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(TLJ V1.0)
Greetings. I almost never document - I'm a C hacker :-)
This archive consists of a bike-light project, a small,
single-cell flasher, and assorted sample schematics that
implement the LM3909 LED-flasher chip.
Well, a few things about the LM3909 are in order. How about
a few quotes from a 1982 National Semiconductor LINEAR book
that I got (and can't live without):
"The LM3909 is a monolithic oscillator specifically designed
to flash Light Emitting Diodes (LEDs). By using the timing
capacitor for voltage boost, it delivers pulses of 2 or more
volts to the LED while operationg on a supply of 1.5V or
less. The circuit is inherently self-starting, and requires
addition of only a battery and capacitor to function as a
LED flasher."
"...will operate in the range -25C to +70C. It has been
optimized for low power drain and operation from weak
batteries so that continuous operation life exceeds that
expected from battry rating." (nice touch)
"Timing capacitors will generally be of the electrolytic
type, and a small 3V rated part will be suitable for any
LED flasher using a supply up to 6V. However, when picking
flash rates, it should be remembered that some electrolytics
have VERY broad capacitance tolerances, for example
-20% to +100%."
"Absolute Maximum Ratings:
Power dissipation: 500 mW
V+ Voltage: 6.4 V
Operating Temp: -25C to +70C"
End of quoting...
The diagram found in the upper-right position in file
L_SCHM1.GIF is as basic as one can get: cap, LED, LM3909,
and a 1.5V battery. This is battery life for that circuit:
+--------------+----------------------------------------+
| Size Cell | Type |
+ |----------------+-----------------------+
| | Standard | Alkaline |
+--------------+----------------+-----------------------+
| AA | 3 months | 6 months |
| C | 7 months | 15 months |
| D | 1.3 years | 2.6 years |
+--------------+----------------+-----------------------+
With this in mind, the file D_CELL.GIF presents a small
flasher that can be soldered and glued to a D-cell. I have
one of these and use it when I got for walks in the park
in the evening... (bonus: acts as a beacon for the thugs :-)
For more charts, see file 3909SPEC.GIF. You'll find a chart
of what capacitance gives what rate, etc.
Lets see... Ok, now for the bike-light file CASE.GIF.
Parts you will need:
1) RatShack case Part # 270-220 or any other small plastic
case. If you want to use it for exterior (is. can get
rained on) applications, make sure it's water-tight or
can be sealed easily with silicone rubber.
2) Minature Toggle ON-OFF Switch (or can be also a push
ON-OFF type). Basicly, ask yourself: "Will this run ALL
the time or only when I need it?" If it's a marker or
a gadget that will run off a car's electical system, you
may not need a switch. If it's a flasher for a bicicly,
definitely invest in a small switch. Get the LOWEST
rated switch you can (Don't have RatShack # for this)
3) Battery holder. If this is for a bike, you want to make
it as shock-proof as possible. It's best if you can
glue the holder inside the project case. If you can't
find one that has a flat back, you can use the one that
has the batteries on the sides - and just use a small
piece of foam to keep it from moving. Using the chart
above (battery life) pick the batteries & holder to best
suit your application.
4) High-Brightness LED. You CAN NOT use standard 1mcd LEDs
as they are almost invisible from any distance. I used
RatShack's Part # 276-087. Please see file LED_SPEC.GIF
for more info. DO NOT buy the 5000mcd LEDs from RatShack.
They are merely the 2000mcd in a bigger, better focused
package (tell me if I'm wrong). Besides, it lists for
$4.99 and you can get three 2000mcd ones for that price
($1.68 a pop) The only other place selling something
similar (I think) is Hosefelt. They have a "T-1-3/4
High Intensity Bright Red" Part # UHO and lists for $.15
If anyone knows other sellers of compareable LEDs, please
let me know. (I just HATE it when mags, MCM and Hosefelt
included, don't list brightness for LEDs!)
5) LM3909. Yup, RatShack has it, Part #276-1705 and lists for
$1.69 (not bad). MCM has it for $5.04 ("Earth to MCM..." :-)
Lets see, it also cross-refs to an ECG-876 (no idea on price)
Anyone know of a cheaper source? Outside USA?
6) Capacitor. If you need low frequency (<500Hz) it has
to be an electrolytic. If you want high freq, a tantalum
or other can be used (note that .30 uF gives 1.1kHz)
As the quote said, a 3V will do but use any voltage
you can get. I used a sub-minature 400uF 6V part in my
bike-light (to get 1.5-2Hz) A small hint: get a bunch
of caps and plug-and-play. Pick the frequency you feel
happy with :-)
7) Perf-board. You don't really need one! It was just handy
for me to attach the LM3909 to it and I also used it to
keep the potting compound from leaking into the battery
compartment. Cut it to fit snugly as shown in file CASE.GIF.
8) Potting compound. You can use wax... if you live in cold
climate :-) Epoxy is both expensive and also (for me)
shrinks when it cures - breaking wires, etc. Besides, it's
VERY permanent (as I found the... er... hard way - I could
even SEE through the clear epoxy where the problem was :-(
I found that packing "peanuts" work just fine.
9) Wire. We're not talking amps - I used wire-wrapping wire.
Don't use more than 18 gauge... unless you're one of the
clowns that claims to "hear" the difference between normal
and oxygen-free speaker cable ;-)
10) Batteries. Obviously, the ones that fit the battery holder!
This is not the time to be cheap - use alkaline! The don't
leak as "much" (you mean they don't "never" leak? Yeah right)
and they last almost twice as long (plus, they have a long
shelf life...
Well, that's about it. Oh, almost forgot, you better know what
solder and flux and, most important, patience are applied. More
assembly instructions are listed in the file CASE.GIF.
By the way, I cheated and used the schematic that appears
in the upper-right position in the file L_SCHM2.GIF - yes,
the one for an incandescent... I found that it give VERY
bright flashes for the 2 LEDs that I used (see CASE.GIF).
Well, that's about it. If you have questions or comments or
bug-fixes (ahmm) please e-mail them to "fmgst+@pitt.edu"
of that bounces (??) send it to "fmg@alpha.smi.med.pitt.edu"
Have fun, don't burn yourself with the iron, and don't inhale!