# What does arrow in LED dimmer circuit mean?

Discussion in 'The Projects Forum' started by TSmith, Sep 21, 2010.

1. ### TSmith Thread Starter Member

Sep 21, 2010
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Please don't laugh, but I have very limited electronics knowledge. I'm trying to build a circuit for a simulated afterburner for a remote control jet. To do that I need to be able to dim a series of LEDs. I can do it by varying the amount of voltage to the LEDs but that does not give much of a range in the brightness. So I decided to stretch my abilities and build a circuit that will pulse the power to the LEDs to give a wider brightness range. However, I'm having trouble figuring out how to wire up the components. In the circuit below it shows an arrow coming out of pin 2 and 6 and pointing to a 50K resistor. What does that arrow mean? Do I run a wire from 2 & 6 to one side or the other of the 50k resistor?

If I can get this to work then I need to figure out how to power about 40 LEDs. The 555 can only handle 200mA which should be good for about 10 LEDs. 40 LEDs I think will draw about 800mAh to 1 amp. But one step at a time as I try to learn some basics here...

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2. ### retched AAC Fanatic!

Dec 5, 2009
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That is the middle pin or wiper on a 50k potentiometer.

3. ### TSmith Thread Starter Member

Sep 21, 2010
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But it's only a standard 50k resistor it is pointing to.

Dec 5, 2009
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5. ### TSmith Thread Starter Member

Sep 21, 2010
57
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Darn. I think I got the wrong circuit. The circuit I wanted to build controlled the LED brightness by varying the voltage to the circuit. The voltage to the LED remained constant but the frequency of the pulses changed as you changed the voltage. Guess I need to go back and see if I can find the correct circuit again. I had so many windows open I think I started working on the wrong circuit.

6. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
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What is your expected result? Do you want to vary the voltage on the LED or do you want to vary the brightness of the LED? This circuit will vary the brightness (perceived brightness) by varying the amount of time the LED is on by varying the pulse width of the output of the 555 timer.

7. ### TSmith Thread Starter Member

Sep 21, 2010
57
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If it is not too complicated I want to build a circuit using the 555 to pulse the output so that there is more perceived change in brightness versus just varying the voltage to the LEDs. To get a better idea of what I want to do with it, here is a video of the LEDs on a RC airplane:

It is very easy to vary the voltage using an extra speed controlled in the airplane tied into the throttle channel. But I thought I may get better results controlling perceived brightness using the 555 vs actual brightness with simply changing the voltage. The circuit with the 555 would need to change the pulse from around 0% at 0 volts to 100% at 9 volts. Might be going over my head to do that. I probably need to use between 20 to 40 LEDs. Half of them are red and the other half red.

8. ### retched AAC Fanatic!

Dec 5, 2009
5,201
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What color are the third half gonna be?

OK, so what you want, is a circuit that changes the brightness of 40 LEDs from 0 to 100% as your speed controller voltage changes from 0v to 9v?

Is this correct?

So 9v would be "Full Afterburner" and the LEDs would be brightest?

Mar 24, 2008
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10. ### TSmith Thread Starter Member

Sep 21, 2010
57
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Possibly blue LEDs for the third half.

Yes, you got it right. Although I could leave off the voltage regulator to the speed controlled used to power the LEDs so that the voltage range would be up to 12.6v. However, over the course of the flight the voltage would fall from 12.6 down to about 9.5v by the time it was time to land. So I thought it may be easier to use a 9v voltage regulator so that the max possible voltage to the LED afterburner system stayed consistent during the entire flight. I built a 9v regulator using a LM7809 and it worked. Not sure if 1 amp is enough for up to 40 LEDs. I guess I could build a 2nd one if needed as the circuit only weighs about 5 grams.

11. ### TSmith Thread Starter Member

Sep 21, 2010
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Everything I know about electronics (very little) I learned in a day browsing a few web sites so your diagram is scaring me Bill. The most complex thing I built was a voltage regulator using one chip and a couple capacitors. It may take me a while to figure out what you posted. I have to say it is fun being able to build circuits and could see this becoming a fun hobby. Unfortunately, I have a lot to learn yet to just understand the jargon.

12. ### Wendy Moderator

Mar 24, 2008
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The first link I gave you is a tutorial for LEDs and 555s. It does assume you understand Ohm's Law, which is a fundimental equation in electronics that never goes away.

If you haven't encountered Ohm's Law before it is in our AAC book here...

Chapter 2: OHM's LAW

Ohm's Law shows the relationship between voltage, current, and resistance, and is used with LEDs to figure out how much resistance an LED needs. All LEDs need a resistor or similar mechanism to keep from blowing themselves up, as they are current devices, and do not control how much current flows through them. That is the resistors job.

