# Best practices using 555/4017 & many LEDs

1. Aug 1, 2012

### Freddythunder

Hello,

I'm new here.. I have a question about running a lot of LEDs in a couple scenarios. I'm building a full-size lightbar for a fake police car to be used in movies. I found a schematic for the red & blue LEDs that I'm going to duplicate using some 555s or 556 here:
http://www.instructables.com/id/41-LED-Flasher-Circuit-using-555-IC/

But I would also like to (easily & cheaply on a no-production-value film budget) get yellow LEDs on the back that would show direction by illuminating at one end and moving across. I've made this before using a 555 and a 4017, but only one LED at a time. I want to make it realistic so I'm dropping about $80 in LEDs so each segment would be 6-10 (30mA) LEDs instead of just one little one. What would be the best, cheapest and most reliable way to do this? Can I use a transistor for the added current or a relay of some sort? I've been searching on and off for months for this solution, but I don't think I know the proper nomenclature to find what I am looking for. Thanks! 2. Aug 1, 2012 ### I_am_learning Both transistor or the relay will do. If you use relays, then you would need to freewheeling diodes to prevent from inductive over-voltage. Or you could use transistors just as well. Make sure the transistor can provide the current required, and you bias the base with appropriate current. I think using transistors would be cheaper and more reliable . And, welcome to PF. 3. Aug 1, 2012 ### mdjensen22 You probably don't want to switch relays as fast as your application will want. In addition the power supply requirements are much higher for driving relays as opposed to driving transistors. Go with the transistors. 4. Aug 1, 2012 ### Freddythunder Awesome, transistors it will be. Thanks for the help. I think that opened a lot of doors. This whole time I had never googled "transistor as a relay" (duh) and got a bazillion results. I'm going to read a few thousand of those and then I'll come back if I have any questions. Thanks! 5. Aug 1, 2012 ### Freddythunder Actually, hang on, I have another question. I ran across this before about a year ago: http://wild-bohemian.com/electronics/flasher.html I remember that transistor configuration to make LEDs blink. I actually tried to get my dad to look at it (actually went to school for this stuff... I didn't) and he never got back to me. Since the LEDs are on the positive side of the transistors, is it feasible for me to run an array of LEDs off of this or will I blow up the transistors because the current still needs to course through them. This is for a car so I'll have tons of current at 12-14 volts. ...Or should I stick to the 555/6 flashers and make arrays through transistors? Thanks again! I'm very happy I found someone who can help me with this stuff! 6. Aug 2, 2012 ### NascentOxygen ### Staff: Mentor How many segments will comprise the full arrow? Each segment being made up of 6-10 LEDS one above the other in a vertical line? 7. Aug 2, 2012 ### NascentOxygen ### Staff: Mentor How many LEDs do you plan to combine for the blue light? That circuit as it stands can probably power at least double the number shown when you operate it off 12 volts, so maybe 40-50 blue LEDs. Much the same for the red. 8. Aug 2, 2012 ### Freddythunder I really want to do this right and have it look as non-cheesy as I can, so I'm actually upping the amount of LEDs from what I said before and doing it right. So, I'd like to have 80 red on one side, 80 blue on the other. If I could use a power transistor or some kind of configuration for this circuit: http://www.555-timer-circuits.com/police-lights.html (noted the wrong one before - sorry!!) The yellow ones in the back would be 10 segments of 20 LEDs each segment. I have put together an online model to explain in full detail what I would like to create. To activate it, click the grey buttons labeled "code3" and "yellow" http://www.freddythunder.com/film/lightbar.html [Broken] Then once all that is done, if I want the yellow lights to blink like the red/blue ones, I'll just duplicate that circuit. I'm thinking that each section of LEDs will be a module with wires coming off of it and I can build a "brain" into one circuit board to control everything with a few 555/4017 combos. Whaduyathink? Last edited by a moderator: May 6, 2017 9. Aug 2, 2012 ### NascentOxygen ### Staff: Mentor In that circuit, they are supplying one of the colours with ten times the current as the other. I haven´t played the video, does it tell you which of their LEDs needs the greater current? On your animation, I think you have the blue flashing differently to the red. In any case, it is too fast for me to figure out: are you wanting to flash any blue simultaneously with any red? Maybe you could slow it down to one quarter actual speed? Last edited: Aug 3, 2012 10. Aug 3, 2012 ### Freddythunder No I'm not sure why any of them would need 10 times the current. I'm too much a noob. On the animation, the way I'm thinking, there's 2 blue and 2 red sections that fire and blink and then shut off, then it alternates to the remaining 2 blue and 2 red sections. The 555 sends a box wave (right?) to the 4017 making it alternate which 'side' fires. I'm sure I can get the circuit to work like the video shows, but my only question is can I use a bigger transistor and run 80 LEDs on each side of that circuit, or do I have to make multiple circuits. As for the speed & the way they blink, quite frankly, I don't care. Just as long as they blink. It's just special lighting for the movies ;) Thanks for your help 11. Aug 3, 2012 ### NascentOxygen ### Staff: Mentor A pair of transistors should be able to drive 40 LEDs, at least. But first we need to know the voltage and operating current of your LEDs, an this will differ for red and blue. Have you chosen the LEDs? There are basically two choices: standard LEDs and high brightness LEDs, the latter costing more and using more current. Different coloured LEDs differ in their efficiency at producing light, and the eye has different sensitivities between red and blue, so to have similar apparent brightnesses it can be desirable to power the different coloured LEDs with different currents. The 555 generates a square wave. 12. Aug 3, 2012 ### NascentOxygen ### Staff: Mentor 13. Aug 3, 2012 ### Freddythunder Thank you for those links! I would have loved to [see if my budget allowed to] get those instead with the larger viewing. I have already bought 110 of the LEDs already from here: http://ledshoppe.com/led10mm.htm [Broken] The white and blue are: Emitted Colour : WHITE Size (mm) : 10mm Lens Colour : Water Clear Peak Wave Length (nm) : N/A Forward Voltage (V) : 3.2 ~ 3.8 Reverse Current (uA) : <=30 Luminous Intensity Typ Iv (mcd) : Average in 750 00 Life Rating : 100,000 Hours Viewing Angle : ±10° Absolute Maximum Ratings (Ta=25°C) Max Power Dissipation : 80mw Max Continuous Forward Current : 30mA Max Peak Forward Current : 75mA Reverse Voltage : 5~6V Lead Soldering Temperature : 240°C (<5Sec) Operating Temperature Range : -25°C ~ +85°C Preservative Temperature Range : -30°C ~ +100°C and the red/yellow are: Emitted Colour : Red Size (mm) : 10mm Lens Colour : Water Clear Peak Wave Length (nm) : 640 ~ 645 Forward Voltage (V) : 1.8 ~ 2.2 Reverse Current (uA) : <=30 Luminous Intensity Typ Iv (mcd) : Average in 10000 Life Rating : 100,000 Hours Viewing Angle : ±10° Absolute Maximum Ratings (Ta=25°C) Max Power Dissipation : 80mw Max Continuous Forward Current : 30mA Max Peak Forward Current : 75mA Reverse Voltage : 5~6V Lead Soldering Temperature : 240°C (<5Sec) Operating Temperature Range : -25°C ~ +85°C Preservative Temperature Range : -30°C ~ +100°C These LEDs have a very narrow viewing angle, but they are as bright as the sun. I'm thinking with the right blinking speed combined with camera height, shutter speed and aperture settings, I'll get real close enough to one of the new$500 lightbars on real cop cars - at least on film.

