# LED Pulse Frequency - Help Please!

Discussion in 'Homework Help' started by Rebecca, Feb 2, 2009.

1. ### Rebecca Thread Starter New Member

Feb 2, 2009
3
0
I am stuck on a question and cannot seem to find assistance anywhere. The question: If an LED is supplied with a flow of pulses at a frequency of 50Hz and this produces a visible output, if the frequency is then raised to 1kHz what will be the effect on the light output.

I've dabbled with a few points ie did it become a laser, and concluded no. Then wondered if it would fall into the infrared scale but again concluded no. My final guess is that the light would become brighter due to more photons being provided?

Can anyone assist me on this please?
Thanks
Rebecca

2. ### GirishC Active Member

Jan 23, 2009
58
0
As you raise pulse frequency fro 0Hz you will see the blinking which is proportional to the frequency of pulse. When you cross the point where eyes can not perceive the on-off you will see continuously lit LED. The brightness of LED would depend on pulse width and frequency.

The light won't fall in the LASER or IR because its a frequency of source turning on and off.

3. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
Look up persistence of vision.

If you actually have the blinking LED to experiment with, try it in various environments, with fluorescent lighting, in front of a TV, and viewing straight on vs. from the side of your eye.

John

4. ### Bernard AAC Fanatic!

Aug 7, 2008
4,240
414
As long as the wave form does not change, ie. duty cycle is constant ; no change in light output.

5. ### Rebecca Thread Starter New Member

Feb 2, 2009
3
0
Thank you very much for your help. May I clarify that the answer is along the lines that the human eye would not be able to detect a change?
Thanks again.
Rebecca

6. ### Bernard AAC Fanatic!

Aug 7, 2008
4,240
414
I increased frequency from 5 cps to 60 kcps, flicker observed up to 35 cps, no change visable on up. Red LED , constant voltage, HP audio sig. gen. rectified with one 1N914.

7. ### Cabwood Member

Feb 8, 2009
20
0
You've nearly answered the question yourself. The human eye is able to perceive flashing at a frequency of up to about 50Hz. The question is really asking this: what does the human eye perceive flashing if the LED flashes on and off at a frequency significantly greater than 50Hz?

You must not confuse flashing red light with the frequency of light being emitted. Red light emitted from an LED or laser consists of electromagnetic waves which themselves oscillate at a frequency of about 450THz, or 450000000000000 cycles per second. The LED is a device that produces waves of this frequency when electrical current passes through it.

When a red LED (or laser) flashes, all that is happening is this: while the LED is powered on, it IS producing electromagnetic waves of 450THz, and while it is off, it isn't producing any waves at all.