I have written a series of 555 tutorial for the AAC book...

Chapter 8: 555 TIMER CIRCUITS

Specifically I have a half written tutorial you might find useful...

Submission: 555 PWM Oscillator

Don't be worried about things you haven't studied yet. This site does a very good job bringing people new to the hobby up to speed.

13. ### TSmith Thread Starter Member

Sep 21, 2010
57
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I do plan to study the information in the links you provided. Who knows, maybe soon in addition to an LED afterburner I may end up with all other types of cool electronic gadgets on my airplanes! Since my eyes (and brain) are not as sharp as they were 30 years ago when I first got into the RC hobby, it is a bit harder at times for me to figure out what my airplane or helicopter is doing up in the air when they are further away. Perhaps powerful LEDs on the airplane would help me to stay oriented during flight. Now I can start a new hobby to help out with my old hobby.

14. ### campeck Active Member

Sep 5, 2009
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If your planes don't have port and starboard lights and headlights that would be super simple to employ.

15. ### TSmith Thread Starter Member

Sep 21, 2010
57
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What type of LED would you recommend that is very bright to be visible at a distance in daylight?

16. ### campeck Active Member

Sep 5, 2009
194
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Um well Its made slightly difficult by the fact that bright LED's actually use a good bit of current. Which means big heavy batteries, which is probably bad for your RC hobby.

But I would go with a wideangle +50lm one. Such as these.
http://cgi.ebay.com/5x-Green-1W-LED...ultDomain_0&hash=item4cf13cb36d#ht_3554wt_911

That one uses 300mA

I would also hook up a CMOS 555 at 50% duty cycle to pulse the LED's to save on power and reduce heat while keeping apparent brightness up. Although being an airplane you could probably deal with the heat with so much air going by...So you could possibly ramp up the current past the rated max of the LED to achieve a higher brightness if needed. maybe 500mA or so. It really depends on battery life and how bright you need them. experiment!

How many of what color LED's do you want?
What is the mAh of your battery?
What is the weight limit of this?
How long do you want to fly for?
And how bright is bright enough for YOU?

17. ### TSmith Thread Starter Member

Sep 21, 2010
57
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The airplane I am putting together now is an electric ducted fan F-16. It weighs about 20 ounces.

With the "afterburner" in the rear, it should be easy to see the airplane flying away from me. I'm guessing a bright white "landing light" LED in the front would suffice to see the airplane coming toward me. Depending on weight and power consumption, a bright red and green LED for port and starboard would be nice too. However, the more juice the light use, the shorter the flight time.

The batteries used will be 3 cell LiPo batteries rated between 1800 and 2200 mAh. Typically I like flights about 6 to 8 minutes long. I have not tested the motor / fan assembly for this airplane yet to determine how much power it uses. That reminds me that I do have a program that will calculate the power consumption of the propulsion system. I need to dig it up as I did not reinstall it after my HD crashed.

This little jet is smaller and faster than the other airplanes I normally fly. It only has a 26" wingspan and stock goes about 80-85 MPH. So I can see it getting very small in the sky very quickly. Therefore, the brighter the better! What amazes me is some guys put a 5 cell LiPo in this kit, a more powerful fan unit, and have been clocked at over 175 MPH. My reflexes are too slow for that kind of madness. This video shows the size of the airplane and how quickly it disappears into the sky.

18. ### retched AAC Fanatic!

Dec 5, 2009
5,201
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Uww... Wow.

I dont trust my shaky hands enough to fly something past myself (or others) at 177 MPH.

A little "twitch" would spell disaster.

That is amazingly fast. Thats 2kw of power on a model sized turbine.

Thats some power.

Ok, to your LEDs.. 1w LEDS will draw around 350mA happily, but not without heatsink.

Im guessing their will be plenty of air passing by these things, especially if you are putting them in the jet wash.

It would be interesting to see how you would keep these superbrights cool for 6 to 8 minutes.

A PWMed circuit would be a must if you are using 5 1w LEDs. You are also going to have to do some experimentation with how "dim" you can operate them for cooling and battery life purposes.

Where are you mounting the afterburner LEDS?

19. ### campeck Active Member

Sep 5, 2009
194
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You know, if you are going to go high power LEDs on the front, port, and starboard, you might as well do them for the rear.
So lets say 300mA x 5 = 1.5A
2.2Ah/1.5A = 1.46hrs = 1:27 of light time. Pulsed at 50% duty cycle doubles that. So 3 hrs running off the battery alone at best. (I did my math right...right?. lol)
Let us know what your Sim says.

20. ### retched AAC Fanatic!

Dec 5, 2009
5,201
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I am waiting to see what that turbine pulls from those batteries.