View Full Version : Dimming a flashlight bulb (3V incandescent)

05-21-2006, 05:12 PM
I need to use a basic stamp to dim several incandescent flashlight bulbs (3-5V DC, less than 1W power). The posts I found on searching this forum all dealt with either using PWM and a capacitor to dim much lower power LEDs or using very elaborate and expensive set ups to dim much higher power AC lamps. Is there anything in between? For my application it's best to have the level be stable (no flicker) and the Basic Stamp free to do other tasks, so I tried a digital potentiometer (CAT5113LI) but it didn't handle enough current for the application. Are there chips that will function as digital potenitometers handling power in the ~1W range? Can a simple amplifier setup work?

Bruce Bates
05-21-2006, 06:41 PM
harmanmd -

What is the point or purpose to using incandescent lamps? There are some decent and affordable white light LED's today. Realism, such as re-creating the lighting of an oil lamp in a model or diorama is about the only thing I can think of where an incandescent lamp might be desirable. In that case I'd just use an LED mix (orange and yellow) to develop the color I was seeking.

In direct answer to your question, the best method, if you don't want the Stamp to be held up, is probably to use an off-chip PWM driver, possibly controlling a MOSFET (if required due to the amperage) and control it from the PBASIC Stamp. Among other manufacturers, I think that Philps Semi and National Semi have PWM drivers and I suspect that Maxim has them as well.


Bruce Bates

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05-21-2006, 06:55 PM
Take a look at the ULN2803A Darlington Array IC--sold in the component shop. At the bottom of the description is a link to: N&V column "Silicon Steroids for the Stamp" #6 (.pdf)--an excellent article on interfacing to higher power loads.

Mike Green
05-21-2006, 09:53 PM
The ULN2803 will let you control the high current load, but, to offload the processing task, you might want to use something like the PWMPAL <http://www.parallax.com/detail.asp?product_id=28020> which has 4 independent channels of PWM although, because it piggybacks on the Stamp, ties up 5 pins.

Chris Savage
05-22-2006, 12:27 AM
There is perhaps something that hasn't been considered here, but it would require some experimentation, or at least careful consideration in part selection. You could use a digital potentiometer to control a power transistor (probably a darlington) to dim the bulb. Depending on it's current this could work. You need to be careful because how much current the bulb requires is directly related to how much base drive current will be required and digital pots don't handle much current. Still I think it could work with careful planning.

Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)

T Chap
05-22-2006, 04:32 AM
If you had the i/o's to spare, you could try this method too:

think of pin 1, 2 3 and 4 as a binary output where you get a total of 16 combinationis, each creating a different value in resistance to the led on the photocoupler. The less resistance, the brighter the led will burn, creating more current available at the transistor output. The greater the resistance, the less current on the output, and the dimmer the bulb. You'd have to play with the values to get it right, but this would give you a decent incrimenal dimming option by outputting 0001, 0002 etc on the 4 outputs. ie 0001 = 1k, 0001 = 2.5k, 0011 - 3.5k, 0100 = 5 k, 0101 - 6 k on and on.

The photo coupler may handle the current depending on your bulb.

T Chap
05-22-2006, 04:56 AM
Oh yeah you would probably want to make the unused "outputs" into inputs so as to not pull down the buss.