LED Enlarger Head - Update

[Steve Smith]

How handy are you with electronic circuit building?
Can you make a pulse width modulation circuit to control the brightness of the LEDs?

I can indeed and suggested this yesterday:

I am then going to work on a switch mode controller with a single control so that fully anti-clockwise the duty cycle is 100% green and 0% blue. Turning the control clockwise will increase the duty cycle of the blue and reduce that of the green so at the mid point they both receive 50% and fully clockwise it is 0% green and 100% blue. This will give me a single Contrast control.

The circuit I will use is one I have used before. It is almost a standard astable multivibrator but the base resistors to the two transistors are connected via a two diode and potentiometer circuit similar to the one in your link. There will then be a suitably large darlington transistor connected to either side to drive the green and blue channels.


Steve.

 

[BetterSense]

If I built an LED enlarger, I would probably use a darlington transistor hooked up to the PWM outputs of a microcontroller, since I have microcontroller infrastructure already. Then you could control the R, G, B lights digitally as well as create automated recipes, or have feedback from a photosensor, creating a closed-loop exposure system.

 

[Steve Smith]

I would probably use a darlington transistor hooked up to the PWM outputs of a microcontroller, since I have microcontroller infrastructure already.

That's quite a logical and sensible way of doing things as you can fine tune it to be whatever you want it to be and have settings to match proper grades etc.

However, I like to keep things simple myself so I'm going to go for my single contrast control!


Steve.

 

[Worker 11811]

So you're using the Darlington transistor so you can get more current out of the system to drive all those LEDs.
Whereas, my idea would be suitable for a couple-few LEDs at most and would peter out if I tried to drive a dozen or more of them, such as you plan to do. Right?

(P.S. - I read you were going to make a "controller" but I skimmed over the words "switch mode".)

 

[jeroldharter]

Good stuff.

Easy for me to say given that I know nothing about electronicsf, but I wish that an entrepreneurial sort here would devise 2 items:

• A simple, adaptable LED variable contrast light source for 4x5 and 8x10
• A simple Jobo alternative, something like an electric screwdriver turning a drum in a water bath.

Now that Aristo is moribund and the bulbs are currently out of production there could be a market for the light sources. Likewise with Jobo and everyone paying $500 for drums.

 

[EdSawyer]

I wonder if something like a Beseler/Minolta 45A head could be converted to LEDs? It's already set up for RGB flash tubes... (which sadly are out of production and hard to find as replacement parts)