# 555 timer flashing led circuit

Discussion in 'The Projects Forum' started by steven2410, Sep 1, 2014.

1. ### steven2410 Thread Starter New Member

Jul 7, 2014
16
0
I just have a question about this simple circuit or example: http://www.555-timer-circuits.com/flashing-led.html. I just don't get 1) why do they use 9V battery to power a single red LED light. 2) why there need to be a 1k resistor before the led? 3) does this circuit hugely affect battery's life ( since i try to blink LED to prolong battery life)

Is there anyway to power a 12V LED (current: 0.08A) by 12V battery efficiently (optimal brightness) or i have to use a slightly higher voltage?

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,921
600
1. They are not using 9 volt battery to power led. The led IS NOT connected to battery.

2. Resistor is used to controll amount of current going through led. Normally the MAXIMUM current though led like that is 20-25 mA. Most people try to run their led (the cheap kind) at about 15 mA.

3. ### to3metalcan Member

Jul 20, 2014
234
25
This is a demo of a 555, not an efficiency-increasing circuit. The usual TTL version of a 555 uses enough current to significantly shorten battery life even when it's "not doing anything." The 9V is a convenient and common choice for a hobby power supply...obviously, you can run an LED on a lot less.

Resistors are necessary because otherwise the current through the LED would burn it out.

Doesn't sound like this is the circuit you're looking for.

4. ### MrCarlos Active Member

Jan 2, 2010
400
135
Hello steven2410

You say:
1) Why do they use 9V battery to power a single red LED light.
Well, they decided to make their circuit with that battery. Sure, they could have done with another different battery voltage.

You say:
2) why there need to be a 1k resistor before the LED ?.
Such resistance is to limit the electric current flowing through the LED.
In this case, the current flowing through the LED is about 9 mA.
I = V / R = 9/1000 = 0.009A = 9 mA.

You say:
3) Does this circuit hugely Affect battery's life (since I try to blink LED to prolong battery life)
Well, yes, of course. but might have another purpose that the LED blink.
For example: "To pay attention."

You say:
Is there anyway to power a 12V LED (current: 0.08A) by 12V battery efficiently (optimal brightness) or I have to use a slightly higher voltage ?.
You can use any DC voltage and current. while not overdo the electrical parameters of the LED in question.

Here we should add a statement:
The LED lights to full brightness when flowing through it a current X.
When X current flows, there is a voltage drop Y between its terminals.

So what if our LED polarize With Z Volts.
Then (Z - Y) / X = R.
Z = 12 VDC
X = 15mA
Y = 1.2V
You can calculate easily the value in ohms of R.
Note:
The important thing here is to know, through the LED data sheets, the electrical characteristics of the LED you are using.
If the battery can supply the power required to turn on the LED there is no problem.

Mar 24, 2008
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6. ### wayneh Expert

Sep 9, 2010
13,427
4,266
Intermittent flashing - also know as a very low duty cycle - can hugely reduced the amount of power needed and thus extend battery life. But it needs to be weighed against the function of the LED. A 10ms flash every 5 seconds might be fine for a "power is still on" indicator, but not good at all for, say, a fire alarm.