# NE 555 timer

Discussion in 'General Electronics Discussion' started by rickster581, Feb 2, 2012.

1. ### rickster581

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0
Feb 2, 2012
Hi Colin,

Rick here from Miami Florida.

I have a question that maybe you can help. I need to make a ckt. to strobe 1 or 2 Ultra bright large L.E.D.'s. from radio shack. I need to have it adjustable from 800 Hz to 4000 Hz. I think that with a 50% duty cycle I can freeze what I am trying to watch. (an R/C airplane prop)

Can you suggest some values. for the variable resistor, and capacitor values?

I need to be able to freeze the prop outdoors, so I may have to add 2, 3, or 4 Ultra bright L.E.D.'s. to the output of the 555 timer. can it handle 100 - 120 Ma?
I will be powering the device with a 1200 mah 11.1 volt LiPo battery.

Thanks...

2. ### jackorocko

1,284
1
Apr 4, 2010
Put a transistor on the output to handle the current for the load. Also, you need to add in a diode to the 555 astable circuit if you want a <50% duty cycle.

http://www.kpsec.freeuk.com/555timer.htm#astable

Without doing the math, the table on that page suggest a 0.01uF cap R2 = 10K - 100K pot. R1= 1K - 10K pot.

Last edited: Feb 2, 2012
3. ### Rleo6965

585
9
Jan 22, 2012
I think you need you need 10 super bright LED for outdoor Stroboscope. You need a transistor to handle total current of 10 LED's. Of course place 1 current limit resistor in series.

Try this link. You can adjust length of LED turn on time which makes LED brighter.

http://danyk.wz.cz/ledstr_en.html

Last edited: Feb 2, 2012
4. ### Harald KappModeratorModerator

11,413
2,619
Nov 17, 2011
Rick,
I don't think 50% duty cycle will give you sharp images.

Assume a propeller of 10 cm diameter (sorry, I'm not into imperial units).
Assume the propeller turns at 5000 RPM.
Assume 800 Hz, 50% dutc cycle.
The LED will be on for 1/800 s * 50 % = 0.625ms.
The tip of the propeller moves at Pi*diameter*RPM = 3.14*10 cm*5000/s = 157000 cm/s. During the ON time of the LED the tip therefore moves 157000 cm/s*0.625 ms = 98 cm, that is approx. 3 full rotations -> blurrrrrrrrrrrrrrrrrrrrrrrrrrrrr
Now assume 4 kHz. ON time is 0.125ms. The tip still moves 19 cm. -> blurrrrrr (with a few 'r's less .

What you'll need is a lighting source (LED is fine) which emits very short pulses of light so you can "freeze" the propeller during rotation (think of a stroboscobic light). Therefore the duty cycle must be very low, more on the order of 0.1 % or less.

There is an upside to this: The mean power converted to heat within the LEDs sinks drastically. Look at the LEDs's datasheet and find out what the max. pulse current is. You may be able to pulse the LEDs with a pulse current much larger than the allowed average current, giving you very bright light pulses without destroying the LEDs (they can recover from the pulse power and distribute the thermal energy generated during the pulse in the 99.9 % idle time.

Regards,
Harald

Last edited: Feb 2, 2012
5. ### Harald KappModeratorModerator

11,413
2,619
Nov 17, 2011
Oops, I goofed.
Sorry.

My calculations are off by a factor of 60. RPM is in minutes, whereas the rest of the calculations is in seconds. Therefore divide the results by 60.
Thus 98 cm -> 1.6 cm -> still blurry.
19 cm -> 0.3 cm. -> could be better.

The good news is that insteaad of 0.1 % a duty cycle of 1% should be good enough and much easier to generate.

Harald

6. ### rickster581

6
0
Feb 2, 2012
Harald, Thanks so much... I was running a test prop on my delwalt drill which turns, about 2500 rpm. and I am preaty sure that I was at about 50% duty cycle. and I was seeing about 45 degrees of the prop as it turned not like flashing a strobe at a fan where you can adjsut the pot to stop the blades completely. and even make it look like it's going backwards. once I saw a blurry 45 degree patern I knew that the on time was too long. not sure I understand the calculation for the correct duty cycle. will the figures you gave me for the cap and vatiable resistor give me an operating window of 800 Hz to 4000 Hz?

Thanks again for your time, and help

7. ### rickster581

6
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Feb 2, 2012
thanks very much for your input...... i am working on it.

8. ### rickster581

6
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Feb 2, 2012
thank you for your time.... will keep you posted.....

9. ### rickster581

6
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Feb 2, 2012
Thanks to all for your time and help

Thank you all very much.

Rick Moro Jr.

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,489
2,830
Jan 21, 2010
When you're testing the circuit the last thing you want to do is kill the LEDs.

The first thing to do is to use a much larger series resistor so that the maximum LED current does not exceed the maximum allowed average current (say 25mA)

Then slow the oscillator down substantially (to the range of 0.1 to 1 Hz) by making the timing capacitor much larger. Once you do this, you'll be able to visually estimate the duty cycle. The difference between 99% and 1% will be very obvious.

Once you're happy that you have the correct duty cycle, crank the speed back up and change the LED circuit to allow higher current.

11. ### rickster581

6
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Feb 2, 2012

Thanks for the input I truly appreciate it.