I am building a toy fan with an LED for fun. The LED spins and
should produce a blink instead of a streak of light when it moves
at high speed. I am having trouble switching a super bright LED
to stay on for 1/1000th of a second at 30 times a second. I'd tried
using a function generator to generate a VVVV wave to drive an
npn transistor which drives the LEDs but instead of turning the
LED on for 1/1000th of a second it doesn't. Instead, the transistor
is creating a square wave (with equal on time and off time) for the
LEDs. Can someone suggestion a practical way to switch a
super bright LED on at 1/1000th of a second at 30 times
a second?

Thanks

Tim Zeifer wrote:
> I am building a toy fan with an LED for fun. The LED spins and
> should produce a blink instead of a streak of light when it moves
> at high speed. I am having trouble switching a super bright LED
> to stay on for 1/1000th of a second at 30 times a second. I'd tried
> using a function generator to generate a VVVV wave to drive an
> npn transistor which drives the LEDs but instead of turning the
> LED on for 1/1000th of a second it doesn't. Instead, the transistor
> is creating a square wave (with equal on time and off time) for the
> LEDs. Can someone suggestion a practical way to switch a
> super bright LED on at 1/1000th of a second at 30 times
> a second?

It's time for you to take a look at using microcontrollers.

If you absolutely must do it the hard way, try driving the base of the
transistor thru a capacitor to shorten the on time to the duration of the
voltage "surge". Use a pulldown resistor after the cap.

Using a micro would allow you to take this project very far. CVS pharmacy
was clearancing these cool waving wand type LED message clocks for $5 each:
http://cgi.ebay.com/tde-PROGRAMMABLE-ME ... dZViewItem
My wife got me one the other day, I'm headed back to clean out the rest of
their stock later. A really cool starting platform for stupid PIC tricks.

Hi Tim,

you could try and use a capacitor in series with the input (base) of the
NPN, and a resistor from the base of the NPN to ground. This creates a
high-pass filter that will shorten the pulse. By changing the value of the
capacitor you can change the length of the pulse. To calculate the correct
capacitor value you would have to know the input current of the transistor,
which you can calculate from the LED current and the current gain of the
transistor (or just measure).

However, if you want the on-time to be very accurate, driving it from the
output of a microcontroller or digital counter would probably be better. Let
me know if you need more help with either approach & good luck with your
project!

Regards, Peter

"Tim Zeifer" <TimZeifer@none.com> wrote in message
news:1yrij.35784$lD6.14035@newssvr27.news.prodigy.net...
>I am building a toy fan with an LED for fun. The LED spins and
> should produce a blink instead of a streak of light when it moves
> at high speed. I am having trouble switching a super bright LED
> to stay on for 1/1000th of a second at 30 times a second. I'd tried
> using a function generator to generate a VVVV wave to drive an
> npn transistor which drives the LEDs but instead of turning the
> LED on for 1/1000th of a second it doesn't. Instead, the transistor
> is creating a square wave (with equal on time and off time) for the
> LEDs. Can someone suggestion a practical way to switch a
> super bright LED on at 1/1000th of a second at 30 times
> a second?
>
> Thanks
>
>
>
>

On Sun, 13 Jan 2008 17:06:05 GMT, "Tim Zeifer" <TimZeifer@none.com>
wrote:

>I am building a toy fan with an LED for fun. The LED spins and
>should produce a blink instead of a streak of light when it moves
>at high speed. I am having trouble switching a super bright LED
>to stay on for 1/1000th of a second at 30 times a second. I'd tried
>using a function generator to generate a VVVV wave to drive an
>npn transistor which drives the LEDs but instead of turning the
>LED on for 1/1000th of a second it doesn't. Instead, the transistor
>is creating a square wave (with equal on time and off time) for the
>LEDs. Can someone suggestion a practical way to switch a
>super bright LED on at 1/1000th of a second at 30 times
>a second?

Set your function generator to produce a 1 mS pulse every 33 mS.

Sounds like you currently have the function generator producing a
square wave (or triangle?).



--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

On Sun, 13 Jan 2008 17:06:05 GMT, "Tim Zeifer" <TimZeifer@none.com>
wrote:

>I am building a toy fan with an LED for fun. The LED spins and
>should produce a blink instead of a streak of light when it moves
>at high speed. I am having trouble switching a super bright LED
>to stay on for 1/1000th of a second at 30 times a second. I'd tried
>using a function generator to generate a VVVV wave to drive an
>npn transistor which drives the LEDs but instead of turning the
>LED on for 1/1000th of a second it doesn't. Instead, the transistor
>is creating a square wave (with equal on time and off time) for the
>LEDs. Can someone suggestion a practical way to switch a
>super bright LED on at 1/1000th of a second at 30 times
>a second?
>
>Thanks
>
>
>
V+
|
|
a LED
k
|
R2
|
|
d
TTL-level sq wave----------C---+--------g 2N7000 or other mosfet
| s
| |
R1 |
| |
| |
| |
gnd gnd


Time constant C*R1 sets the led on time. Try 0.1 uF and 15K. A healthy
gate drive (5 volts min, maybe a bit more) would be good to turn a
2N7000 on hard.

