Flashing a lot of LED's.

[colin55]

If you drive the LEDs at 25mA for 25% duty cycle, the current consumption is less than half but the illumination appears to be as bright as a constant current due to the persistence of vision and this just applies to the ON period. The frequency of operation can be 2kHz or more.

 

[audioguru]

If you drive the LEDs at 25mA for 25% duty cycle, the current consumption is less than half but the illumination appears to be as bright as a constant current due to the persistence of vision and this just applies to the ON period. The frequency of operation can be 2kHz or more.

2kHz is much too fast for visible blinking,
Your method of using pulse-width-modulation dims the LEDs (which is how I dim the LEDs in my LED projects) because our vision averages brightness and does not work like a peak detector.

 

[Andy1845c]

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.

Sorry. Poor wording on my part. I should have said check the Vf at close to your targeted If. Your correct, you do need to know the exact Vf to do the math to get exact If. Basically what I am getting at is the Vf will change a bit with the current, so don't measure them at 10mA if you are planning to run them at 30mA.

The DMM probe sould go on either side of the LED, anode and cathode.

 

[voodoo5293]

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.

Well then, that seems fine. I would obviously have to run 10 strands of 3 as opposed to the other methods, but it should be able to work for about 5 or 6 rides before I need to recharge them. For whatever reason I was thinking NiMh was 1.5V in my rough calculations.

You guys are loosing me with the PWM talk. I know what PWM is, I just don't quite get how it applies to dimming an LED. Is that a viable option? If so, would it extend battery life? Thanks.

 

[audioguru]

Dimming the LEDs reduces their current so the battery charge lasts longer.
Reducing the number of LEDs also dims the total amount of light and reduces the current.

 

[Andy1845c]

You guys are loosing me with the PWM talk. I know what PWM is, I just don't quite get how it applies to dimming an LED. Is that a viable option? If so, would it extend battery life? Thanks.

It doesn't really dim them, it just makes them look dim to the human eye.

It would extend the battery life because the LEDs will not be on 100% of the time, but rather will be flashing very rapidly. Faster then the human eye can detect. Adjusting the duty cycle (on time vs. off time) will make them brightness look different. The PWM would be applied to the power the LEDs are running off of and you would still have to have a way to do your 6Hz blinking to get the effect you are looking for.

I don't know if I was able to explain that very well.

Personally I would forget PWM for this project as it will just make it more complex. Its no fun when you get bogged down in too many details and things get more complicated then they need be.

 

[colin55]

2kHz is much too fast for visible blinking,
Your method of using pulse-width-modulation dims the LEDs (which is how I dim the LEDs in my LED projects) because our vision averages brightness and does not work like a peak detector.

Let me correct you here:

I have designed hundreds of circuits and sold thousands of kits, so my statements are correct.
The blink rate or flash-rate is a few Hz. I am not changing this. When the LEDs are illuminated, we introduce the clever concept of turning them on and off very quickly so that they appear to be ON. We drive them at a few kHz so they do not appear as striations when moving across the viewer’s area of sight. You can drive them at any frequency above 100Hz for this feature to take affect but at low frequencies they can be seen to oscillate when the tail light is waved in the air.
If the LED current is increased slightly, the brightness of the LEDs will increase and this brightness will be seen as "constantly illuminated" due to the phenomenon called "Persistence Of Vision."
Before you make any further comments, try it.

 

[voodoo5293]

Sorry. Poor wording on my part. I should have said check the Vf at close to your targeted If. Your correct, you do need to know the exact Vf to do the math to get exact If. Basically what I am getting at is the Vf will change a bit with the current, so don't measure them at 10mA if you are planning to run them at 30mA.

The DMM probe sould go on either side of the LED, anode and cathode.

Thanks for clarifying.

 

[voodoo5293]

It doesn't really dim them, it just makes them look dim to the human eye.

It would extend the battery life because the LEDs will not be on 100% of the time, but rather will be flashing very rapidly. Faster then the human eye can detect. Adjusting the duty cycle (on time vs. off time) will make them brightness look different. The PWM would be applied to the power the LEDs are running off of and you would still have to have a way to do your 6Hz blinking to get the effect you are looking for.

I don't know if I was able to explain that very well.

Personally I would forget PWM for this project as it will just make it more complex. Its no fun when you get bogged down in too many details and things get more complicated then they need be.

I agree 100%. I will not worry about PWM. Maybe I will play around with that for a different project.

