Re: Programmable IR LED Flasher Circuit -- How to Build?


kpkilburn@xxxxxxxxx wrote:

First of all, death to the 9V battery. Design with AA cells.

Doesn't really matter. I was just going off of the design of the one
I've used.


Second, by sequence, you do mean the lights flash sequentially? Or is
there a cadence to the flashing?


It's a programmable blinking pattern. I guess sequence was the wrong
word. Whatever you tap out on the leads is repeated (about a 5-second
period that repeats). You can actually see it from quite a ways off.
The one that I've used (Phoenix Beacon) has 3 LEDs in it. When you're
looking through night vision devices, you can see it.

I doubt you could see an individual
LED from any distance, so a pattern of flashing would be dumb. Cadence
is another story. That you could detect over distance.

It's for assembling on a drop zone at night. When you have multiple
units in the area and it's dark, a pattern of flashing is very useful.
OK, cadence too. For example, it's long-long-short for one unit.
Short-short-short for another. So by pattern, I mean long and short
bursts.


While a shift register would work, all you are really building is a
state machine. This can be done with D flip flops and an EPROM.

Isn't a shift register just flip flops strung together? Or would it be
better to use them individually? I was just assuming shift register.
It's been years since I've done anything like this.

First of all, it is often hard to build "one of" and make it for less
than a commercial unit. Even metal boxes are $10 these days. So I would
build it only if the item were superior, or you want to learn. I
generally build things that can't be bought.

Now how many patterns do you really need? Lets say the pattern lasted 4
seconds. In any one second period of that 4 seconds, the led could be
one or off. So enumate the possibilities, with each digit below
representing one second period:
0000
0001
0010
0011
....
1111
For example, 0010 would be two seconds off, one second on, then one
second off.

So there are 16 unique cadences. One solution would be to build a state
machine that counts as follows:
1000
0100
0010
0001

Now each digit here would be followed by a switch that would enable the
LED. You could do this with a shift register, but the problem is to
make sure you only get one "one" in the register. A counter followed by
a one of N decoder would work. You could also get 4 D FFs and make a
state machine

Present| next
0001|1000
0010|0001
0100|0010
1000|0100
all other states are "don't care"

Plug these into a k-map and reduce the logic.




Some of
the address lines would be used in making the state machine, and others
would be used in selecting the cadence pattern. One bit in the memory
needs to be reserved for LED on or off. Just write out the K-amp by
hand, then program the memory (assuming you can find a burner).

The device has to be "field programmable". You short the two leads and
it starts a 5-second time period. Whatever you tap goes into "memory"
and then that pattern repeats indefinitely (or until the battery dies).

That is much harder to do and would probably require a microprocessor.



I'd suggest designing a current source to feed the leds. This will keep
the light output uniform as the battery is drained. You could use two
AA cells and a boost converter instead of the 9V battery. Your choice,
but two AA cells are the same size as a 9V battery, but have much more
juice.

AAs would be ok. What's a boost converter?

A boost converter takes the AA cell voltage, say between 2 and 3V, and
boosts it to a voltage where logic would work. In the case of this
design, you may be able to run the logic from 2 AA cells if it is CMOS,
since the speed is very slow. However, the LEDs need more than 2V to
turn on. Maybe 3 AA cells would do the job, but it doesn't give you
much head room to regulate the current in the leds. If the LEDs are in
series, you need considerable voltage, though plenty of companies make
chips for LED flashlight that could be used.



I built a cheesy IR flasher by taking a made in China red led flasher
and replacing the visible leds with IR leds. The Chinese design was so
bad that they paralleled the LEDs. I guess they don't know about
current hogging in China.

:-)


Hint: This isn't a big bucks design. High school kids in the valley can
do this.

I figured as much. That's why I don't want to spend $70-80 on
something like this if I can build it. Plus, I just want to see how it
works.

Thanks for the helpful reply.

.

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