# LED Flasher with Variable Flash Times

Discussion in 'Electronic Basics' started by W. Watson, Jan 13, 2006.

1. ### W. WatsonGuest

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.

(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

2. ### Roger DewhurstGuest

Go for a chip that will count to a large number and trigger it with a 555
that can easily be adjusted to provide a pulse frequency that makes the
counter complete its sequence in the time that you require.

R

3. ### W. WatsonGuest

If you'll provide me with the details, I'll do it. I can generally construct
circuits, but don't know how to design them.

(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

4. ### Jasen BettsGuest

4060 chip if they have it.

Bye.
Jasen

5. ### Bob MonsenGuest

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

6. ### John FieldsGuest

---
When you say "flasher" do you mean that your camera can be triggered
by a flash of light and you need something that flashes an LED every
once in a while for a certain amount of time or do you need a
contact closure to trigger the camera?

In either case, what should the width of the trigger be?

7. ### W. WatsonGuest

Yes, it is a video camera hooked to a device that continually looks for sky
motion. Probably 1/4 second duration or better would do the trick.

(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

8. ### W. WatsonGuest

Thanks for the circuit. What is an SCR? I would guess these parts are
available from RS. I probably have the cap/res and maybe even the IC. I just
responded to someone with the length of the pulse. Probably a 1/4 second or
bit longer. Less than 1 second should do. The camera is shielded by a
plastic dome about 18" in diameter. I'm not quite sure about brightness.
Something that can be seen in the daylight at a distance of about 12" should
work. The video camera is attached to a device that detects movement, even a
flash. It will run 7/24. I have a red safety flasher about 1.5" in diameter
that blinks every second that likely would be detected. It is enclosed in
faceted material like that found on the rear light of an auto. Of course,
the rate is too high for this use. Small insects crawling across the dome in
the day time will trigger the camera detector. (BTW, the device does not
download the continuing steam of images unless there is motion. A flash
would be considered motion.)

BTW, do you draw these circuits with Word and then convert them to a txt
file, or maybe some other tool?

(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

9. ### John FieldsGuest

---
Try this: (view in Courier)

+---------+-----+------+
| Vcc | | |
[100K] | +-->[100k][0.12µF]
| [100K] | |
| | +-------+------+------+
+-------|Rs Rtc Ctc | Vcc
| | | Vcc |
+--|MR Q3 Q4 Q13 | | [75R]
| +-------+----+----+---+ [4k7] |
| HC4060 | | | | [LED]
| +----|----|---[<1N4148]--+ |
| | | | C
+---[1N4148>]---+ +---[<1N4148]--+---B 2N4401
| | E
+---[1N4148>]--------+ |
GND

With the 100k pot adjusted to about mid-scale and Q3 toggling at
1Hz, the LED will flash once every 17 minutes for one second.

The 75 ohm current limiting resistor for the LED is assuming a white
LED with a Vf of 3.5V at an If of 20mA.

10. ### John FieldsGuest

---
Ooops...

That 0.12µF cap needs to be 0.47µF:

+---------+-----+---------+
| Vcc | | |
[100K] | +-->[100k] [0.47µF]
| [100K] | |
| | +-------+---------+---+
+-------|Rs Rtc Ctc | Vcc
| | | Vcc |
+--|MR Q3 Q4 Q13 | | [75R]
| +-------+----+----+---+ [4k7] |
| HC4060 | | | | [LED]
| +----|----|---[<1N4148]--+ |
| | | | C
+---[1N4148>]---+ +---[<1N4148]--+---B 2N4401
| | E
+---[1N4148>]--------+ |
GND