You can do the same with an NPN, but you'll need a much bigger cap and
the on time is less well defined.

John

There is an attachment as a wordpad doc which I created that will get
you close. It uses a 555 timer and a diode. Good Luck, Yag 1064

On Sun, 13 Jan 2008 17:06:05 GMT, "Tim Zeifer" <TimZeifer@none.com>
wrote:

>I am building a toy fan with an LED for fun. The LED spins and
>should produce a blink instead of a streak of light when it moves
>at high speed. I am having trouble switching a super bright LED
>to stay on for 1/1000th of a second at 30 times a second. I'd tried
>using a function generator to generate a VVVV wave to drive an
>npn transistor which drives the LEDs but instead of turning the
>LED on for 1/1000th of a second it doesn't. Instead, the transistor
>is creating a square wave (with equal on time and off time) for the
>LEDs. Can someone suggestion a practical way to switch a
>super bright LED on at 1/1000th of a second at 30 times
>a second?
>
>Thanks
>
>
>

Rick G wrote:
>There is an attachment as a wordpad doc which I created

Groups without the word *binary* or *binaries* in their names
do NOT support attachments.

"Tim Zeifer" <TimZeifer@none.com> wrote in message
news:1yrij.35784$lD6.14035@newssvr27.news.prodigy.net...
> I am building a toy fan with an LED for fun. The LED spins and
> should produce a blink instead of a streak of light when it moves
> at high speed. I am having trouble switching a super bright LED
> to stay on for 1/1000th of a second at 30 times a second. I'd tried
> using a function generator to generate a VVVV wave to drive an
> npn transistor which drives the LEDs but instead of turning the
> LED on for 1/1000th of a second it doesn't. Instead, the transistor
> is creating a square wave (with equal on time and off time) for the
> LEDs. Can someone suggestion a practical way to switch a
> super bright LED on at 1/1000th of a second at 30 times
> a second?
>
> Thanks
>
Keep in mind that the fan may not always spin at the same speed, in which
case you'll still get movement in the position of the LED flash, unless you
also have accurate speed control, or maybe have the fan trigger the LED once
per revolution. Just a thought.

.... Steve

I mocked up a circuit in PSpice using a 555 timer. It seemed to
satisfy your conditions. The circuit consists of 4 Resistors, 3 caps,
1 diode (gen. purpose), and a 555 8 pin timer IC. The output(pin3)
produces a 1ms negitive transition every 33ms period. Connect the
cathode of your LED to the output. I'll describe the schmatic using a
net-list approach.
Vx 1, 0 , {any voltage}
R1 1, 4, 1K
R2 4, 2, 18K
C1 2, 0, 3.9uf
C2 5, 0 0.01uf
C3 1, 0, 0.01uf
D1 7, 2, 1N4xxx series
R3 1, 6 10K
R4 6, 0 1K
555 Pin Node
1 0
2 2
3 6
4 1
5 5
6 2
7 7
8 1
Hope this will give you an easier starting point. Good Luck

Yag1064

On Sun, 13 Jan 2008 10:57:44 -0800, Peter Bennett
<peterbb@somewhere.invalid> wrote:

>On Sun, 13 Jan 2008 17:06:05 GMT, "Tim Zeifer" <TimZeifer@none.com>
>wrote:
>
>>I am building a toy fan with an LED for fun. The LED spins and
>>should produce a blink instead of a streak of light when it moves
>>at high speed. I am having trouble switching a super bright LED
>>to stay on for 1/1000th of a second at 30 times a second. I'd tried
>>using a function generator to generate a VVVV wave to drive an
>>npn transistor which drives the LEDs but instead of turning the
>>LED on for 1/1000th of a second it doesn't. Instead, the transistor
>>is creating a square wave (with equal on time and off time) for the
>>LEDs. Can someone suggestion a practical way to switch a
>>super bright LED on at 1/1000th of a second at 30 times
>>a second?
>
>Set your function generator to produce a 1 mS pulse every 33 mS.
>
>Sounds like you currently have the function generator producing a
>square wave (or triangle?).

On Tue, 15 Jan 2008 01:54:10 -0500, Rick G <yag1064@yahoo.com>
wrote:

>
>There is an attachment as a wordpad doc which I created that will get
>you close. It uses a 555 timer and a diode. Good Luck, Yag 1064

---
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--
JF