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?
|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.
Have a look at . 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.
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
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.
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.