Thanks!

Last edited by a moderator: May 6, 2017
14. Aug 3, 2012

### Staff: Mentor

Those blues are so dim in comparison with your reds that you may decide to buy brighter blues. Could you power a handful side by side with an equal number of reds, and view them head-on from a distance to see how they'd go? The yellows and reds sound good.

At 10mm, they are big ones. You've bought 100, but on the figures you gave you'll be needing at least 400?

Yes, you could work within viewing angle restrictions by always filming head-on.

15. Aug 4, 2012

### Staff: Mentor

Just check your figures on your whites and blues, as that site currently doesn´t list 750 mcd LEDs, certainly not in 10mm.

At first glance, I think it will be feasible to use a driver module, and with care operate parallel strings for the blinking LEDs, e.g., the decade counter can control two 4015 modules each driving 84 LEDs to produce the blinking. To keep down cost, the yellow LEDs can be switched by transistors.

Have a look at the 4015 BoostPuck here. Without boosting the +12v from the car battery, you´d be limited to powering 3 of the blue LEDs in a string; by boosting it to +28v you can power 7 blue LEDs per string.

16. Aug 11, 2012

### Freddythunder

Okay. Sorry for the delay. Got laid off from my job and have been job hunting ever since.

I read somewhere on here that I can't count and it seems I cannot. Sorry my led count was off. But it looks like I still have some planning to do. I took a look at a real lightbar a couple days ago and I think the LEDs are ten on top, five on bottom so it may be less LEDs anyway.

