Flashing a lot of LED's.

[audioguru]

If your LEDs have a max allowed current of 30mA then use 25mA to be safe.
Add the forward voltages of the LEDs (each LED will have a different voltage since the spec'd voltage is a range of voltages) and subtract the total from the supply voltage to have the voltage across the current-limiting resistor, then use simple Ohm's Law to calculate the current-limiting resistor.

Post the datasheet of your LEDs.

 

[voodoo5293]

Here is the tech doc. I was re reading it, and I really couldn't make to much sense of it. But If I am reading it right, it looks like the forward voltage is 1.9, not 2.2. If you guys can make any more sense of it, I would be much appreciative. Oh yeah, and the LED in question is the HLMP-EG15-RU000.

 

[voodoo5293]

Hi vodoo,

I like cool electronics, meaning no hot running ICs, and even power supplies I designed for industrial applications don't run hotter than 40deg/C.

Here is my suggestion: Driving 30 LEDs from a 12V power source you might connect up to five LEDs in a string resulting in six strings for 30 LEDs.

Calculating the voltage drop of five LEDs per 2.2V the required forward voltage will be exactly 11V. Considering the worst case of 0.7V voltage drop in a transistor there is a leftover of 0.3V to "play" with. Using 20mA forward current for the LEDs the current limiting resistor for each string would then be 15Ω.

Without any doubt a timer IC NE555 can drive six transistors with a base current of 2mA each.

Each transistor (BC337) can handle up to 600mA collector current. Again this is very safe since the collector current won't be higher than 20mA.

Here is a schematic with a timer circuit adjustable from 1 to 2Hz.

Boncuk

Thank you for this. I didn't catch this post last night. The schematic was awesome, except for it is a bit to slow. What resistors and capacitors would I need to change in that schematic to change the frequency?

 

[Andy1845c]

The NE555 IC controls the frequency. If you do a quick google seach on them, you will find alot of info on how they operate.

Here is a quick and east calculator to calculate frequency. Note the frequency is given in kilohertz.

**broken link removed**

R1, R2 and the capacitor all work together to adjust the frequency and duty cycle.

 

[voodoo5293]

I have been doing some reading on the 555. It looks like all really need are two resistors, and a capacitor to make it astable. I believe that is what I want. If I use a high (100-500k)pot in place of R1, and make R2 about 100k, and about .5uF for C1, I should be able to make the frequency anywhere from about 6 to like 15 hz. Unless I messed up in my math.

 

[MrDEB]

frequency of 555

with the 100K,100K,.5uf = 9.6 hz
with 500K,100k, .5uf = 4.11 hz

here is a pretty easy caculator I use
**broken link removed**

 

[voodoo5293]

I checked out that calculator. So if I use a .3uF capacitor, then it should give me between 7hz and 16hz with a 500k pot for r1 and 100k resistor.

How exactly does a pot work? Does it have like a maximum resistance at one end, then go down from that? Like if I get a 500K pot, would it go all the way down to 100K? How would I pick out a pot that has a variable resistance in that range?

Thanks again guys. You guys are really helping out a lot. It is all starting to make a lot more sense to me. I am going to work on a schematic tomorrow morning and throw it up on here so you guys can let me know how it looks and if it will work. Thanks again. You guys are great.

 

[Boncuk]

I checked out that calculator. So if I use a .3uF capacitor, then it should give me between 7hz and 16hz with a 500k pot for r1 and 100k resistor.

How exactly does a pot work? Does it have like a maximum resistance at one end, then go down from that? Like if I get a 500K pot, would it go all the way down to 100K? How would I pick out a pot that has a variable resistance in that range?

Thanks again guys. You guys are really helping out a lot. It is all starting to make a lot more sense to me. I am going to work on a schematic tomorrow morning and throw it up on here so you guys can let me know how it looks and if it will work. Thanks again. You guys are great.

Don't you think blinking two lights alternating at a frequency of 7 to 16Hz will look a bit nervous? Remember that you are dealing with filament type lamps which will glow dimly because they never get full power and never zero power for their filaments to light up and extinguish.

If you want a variable resistance from 100K to 500K you should combine a fixed value transistor and a trim pot. Use 100K fixed value and 470K trimable. (These are standard values).

Boncuk

 

[Nigel Goodwin]

Where do you buy a 470k trim pot?

Any component supplier - it's a standard component (may also be called 500K).

 

[Boncuk]

Thank you for this. I didn't catch this post last night. The schematic was awesome, except for it is a bit to slow. What resistors and capacitors would I need to change in that schematic to change the frequency?

If you change the value of C3 to be 330nF the frequency comes pretty close to your desired rate and span.

Boncuk

 

[Hero999]

There seem to be three preferred value series for potentiometers.

10, 25 and 50.
1, 2 and 5.
10, 22 and 47.

Most manufacturers will use either one of the three listed above.

