocaCola

Apr 7, 2012
3,635
I'm not sure that helps. It still doesn't explain why the LED series resistors have a 2:1 ratio.
A 2V drop from 12 to 10 or even 9 to 7 would not require a 2:1 ratio.
I can only presume there is an unmentioned difference in the current required for each LED

In addition to the differences in LED, it might very well just be what was available to the author...

For example here is a list of the store stocking ¼ Watt resistor values at Radio Shack in that general area 1K, 470, 330, 220, 100

Easy to see where the jump from 220 to 470 can arrive if that is all you have available and want to keep it simple and not get into parallel or series resistors...

#### duke37

Jan 9, 2011
5,364
LEDs need a defined current drive, perhaps peja5081 could tell us why he needs more voltage.

#### CocaCola

Apr 7, 2012
3,635
LEDs need a defined current drive, perhaps peja5081 could tell us why he needs more voltage.

My guess is that he/she purchased pre-resistored 12 Volt ready LEDs... There is a big problem with the way these are marketed to the masses... Due to the ignorance of the general population these 12V pre-resistored LEDs all the sudden becomes a "12V LEDs" proper... Most don't have the slightest clue that LEDs are current driven, and only require a minimum forward voltage...

#### peja5081

Jul 21, 2012
2
My guess is that he/she purchased pre-resistored 12 Volt ready LEDs... There is a big problem with the way these are marketed to the masses... Due to the ignorance of the general population these 12V pre-resistored LEDs all the sudden becomes a "12V LEDs" proper... Most don't have the slightest clue that LEDs are current driven, and only require a minimum forward voltage...

that's right..look like this

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