It takes up to an hour for our eyes to fully adapt to the dark and achieve maximum light sensitivity with the iris fully open. Astronomers use red light to avoid interfering with this adaptation process. A lamp for stargazing should also have several other features. Some of the features of the lamp described here are:
Red light for observation Dimmable Easy operation (including with gloves if necessary) White light for erecting and dismantling the telescope Reliable protection against operator errors (no accidental white light) Existing lamps can be remodeled The lamp is controlled by a single button and responds to button presses as follows: With the lamp off, pressing the button for less than 5 seconds switches on the red light With the lamp off, pressing the button more than 5 seconds switches on the white light With the red light on, pressing the button for less than 1 second switches off the lamp With the red light on, pressing the button for more than 1 second alternately brightens or dims the lamp With the white light on, pressing the button switches off the lamp The lamp also remembers the red light setting.
The starting point for the remodeled lamp is an inexpensive headlamp from a DIY outlet, which has seven white LEDs and a splash-proof button. The lamp has a battery module that holds three AAA cells (4.5 V), with two spring contacts that press against contact surfaces on the built-in PCB. This board holds the control button for a lamp. Three wires lead from this board to another PCB with the LEDs and the LED driver IC. They are ground (GND), +4.5 V (VCC), and Button (contact closure to ground). In the remodeling process, the original PCB with the LEDs and a LED driver was replaced by a PCB with the author’s circuitry. The original portion of the lamp circuit (battery holder and button) appears at the left in the schematic diagram. The new LED board is fitted with an ATtiny45 microcontroller and three LEDs with series resistors, consisting of two diffuse red LEDs and a white LED. The latter LED can be salvaged from the original LED board (maximum LED current around 50 mA). As the microcontroller’s rated output current is only 20 mA per pin, the white LED is connected to two pins. Buffer capacitor C1 may be omitted if space is tight. The firmware (including the source code in assembly language) may be downloaded from the web page for this project , where you may also order a pre-programmed ATtiny45 microcontroller. If you wish to program the microcontroller yourself, you can select various ATtiny microcontrollers or the AT90S2343 (the type originally used by the author) in the software. The firmware occupies only a small part of the microcontroller’s program memory, so there’s plenty of room for extensions.
e values of resistors R1 to R3 can be adjusted to match the forward voltages of the LEDs actually used. The voltage drop across the microcontroller is practically negligible. The remodeled lamp is switched off exclusively by the microcontroller, which according to the datasheet draws less than 1 µA in sleep mode, nearly the same as the self-discharge rate of the batteries. The microcontroller is awakened by pulling PB2 to ground (when the button is pressed). www.elektor.com/090550