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# Driving Leds

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#### Kingpin094

##### New Member
Well first I guess I should explain what I am trying to do. Everyone has
seen the effect of things infront of a stobe light. Or using a strobe light to
adjust the timing on a car engine. What I am trying to do is to make an
80mm fan appear to stop in mid-air. I have seen several schematics on
the internet for them but the all uses 5+ chips. I have used a PIC 16F628
to generate the pulses and the unit works. Here is the problem I can't get
enough power to the leds to get to full brightness or even 10% for that
matter.

The PIC generatess a PWM signal that is in sync with the fans tach output.
The actual pulse train has roughly a .1% duty cycle at 150Hz to 200Hz (4
pulses per revolution at ~2800rpm).

The question is how can I get more power to the leds while still keeping
the same or shorter duty cycle? I can get the leds to light but you have to
have the room pitch black in order to see the fan. I can post the code if
that would help.

im doing a car LED lighting project too, if you are controling the LEDs with resistors then putting in resistors with lower resistances will brighten it up

i was wondering if you couldnt take the pulse that goes to the LED and go to a transistor instead.so if you stepped up the pulse with the transistor to 12 volts ,you would just have to add a resistor of about 470 ohms in between the trans and the led.sounds like you have an interesting idea

LED driver

hi,

use a transistor to drive the LED. connect your ic output to the base of the transistor and connect your LED to the collector side with the corresponding current limiting resistor.

for 12v @ 25ma. use 470R or for 9v @25ma. use 390R

Yes, you should use a transistor to feed the LED. To provide more power you can use two in series (off a 5V supply), at no extra power cost because you then lose less in the series resistor.

You can also drive the LED with much higher current than it's specifications allow - as it's only pulsed on for short periods, it's not a problem. IR remotes commonly pulse at over an amp!.

Bear in mind though, the duty cycle in use - if it's on for 10% of the time, and off for 90%, you can pulse it at 10 times it's rated current. But you must be certain that the 10% isn't exceeded.

Ok maybe I am doing something wrong, I tried to use a transistor with no increase in brightness.
I also hooked up a ULN2003 with no luck on that one either. I raised the led voltage to +12v and
didn't see much improve ment. Is it possible that I am switching the led too fast or not leaving
it on long enough?

Pic code: With internal RC osc. at 4Mhz
;
LIST p=16F628
include "P16F628.inc"
__CONFIG _CP_OFF &_BODEN_OFF &_PWRTE_OFF &_WDT_OFF &_LVP_OFF &_MCLRE_OFF &_INTRC_OSC_NOCLKOUT

;Setup anything that needs to be setup

#DEFINE pulse PORTA,0

cblock 0x20
temp
endc

org 0x0000

;Turn of comparators
movlw 0x07
movwf CMCON

bsf STATUS, RP0
movlw b'00000000' ;Set PortB to outputs
movwf TRISB
movlw b'00000001' ;Set PA0 input
movwf TRISA
bcf STATUS, RP0

;Start Main
movlw b'00000000'
movwf PORTB
movwf temp
incf temp
loop:
btfsc PORTA,0 ;Is pulse input = 1?
goto test_temp ;Yes output 1
goto wait_zero ;No output 0

test_temp:
btfsc temp,0 ;Is pulse and temp = 1?
goto loop ;temp = 1
movlw b'11111111'
movwf PORTB ;temp = 0 activate output

;THere are actually 320 nop here I was to lazy to write a loop
nop

movlw b'00000000'
movwf PORTB
incf temp
goto loop

wait_zero:
btfsc temp,0 ;Is pulse and temp = 0?
goto fix_temp ;Temp = 1
goto loop ;

fix_temp:
decf temp
goto loop

end
;end main

schematic

I have seen several schematics on
the internet for them but the all uses 5+ chips. I have used a PIC 16F628
to generate the pulses and the unit works.

would it be possible to post a schematic or point me in the direction of some (using the chips or pics) as im very interested in doing this myself!

Thanks
Kane

This is were I got the idea from.
https://bit-tech.net/article/80/

I am actually using the tach out of the dc fan to sync. the leds to the fan. The above circuit uses
a pot and a best guess approch to sync. with the fan.

I am using a pic to create a circuit that does not change frequency over time so the fan appears to be
stopped in mid air. I can post a schematic of what I have so far but it is just the fan output going into
one pin and the an led coming off another pin. I am trying to get more power to the led so that they
will be brighter.

A simple way to do it would be to use a LED with a constant output (ie. not pulsing) shining through the fan with a photo transistor on the other side to receive its light. Thus the photo transistor would turn on/off as the gaps between the fan bades passes it.

The output of the Photo transistor would then be used to trigger a monostable which would drive the Strobe LED (via a transistor to provide the gain) The length of the monostable pulse would be selected to provide make the LED pulse whatever length you want.

As Nigel said, you could have two or more LEDs to increase the light output and the LED current can exceed the LED's rating given that it is a low duty cycle.

Len

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