11. ### Bob MonsenGuest

^^^^^^^^^^^^^^^
An SCR is a kind of 'thyristor'. It is a switch in which a triggering
pulse will make it turn on, and stay on until current through it drops off
to some small amount. Triacs are two way SCRs.
You probably can't get an SCR from radio shack. However, you can replace
it with two transistors, an NPN and a PNP, as shown in the prior message.
This circuit can be modified to give longer pulses by using a resistor in
series with the LED, and a bigger storage cap. As it is, it'll only give
about 2ms of very intense flash. I bet this will be fine, but you never
know until you try.
You should experiment to find out.
You might want to ask the manufacturer about how to trigger it on a timely
basis, and also why it is stopping unexpectedly.
I use picture-mode in emacs these days. I used to use AACircuits, but that
doesn't work very well under linux. If I was using windows on a regular
basis, I'd still be using that program, which is quite nice, and easy to
use.

--
Regards,
Bob Monsen

"I cannot persuade myself that a beneficent and omnipotent God would
have designedly created parasitic wasps with the express intention of
their feeding within the living bodies of Caterpillars."
-- Charles Darwin

12. ### John FieldsGuest

---
Oops again...

+---------+-----+---------+
| Vcc | | |
[100K] | +-->[100k] [0.47µF]
| [100K] | |
| | +-------+---------+---+
+-------|Rs Rtc Ctc | Vcc
| | | Vcc |
+--|MR Q3 Q4 Q13 | | [75R]
| +-------+----+----+---+ [3k6] |
| HC4060 | | | | [LED]
| +----|----|---[<1N4148]--+ |
| | | | C
+---[1N4148>]---+ +---[<1N4148]--+--[1N4148>]--B 2N4401
| | E
+---[1N4148>]--------+ |
GND

Because of the drops in the diodes connected to the 3.6k resistor,
with either Q3 or Q13 low the bottom of the resistor will go to
about 0.7V plus the Vol of the chip's outputs which, without the
diode in series with the transistor's base, could keep the
transistor from turning off, leaving the LED on all the time.

13. ### W. WatsonGuest

Bob Monsen wrote:

....
Google didn't find anything by that name except a forum.

(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

14. ### ehsjrGuest

AAcircuit - no s on the end.

Ed

15. ### Bob MonsenGuest

Emacs is a text editor which uses monospaced fonts, in the traditional
unixy way. It enables one to write extensions called 'modes' in lisp,
which enhance the capabilities of the editor. Picture-mode is a mode that
comes with it. Another mode is cc-mode, which colors, formats, and indents
C code.

Emacs is a great editor for programmers, and has lots of cool features.

--
Regards,
Bob Monsen

various scientific books, that science consists in grouping facts so
that general laws or conclusions may be drawn from them."
-- Charles Darwin

16. ### Bob MonsenGuest

On Sat, 14 Jan 2006 01:45:21 -0800, Bob Monsen wrote:

<Drivel about SCRs, in which the following circuit was presented as an
equivalent for an SCR:>
Sadly, it doesn't work. Here is the real thing:

flow
|
|
e
PNP b----.
c |
| c
in-----o----b NPN
e
|
|
GND

When 'in' is 0, the NPN is off, causing no flow to the PNP base. When
'in' momentarily spikes, the NPN is turned on, draining current
through the PNP's e-b junction, turning it on. That will pull 'in' up
(assuming it isn't being held at some voltage), and allow the flow to
continue even after the signal is gone. Once the flow abates, nothing
keeps the NPN on, so it turns off, awaiting another pulse.

Your circuit can be done as so:

5V-----------[220k]------o---------.
| |
--- |
LED \ / |
--- [100]
| |
e |
0.1uF PNP b-. --- 100uF
c | ---
|| | c |
in--||---o----[100]------o-b NPN |
|| | e |
--- | |
/ \ 1N4001 | |
--- | |
| | |
Gnd------o------------------o------'

Where 'in' comes from the timer circuit I posted earlier.

--
Regards,
Bob Monsen

"False facts are highly injurious to the progress of science, for they
often endure long; but false views, if supported by some evidence, do
little harm, for every one takes a salutary pleasure in proving their
falseness."
-- Charles Darwin