Long Duration LED Flashers

This is a continuation of a thread I saw a while back, which got me thinking. Several problems with some concepts I had were pointed out, so this is my corrected diagram. I've got all the parts, I'm planning on breadboard this using 2 AAA batteries and see how it lasts. I'm also planning on building a different version for Blue LEDs, with their higher voltage drops.



Parts List
U1 - CMOS 555
Q1,2 - 2N2222
D1 - High Intensity Red LED
C1 - 1 µF
R1 - 3.0 MΩ
R2 - 47 KΩ
R3 - 27 Ω
R4 - 18 KΩ

If this format looks familiar it will eventually wind up as part of my 555 series of experiments. I'm looking for holes in my logic. Thanks.

Hi Bill,
Your schematic shows PNP transistors. The 2N2222 is an NPN transistor.

DOH!

**************

OK, fixed. Thanks.

Let's go through the math. The basic 7555 circuit is designed for a 30ms pulse every second. I picked this because of persistance of vision. I'm also aiming as high as practical for every resistance I can to reduce current draw. This is why I went with the Darlington's, to reduce the base current needed.



This is the modification I'm using for the blue or white LED.

I'm picking C2 based on Radio Shack parts, otherwise I would have gone with 300µF or so. The Blue LED doesn't conduct at 3V, so it is basically out of the circuit. The capacitor charge to full value of the battery through R5 (calculated using 5 RC time constants), and when the transistors switch on the - side of C2 goes to -3V, which drops 6V across D1 through R3. The max current is in the neighborhood of 30ma, and the pulse should last the full 30ms.

This design strongly resembles the LM3909.

Any holes in the math or logic? Thanks.

Bill_Marsden said:

Let's go through the math. The basic 7555 circuit is designed for a 30ms pulse every second.

Click to expand...

The cap should be designated as a low-leakage type, or one that's been recently reconditioned. An electrolytic cap that has not been recently reconditioned may have too much leakage current to allow charging high enough to trigger the timer.

I picked this because of persistance of vision.

Click to expand...

Keep in mind that PWM is used to dim LEDs. I got bitten by this very same item on a similar experiment several months ago. If the flash is brief, it must be very intense or it won't make an impact. From my rough calculations, if you were using an old-technology red LED that had a Vf of 1.7v with that 27 Ohm resistor, the current through the LED would be around 48mA. More modern red LEDs might have a Vf of 2v to 2.2v.

My original calculations were for a high intensity at 2.0V @ 30ma, figuring .2V drop for the transistor. For a 2.2 Vf I'm figuring 22ma for a high intensity, and around 48ma for a old style LED. Think I need to drop the resistor?

I didn't think I'd need to worry about it for such a low value cap. 1µF should be pretty low leakage I would think, especially if bought new from Radio Shack.


... 272-1055 ..... 272-1434

The 1st one is probably the better of the two.

What do you recommend as a reasonable duration for the flash?

Parts List
U1 - CMOS 555
Q1,2 - 2N2222
D1 - High Intensity Blue LED
C1 - 1 µF
C2 - 470 µF
R1 - 3.0 MΩ
R2 - 47 KΩ
R3 - 82 Ω
R4,5 - 18 KΩ

Hi,


You have to turn the (blue) LED around if you want it to emit light.


You won't be able to run the batteries too far down if you're using a darlington, due to it's ce-drop, which is at least ~0.7V more than a regular (i.e. a non-darlington) transistor.
With a drop of say 1.2V, a LED_Vfwd of 3.6V you need at least 4.8V plus a bit for current regulation, taking the number to at least 5V (and that won't be very stable, considering the changing battery voltage over time.
Add to this that the "doubler" only doubles as long as you don't load it - but even if a 100% efficiency is assumed, you cannot run the circuit below around 1.25V/cell (so you can totally forget Nixx cells as a source).

It might be an idea to let each pulse ping a resonant (LC) circuit to drive the LED (a pulse transformer could be used for the "L" if needed).


Out of curiosity: What are your design goals for this circuit?

He's trying to more-or-less emulate an LM3909.

I agree, the transistors he's using could be replaced with a single 2N4401 with much lower Vce and decent gain.

Hi,

SgtWookie said:

He's trying to more-or-less emulate an LM3909.

Click to expand...