 

[voodoo5293]

Let me correct you here:

I have designed hundreds of circuits and sold thousands of kits, so my statements are correct.
The blink rate or flash-rate is a few Hz. I am not changing this. When the LEDs are illuminated, we introduce the clever concept of turning them on and off very quickly so that they appear to be ON. We drive them at a few kHz so they do not appear as striations when moving across the viewer’s area of sight. You can drive them at any frequency above 100Hz for this feature to take affect but at low frequencies they can be seen to oscillate when the tail light is waved in the air.
If the LED current is increased slightly, the brightness of the LEDs will increase and this brightness will be seen as "constantly illuminated" due to the phenomenon called "Persistence Of Vision."
Before you make any further comments, try it.

Interesting. So you have the 6hz pulses actually flashing at 1khz 6 different times per second? You can do all that from a single 555?

 

[colin55]

No. You need 2 Oscillators. But that's why I have never used a 555. I only use a hex schmitt trigger IC (40106 74C14), that has 6 gates and these can gated to produce all sorts of effects.

 

[voodoo5293]

No. You need 2 Oscillators. But that's why I have never used a 555. I only use a hex schmitt trigger IC (40106 74C14), that has 6 gates and these can gated to produce all sorts of effects.

I see. How much more efficient would that be that just using the 555? I mean would I gain more than 10% more battery life? If not I don't really think the extra complications would be worth it for my project.

 

[audioguru]

Before you make any further comments, try it.

I tried it many times.
I have three LED projects that use PWM at 3kHz for dimming the LEDs.
When the pulse width is next to nothing then the LEDs cannot be seen and the average current is extremely low.
When the pulse width is a little more then the light looks extremely dim.
When the pulse width is about 15% then the light looks like it is at half of max brightness.
When the pulse width is half then the light looks like a little less than max because our vision's response to brightness is logarithmic.

Your eyes are not a peak detector. They respond to the average of the duty-cycle.

Incandescent light dimmers also use pulse-width-modulation for dimming.

Multiplexed LEDs are very dim unless their current is way above continuous current to look at the same brightness.

 

[voodoo5293]

I found this schematic for powering an LED array from 12V using a 555 to make it flash. I am wondering how exactly it works. Like, what are the transistors there for? I still don't quite get how they help power things. Or how they amplify. Are they being used as an amplifier in that schematic? Or are they serving some other purpose? Any help understanding that schematic would be GREATLY appreciated.

 

[voodoo5293]

Forgot to post the image. Here it is:

 

[voodoo5293]

I tried it many times.
I have three LED projects that use PWM at 3kHz for dimming the LEDs.
When the pulse width is next to nothing then the LEDs cannot be seen and the average current is extremely low.
When the pulse width is a little more then the light looks extremely dim.
When the pulse width is about 15% then the light looks like it is at half of max brightness.
When the pulse width is half then the light looks like a little less than max because our vision's response to brightness is logarithmic.

Your eyes are not a peak detector. They respond to the average of the duty-cycle.

Incandescent light dimmers also use pulse-width-modulation for dimming.

Multiplexed LEDs are very dim unless their current is way above continuous current to look at the same brightness.

I don't get what you mean by pulse width percentage? Percent of what? Could you go into a little more detail about that please?

 

[colin55]

By increasing the current 25% and turning a LED on for 25% of the time, produces the same brightness as a constant current but draws less than 50% of the current.

 

[audioguru]

I found this schematic for powering an LED array from 12V using a 555 to make it flash. I am wondering how exactly it works. Like, what are the transistors there for?

It is a stupid circuit because it has nothing to limit the current so the LEDs will blow up.
The transistors amplify the current:
1) The current from the 555 into the base of the 2N2222 transistor is limited by the 1k resistor to about 5mA.
2) With a 5mA base current then the emitter current of the 2N2222 transistor is about 600mA.
3) With a 600mA base current then the collector current of the 2N3055 transistor is about 8A.

 

[audioguru]

By increasing the current 25% and turning a LED on for 25% of the time, produces the same brightness as a constant current but draws less than 50% of the current.

No.
Your vision's response to brightness is logarithmic so increasing the LED current only 25% will make only a small increase in brightness.
Decreasing the duty-cycle to 25% will make the LED appear dimmer than the increase in brightness caused by the small change in current.

If the average current is half then the LED will look like an LED with half the current: dimmed a little.

 

[colin55]

"With a 600mA base current then the collector current of the 2N3055 transistor is about 8A."

This is not true.

The current into the base of the 2N3055 is limited to the current taken by the load in the circuit under investigation.