Since most potentiometers have such a wide tolerance, it's perfectly acceptable to use a 500k instead of 470k or 20k in stead of 22k or 25k.

 

[voodoo5293]

Don't you think blinking two lights alternating at a frequency of 7 to 16Hz will look a bit nervous? Remember that you are dealing with filament type lamps which will glow dimly because they never get full power and never zero power for their filaments to light up and extinguish.

If you want a variable resistance from 100K to 500K you should combine a fixed value transistor and a trim pot. Use 100K fixed value and 470K trimable. (These are standard values).

Boncuk

Blinking two lights? I am going to be blinking all the lights at the same frequency, not an alternating frequency. That is the main reason I want it to be variable, so I can adjust the speed to what seems like a good rate. 6 to 15hz seems like it should be doable with just a .3uF capacitor, a 100k resistor for R2 and a variable Pot for R1.

Also,, I don't get what you mean by combine a fixed value transistor with trim pot. How would I do that? Did you mean a fixed value resistor?

 

[audioguru]

Some people have an epilepsy attack when they see lights flashing at the frequency you want.

 

[voodoo5293]

Some people have an epilepsy attack when they see lights flashing at the frequency you want.

Seriously? It isn't like they are going to be staring at it. It is going to be mounted on the back of a bike. I highly doubt anyone will be looking at it long enough to induce an epileptic fit.

 

[voodoo5293]

You could put 4 red 2.2V LEDs in series which is 8.8V and in series with a current-limiting resistor that will have 3.2V across it. But the LEDs will not all be 2.2V. Some might be 1.8V and others might be 2.4V. You will need to measure them and sort them into strings. Each string might need a different current-limiting resistor which is calculated by Ohm's Law.

I was curious about how you would go about measuring the forward voltage of individual LED's? I think I know how, but I am not sure.

 

[audioguru]

You are using 30 LEDs on the back of a bike? Most blinking LEDs on the back of bikes use only 3 or 4 LEDs.
You will need to haul a trailer to carry the battery.

EDIT:
The viewing angle of your LEDs is too narrow at only 15 degrees. They should be 30 degrees or more.

 

[Andy1845c]

I was curious about how you would go about measuring the forward voltage of individual LED's? I think I know how, but I am not sure.

Its pretty easy.

Credit to philba for posting that in another topic.

Make sure you size the resistor so the led runs at your target current (If).

 

[voodoo5293]

You are using 30 LEDs on the back of a bike? Most blinking LEDs on the back of bikes use only 3 or 4 LEDs.
You will need to haul a trailer to carry the battery.

EDIT:
The viewing angle of your LEDs is too narrow at only 15 degrees. They should be 30 degrees or more.

Hmm. Well that is more why I was wanting help with how to design the circuit than with actually designing the circuit. I don't have my parts or anything finalized yet. I may try some of these LED's, I am thinking that for my use, the beam from the LED's should be pretty wide at a hundred feet or so away. If not, I can always go to a wider LED.


As for the battery, I should imagine a small motorcycle battery would suffice. Maybe a couple 6V lantern batteries in series depending on how long they last? It isn't like it will be used for hours and hours at a time. If anything, maybe 8 NiMh AA batteries in series, so I can recharge them. It should be drawing no more than a few hundred mA, and some 2500 mAH batteries should be able to run it for a good 4-5 hours. Then I can just recharge them. Unless I am way off in my rough calculations, I should be ok as far as batteries go. Let me know if you see any errors in my reasoning.

 

[voodoo5293]

Its pretty easy.

Credit to philba for posting that in another topic.

Make sure you size the resistor so the led runs at your target current (If).

Don't you have to know the Vf to size the resistor? Regardless, that doesn't make a lot of sense to me? Where would I put the probes to find the forward voltage? On each side of the LED? Or would I take a measurement of the voltage from V+ to ground, before I added the LED and resistor, then after I add the LED and resistor, then subtract after from before? As for the resistor, I assume I would just use the typical Vf for that LED to size it?

Thanks again for answering all my newbie questions. I just want to make sure I have a good understanding before I start building anything.

 

[audioguru]

Red LEDs are 1.8V to 2.2V.
8 Ni-MH cells are 9.6V and drop to 8V when they are nearly dead.
So three LEDs in series make 5.4V to 6.6V and when the battery is 9.6V and the current is 25mA then the current limiting resistor is (9.6V - 5.4V)/25mA= 168 ohms. The nearest standard value is 175 ohms and then the current is 24mA.

If the LEDs are 2.2V then their current with a 175 ohm current-limiting resistor is 17mA.
If the LEDs are 2.2V and the battery is nearly dead at 8V then the current is only 8mA.

When the LEDs are 1.8V then a charge on 2500mAh battery cells will last for 2500/(24mA x 10)= 10.4 hours. If they blink then they might be turned off for half the time then the battery will last twice as long. Higher voltage LEDs use less current and are dimmer so the battery will last longer.