Dusk to Dawn Flashing LED

I would like to have somebody knowledgeable in electronics construct a device
that would turn on at dusk and flash until dawn. I want to power it with AA
batteries so I assume the best type of light would be a red LED.

Possibly such a device might be commercially available.

 

What's the application? How far away (and in what direction, or
rather in how many directions) does it need to be visible? How long
do you need it to run unattended (i.e., before you could change the
batteries)? What environment? Is there no possibility of AC power
at all?

Bob M.

 

a device that would turn on at dusk and flash until dawn.

Another weenie with a half-formed idea.

|How far away (and in what direction, or rather in how many directions)
|does it need to be visible?
|How long do you need it to run...before you...change the batteries ?
|What's the application?
| Bob Myers
|
....and what is the duration of the "flash"? How often will it flash?
Duty cycle will be important for battery life.

 

4.5V (3x AA battery)
---------o----o----------o-------------------o--------.
| | | | |
| | | .-. V 1N4148
| | | 470k | | -
.-. | | | | |
220k | |<--' | '-' |
pot | | | | V 1N4148
'-' | | -
| | >| |
| | 2N3906 |------o
| CMOS | /| |
| 555 | | |
| __ | | |
.------|--------1| |o-' RED LED | |
| | .----o| |o-------------|<------o |
| | | -o| |o---------------------o |
| o--------o|__|o------. | |
| | | | | |
| | | | | |
| | '----------------|--------------o |
| | | | |
| .-----. | | |
| | LDR | | | .-.
| | | --- 1uF --- 1uF | | 100k
| '-----' --- --- | |
| | | | '-'
| | | | |
-----o------o--------------------o--------------o--------'
Ground

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Buy an LDR (a light sensitive resistor) from radio shack or wherever,
and measure it at the darkness you want the thing to turn on at. Then,
buy a pot that is about twice that value of resistance. Build the
circuit, and adjust the pot so the 555 turns on when it gets dark.

The larger the resistance of the LDR, the better, from a battery
perspective. You can also use a 9V battery for this circuit, which might
be more convienient, but won't last as long.

It will flash every second or so. You can adjust the flash rate by
changing the 470k resistor. Using a 9V battery may increase the time
between flashes.

YMMV. I haven't constructed it.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

Have a look at <www.geocaching.com>. The LED will mark a cache meant to be found at night probaly
in a wooded area. The light should be visible for 180 degrees at about 50 feet and run for a
reasonable length of time on battery power.

Thanks to those who have responded.

 

There are a couple of changes to my circuit for those requirements.
First, the use of reset is problematic, because it allows leakage when
it's light, decreasing battery life. Here is a circuit that will supply
9V when it's dark:

P-MOSFET like
IRF7104
9V battery
---o-----o--------+^+----- To 555 circuit Vcc
| | |||
| | ===
| | |
| | |
.-. .-. |
100k | | | |100k |
| | | | |
'-' '-' |
| | |
| '------- o
| |
| |/
o------------|
| |>
| |
| |
.-----. |
| LDR | |
| 10k | |
| Max | |
'-----' |
| |
| |
| |
| |
---o--------------'
GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

When the LDR changes from 1k during the day to 10k at night, the NPN
transistor will be biased into operation, which will turn on the
P-MOSFET pass transistor. The leakage during the day is probably less
than 100uA.

As to the 50' radius thing, you are going to have to blast away with an
LED to get this kind of distance. Thus, the little scheme I suggested
before won't work, because there won't be enough current to power the
LED at a high enough output. However, by simply replacing the LED in the
prior diagram with a 10k variable resistor, you can use the output to
switch on a PNP transistor at the 9V rail, which will give you a switch
at the 9V rail. Put some of those bright white LEDs between the
collector and ground, and you can flash pretty brightly. You may need to
limit the current, which can be done by varying the resistance between
the output and the base of the PNP transistor using P1. You can also
limit it using a resistor in series with the LEDs.

P2 controls how long the flash lasts. This circuit will fry most LEDs if
the current through them is set too high by P1.


From power cutoff circuit above
-o-------|----------------o-------------------o--------.
| | | | |

|---------o----. | 470k | | -
/| | | | | | | |
| | | | | '-' |
| | .-. | | | V 1N4148
| | | |<--' | | -
| | | | P1 47k | >| |
V-> | '-' | 2N3906 |------o
- | | CMOS | /| |
| | | 555 | P2 .---. | |
| | | __ | 10k | | | |
| | .-|-----1| |o-' _V_ | | |
| | | | .---o| |o----------|___|-o----o |
| | | '-----o| |o---------------------o |
| |---------o|__|o------. | |
V-> | | | | |
- | | | | |
| | '---------------|--------------o |
| | | | |
| | | | |
| | | | .-.
| | --- 1uF --- 1uF | | 100k
| | --- --- | |
| | | | '-'
| | | | |
-o---------o-------------------o--------------o--------' GND

P1 controls brightness
P2 controls length of flash
Use 2 bright white LEDs, that can handle at least 200mA for a bit

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

there's a chip (LM3909)that will flash an LED for a long time using a
single 1.5v cell. the 180 degree issue might mean using several of them
depending on the coverage of the LED you pick. There are inexpensive
ready made flashers for cyclists/hikers but they don't have the on at
dark/off at dawn option as far as i know.