Then Bill might wanna take a peek at http://home.cogeco.ca/~rpaisley4/LM3909.html and http://www.redcircuits.com/Page87.htm

Søren;104729 said:

Then Bill might wanna take a peek at http://home.cogeco.ca/~rpaisley4/LM3909.html and http://www.redcircuits.com/Page87.htm

Click to expand...

Yes, I know ... but he wants to do it with a CMOS 555 timer.

Hi Bill,

On the blue LED flasher, every time the transistor conducts you are effectively loading the battery with a 82Ω resistor. Because of the long the timing of the 555 happens with 3MΩ/1μF, perhaps you can increase this resistor to a higher value to save some juice from your batteries.

Also please note the polarity of the LED.

I'm not sure about the conductance drop for the CE of Q2, I think it will be around .2V. I'll have to run an experiment to verify this. Basically the reason I want the Darlington is to reduce the Base Emitter current required. I may have to do a rethink on this.

Yes, I had a brain fart on the LED, I'll fix it.

I'll get back with ya'll on this, thanks for the input.

<sigh> Looking at it I can see some other issues, notably the hard ground when the transistors conduction takes out the current path I was thinking about. Back to the drawing board. Dagnabit, it ain't that hard...

Thanks.

OK, I ran some experiements on the transistors using this as a reference...



The results for #1 were interesting.

Vcc = 8.2VDC ..... TP1 = 1.19V
Q1,2 = TN2222A .. TP2 = .63V
R1 = 20KΩ .......... TP3 = 6.3V
R2 = 330Ω

I tried a 75KΩ for R1, it didn't turn the transistors on all the way.

I looked at the specs for 2N4401, they didn't look that much better on gain, why do you recomment them Wookie? Both the 2N2222 and 2N4401 meet my basic criteria, avaible from Radio Shack (remember, this will be the AAC book experiment) and I have them in plenty.

I'll be running several experiements to pick some values for these parts.

#2
Vcc = 8.1VDC .TP1 = 0.68V
R1 = 2.4KΩ ... TP2 = 0.01V
R2 = 330Ω .... TP3 = 6.18V
Q1 = TN2222

For the heck of it,
#1
Vcc = 8.1VDC . TP1 = 1.3V
R1 = 2.4KΩ .... TP2 = 0.64V
R2 = 330Ω ..... TP3 = 6.10V
Q1,2 = TN2222

2222 vs 4401
In this case, I got "bit" by an old SPICE simulator, which had lead me to believe that 4401's had more gain than 2222's.

You could actually use 2N2222/PN2222/MPSA2222, 2N3904 or 2N4401 in your circuit without noticing much (if any) difference. These three transistors are generally included lately in Radio Shack's "NPN Transistors (15 pack)" Model: 2N2222 (5 of each); however if you should happen to get an older assortment, your mileage may vary significantly, as they used to include "house marked" transistors.
http://www.radioshack.com/product/index.jsp?productId=2062586
The Radio Shack assortment is an expensive way to build a transistor collection, seeing as how you can buy them individually from someplace like Digikey or Mouser for about a dime apiece. But if you're in a hurry, RS just might have them on hand in a local store.

The 2N3904 is widely used in audio circuits, but it doesn't have the current handling capabilities of the 4401 or the 2222. It runs out of steam right after 100mA. The 4401 and 2222 can handle several times more current.

Hi Bill,
Your transistor tests show that either your transistors are failing counterfeits or their collector and emitter are connected backwards.

For the single transistor you have a collector current of 18.7mA and a base current of 3.1mA. The transistor should saturate with a base current of only 1mA or less.

Your darlington transistors failed to saturate when their beta was 183.
An MPSA13 and MPSA14 little darlington transistor saturates well with a minimum beta of 1000.

The LMC555 and TLC555 Cmos ICs have little high output current. The low output current is 4 to 5 times more so an ordinary high gain PNP transistor like a 2N3906 should be used.

Hi Bill,

Why are you using 8V instead of 3V (2x1.5 batteries in original project schematic) to test out your transistors design?

Things will be a whole lot difference when you reduce the available voltage down to less than 3V.

Audioguru said:

Hi Bill,
Your transistor tests show that either your transistors are failing counterfeits or their collector and emitter are connected backwards.

Click to expand...

If they were connected backwards, TP2 for circuit 2 would be a lot higher.

For the single transistor you have a collector current of 18.7mA and a base current of 3.1mA. The transistor should saturate with a base current of only 1mA or less.

Click to expand...

Well, it looks to me like it's pretty well saturated. He's only measuring a Vce of 10mV.