I'd like a flasher that I can control flasher time intervals say at 1 min, 5
min, 15 min, and 1 hour. I need the flasher to trigger a night camera every
so often to see where it stops detecting the flash. It seems to operate for
about 3 days and then stops. Realistically, 15 minutes would probably be
sufficent. One of F. Mimn's RS books shows the details for a 3909 IC with a
capacitor. He gives no indication of what value it would need for a specific
interval. I'd go for a 3909 or some already built device. RS doesn't seem to
have any such devices.
One possibility would be a CD4541 chip. It is both a timer and a counter,
combined into a single IC. You use a cap and two resistors to set the
clock frequency, and two pins to set the output division ratio. There are
4 possibilites: 8192, 1024, 245, and 65526.
The clock frequency is related to the timing cap and resistor as
f = 1/(2.3*Rt*Ct).
So, lets assume you want a 15 minute flash. Then you'll need a rising
pulse every 15 minutes, so the end frequency is 1/900. By using the 65536
divider, need the timer to produce a frequency of
1/900 * 65536 = 72.8Hz. So, using the formula above,
f = 72.8Hz = 1/(2.3*Rt*Ct). Choosing Ct=0.1uF, we have
Rt = 59k. Picking a resistance value that is close, you use 62k. The other
resistor should be 2x the first, so pick 120k.
The other problem is what to do about the pulse. The chip will give you a
nice 1/900 Hz square wave, meaning it'll be high for 7.5 minutes, and low
for 7.5 minutes.
One easy solution to this is to use an SCR. Trigger it through a
capacitor, and
Vcc (4.5V?) ----------------------------.
|
|
[220k]
|
WHITE LED |
.----|<|--[Rx]--o
| |
|| - | +
From 4541 pin 8---||-o-\V SCR ----- 220uF
|| | - -----
- | |
^ | |
| | |
GND -----------------o--o---------------'
What happens is that in the 7 1/2 minutes between cycles, the 220uF
cap charges up to Vcc. When the timer chip output goes from 0 to Vcc,
the SCR is turned on, and discharges all of the charge on the big cap.
The whole circuit is like this:
4.5V---------------------------o---------------------------.
| |
| |
.-----------. | [220k]
.-[64k]----|1 14|----o |
| 0.1uF | | | |
o--[CAP]---|2 CD4541 13|----o |
| | | | |
'-[120k]---|3 12|----o |
| | | WHITE |
|4 11| | .--LED--o
| | | | |
.----------|5 10|----o | |
| | | | .---------\--- |+
o----------|6 9|----' | \V SCR --- 220uF
| | | 1uF | --- --- Electrolytic
o----------|7 8|---------||-o-|<|--. | |
| '-----------' | | |
G--o------------------------------------------o----o-------'
If you don't have any SCRs (triacs will also probably work) then you
can use two bipolar transistors connected up like this:
.-----o-----
| |
e |
PNP b----o
c |
| c
in-----o----b NPN
e
|
GND
If you find you are blowing up LEDs, make the 220uF cap smaller, or
put a current limiting resistor in series with the LED.
--
Regards,
Bob Monsen
"But when on shore, & wandering in the sublime forests, surrounded by
views more gorgeous than even Claude ever imagined, I enjoy a delight
which none but those who have experienced it can understand - If it is
to be done, it must be by studying Humboldt"
-- Charles Darwin