Howdy everyone. I have it in my mind to make my own 8 Led lantern with a 360 degree output. The thing I lack is the electronics knowledge to come up with a circuit to do it. What Im looking for is a way to drive 8 Leds from 4 D batteries and I would like to be able to dim them with a variable resistor. I was hoping to be able to use the Linear Technologies LT1932 led driver and they even have an example circuit for an 8 led driver at http://www.linear-tech.com/pdf/1932i.pdf But I guess my I'm a bit lost as I don't know how to make that "dimmable". Does anyone have a circuit design or know where I might find one that uses this chip or any other? I don't really care what is used as long as I can run it off 4D batteries, use 8 superbright nichtia (sp) leds and can be dimmed with a variable resistor. Thanks!
The LT1932 would be perfect for what you want to do. It shows how to dim it right in the application note (figure 6). Personally, I would go with the PWM method as white LED's prefer to have full current or no current. The light and color output can become somewhat unpredictable at lower current levels. By pulse width modulating, you would switch the current on and off very quickly but be able to control the ratio of on time vs off time. The peak current thru the LED would still be the same, just the amount of time it is on would change. For example, if the PWM duty cycle was at 10%, the LED's would be on for 10% and off for 90$ of the time of the PWM frequency so the LED's would appear ~10% as bright as full intensity. 50% duty cycle would be half intensity, etc... The circuit you PWM the LT1932 with would depend on what your battery voltage is. You said you were going to use 4 D cells, but how? In series for 6V, or in parallel for 1.5V, or series-parallel for 3V? If 6V, you might want to experiment with something like figure 40 in this. Just use a potentiometer across your battery voltage with the wiper connected to +VC and Vout tied to the SHDN/ pin on the LT1932. You wouldn't necessarily have to use an LM339 (much cheaper and easier to get than an LM139) since you don't need all 4 comparators. You might be able to get by with an LM2903 or LM393 since they will work down to 3V. Just ground pins on the unused comparator. Gee, I think I've rambled on enough for now. Hope it helps.