So what you are suggesting instead of using the bc547 transistors (or alternate higher power transistors) I could use these boost puck thingies? Looks like a new world.

And can you show me how I would get the yellow ones to work like that using just transistors? Maybe a link to a tutorial or something?

I'm real close to just making the red and blue blink back and forth with just transistors to save time and money. As for the brightness, no matter what the numbers say, both the red and blue are bright enough for filming. They may actually be too bright and I may have to use larger resistors, that is if I ever get that far.

Thanks for the help!

17. Aug 12, 2012

### Staff: Mentor

Nasty. Not something you saw coming?
I was suggesting this, it would save transistors and resistors and a lot of connections. However, in light of your opinion that at full current the LEDs may be too bright, this eases the design constraints I was working within.

It's tempting to connect 3 of the blue LEDs in series with a resistor and power that string from the +12v. But there's a difficulty in designing for say, 22mA of LED current at a battery voltage of +12, while avoiding having that current soar to almost double that value as the battery warms up under charge and its terminal voltage rises to around +13.5v.

That regulator module I pointed to would have solved this. However, until you work out precisely how many LEDs need to be powered in unison, this can remain as "to be decided".

Regardless, some tests for you: connect one of your blue LEDs in series with a 390Ω resistor to a +12v supply, and record the voltage across the LED. Now substitute one at a time another fifteen blue LEDs, recording their voltage each time. That will indicate the voltage spread to allow for.

Can you do the same for your YELLOWS and, separately, the REDS.

At this stage I'm needing to know how many RED and BLUE together will flash in unison, to see whether they can all be economically powered via a single transistor.

As for the job hunting -
http://img502.imageshack.us/img502/7752/goodluckemoticon.gif [Broken]​

Last edited by a moderator: May 6, 2017
18. Aug 12, 2012

### Freddythunder

As for the job, I had an idea it was coming soon. But two Fridays ago my boss said not to worry there was plenty of work and then Monday the company couldn't pay me anymore. I got feet on the pavement though.

Anyway, I'm not sure I understood what you meant about testing voltages. I started with a 330 ohm resistor and one blue, the voltage was 3.29. 2 went to 3.18. When I got to three, the first led was 3.05 and the third was 3.18.
4, 1st was 2.92 and the last was 3.06
5, 2.76 and 2.86
6, 2.54 and 2.56
By the time I got 7 LEDs hooked up in series, it was just under 2 volts and they were barely on. At that point I figured I'd check to see what you ment cause I think I'm testing things wrong.

I'm going to do some camera tests tonight to find out exactly how many LEDs will be in each section. I have 10 red and ten blue plugged into my breadboard just running solid at I think 30ma each. Its painful to look at so I might drop it down to 10 LEDs per section as its for movies and not real traffic visibility. I'll keep you posted!

19. Aug 12, 2012

### Staff: Mentor

By "substitute" I mean remove the first LED and replace it with a different one, so that at any one time there is just the resistor and one LED. Different LEDs may have slightly different voltages, those differences are what I'm interested to know.

Since you're starting over, can you use a 470Ω resistor this time, the results will be a little more representative; I'm getting the feeling we can aim for less than the full 30mA.

To answer an earlier question in this thread, I'm sure you'll be able to operate all the sequences off the one 555/4017 counter, you won't need to duplicate the clocking part. This means your sequences will all be fully synchronized.

Just confirm: the yellow strip comprising 10 blocks, when it's activated, will have the left-most block constantly lit, and the others will sequentially fire and each then remain lit until the right-most block has been lit, then 9 blocks will go dark together but the left-most block will never go dark at any time?

(Suggestion: if that is the case, then it may be worth making the yellow strip 11 blocks, since the counter can operate 10 and your left-most block does not need to be clocked if it is to stay lit constantly.)

I haven't been able to figure out the whites—what they do, why they are located where they are, and how many there'll be.

20. Aug 12, 2012

### Freddythunder

I really appreciate your help with this. I really have no one else to ask this stuff to.

Okay, now I know what voltages you need. Here are the results with the 470 ohm resistor (yellow purple brown)
Yellow, 2 volts
Red, 2.1 volts
Blue, 3.27 volts

That is awesome that the red and blue is only one 555/4017 they cab be synced, or not, won't matter.

As for the yellow, that is completely right about the sequence. I think it would be more realistic if the first one on the left shut off when it resets, but whatever gets me close enough is close enough.

I'm going to go test the 20 LEDs I have on the breadboard to see how it looks on the camera. After that I'll post back exactly the final count of how many LEDs