Building a Variable Contrast LED head for a Durst 138

[Larry Kellogg]

Background link with information on creating a variable contrast led head for the Durst 138

https://www.trippingthroughthedark.c...8s-condensers/

and this comment from Larry Gebhardt, the guy who built it, to kick off this discussion.

"

My plan was to create a printed circuit board based on the prototype I built. However I've been using the prototype for two years and haven't finished the printed board design. I've had a few new ideas since the original design, so I'll see if I can get them integrated and the board designed and printed. Then I'll sell some of the blank boards at a low cost.



If you are willing to build a prototype board I can certainly provide some pictures and a description of what needs to be done. It's really not much, especially if you don't want it to switch AC for safelights. You need an Arduino, keypad, 20x4 display and some Buck Block drivers with a few transistors. All of it's pretty standard stuff in the Arduino universe.



I don't really want to get involved in turning this into a business. I'm mostly afraid of the liability of sending out electronics that use AC power (I don't want to fry someone). But I've found some new drivers that will boost a 12V DC power supply to higher voltage to drive the LEDs. So something could be built that uses a standard PC power supply, which would simplify things a lot."


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Ok, I'm trying to figure out what to buy in order to build a prototype board. Would I buy a Poor Man's Buck Block?

http://m.instructables.com/id/Poormans-Buck/

How many?

Any tips for where to buy the stuff?

[Tin Can]

I am very interested. This is a great thread!

[Peter De Smidt]

Good stuff, Larry.

[Tin Can]

Background link with information on creating a variable contrast led head for the Durst 138

https://www.trippingthroughthedark.c...8s-condensers/

and this comment from Larry Gebhardt, the guy who built it, to kick off this discussion.

"

My plan was to create a printed circuit board based on the prototype I built. However I've been using the prototype for two years and haven't finished the printed board design. I've had a few new ideas since the original design, so I'll see if I can get them integrated and the board designed and printed. Then I'll sell some of the blank boards at a low cost.



If you are willing to build a prototype board I can certainly provide some pictures and a description of what needs to be done. It's really not much, especially if you don't want it to switch AC for safelights. You need an Arduino, keypad, 20x4 display and some Buck Block drivers with a few transistors. All of it's pretty standard stuff in the Arduino universe.



I don't really want to get involved in turning this into a business. I'm mostly afraid of the liability of sending out electronics that use AC power (I don't want to fry someone). But I've found some new drivers that will boost a 12V DC power supply to higher voltage to drive the LEDs. So something could be built that uses a standard PC power supply, which would simplify things a lot."


------------------------------------------------------------

Ok, I'm trying to figure out what to buy in order to build a prototype board. Would I buy a Poor Man's Buck Block?

http://m.instructables.com/id/Poormans-Buck/

How many?

Any tips for where to buy the stuff?

Bob Mann is one of the respondents to the tripping through the dark link , you may want to consider his recent posts in my thread here.

[Larry Gebhardt]

What I used were four of these 1 Amp BuckBlocks. I needed 24 LEDs (12 blue and 12 green) for the diffusion head. The forward voltage of the LEDs was about 3.5V which limited me to under 12 per driver (32V max), so I used two for each channel.

If I was to do this again I would use a standard PC power supply which can provide a lot of power at 12V. I would then use a 700mA FlexBlock. This can take 12V in and boost the output to 48V, letting use drive 12 LEDs per driver. We'll need to use different LEDs form what I originally used to account for the lower current if we want to maintain brightness.

I would also use the 3 UP LEDs on one star if I was building again. I'd get the Cree XP-E Royal Blue, Green and Red so I could experiment with color (and I'd add a third channel for red). But you really only need the blue and the green. If you don't want a diffusion head I found the 3 green and three blue were quite bright with the condensers. But I'd go with 6 of the RGB stars on a rebuild. Note that there are stars with 4 LEDs (RGB + White, but these all seem to use the blue and not royal blue, so I don't know how well they would work). There are also other brands that will probably all work equally well.

I based my setup on William Brodie-Tyrrell's f-stop timer so there's a bit of construction related details over there. However I used an Arduino Mega since I needed more memory for the variable contrast logic, and more pins for SD Card reader and the 4 driver controls. I also used a 4x20 display instead of the 2x16 display.

What little I've written up is here. I'll try to add some more posts, especially as I rebuild it.

[Larry Gebhardt]

Bob Mann is one of the respondents to the tripping through the dark link , you may want to consider his recent posts in my thread here.

I saw Bob's post, but didn't put the name together with his comment on my site. Small world.

[Larry Gebhardt]

Ok, I'm trying to figure out what to buy in order to build a prototype board. Would I buy a Poor Man's Buck Block?

http://m.instructables.com/id/Poormans-Buck/

How many?

The Pooman's Buck looks pretty cool. But you'd need to use a higher voltage power supply (but not the commonly available 24V ones) like I did in my prototype to drive more than 4 or 5 LEDs.

[Jim Jones]

I don't understand the need for higher voltage than 12 Volts to drive LEDs. If each color is wired in a series-parallel configuration, less than 12 Volts should suffice.

[Larry Gebhardt]

I don't understand the need for higher voltage than 12 Volts to drive LEDs. If each color is wired in a series-parallel configuration, less than 12 Volts should suffice.

The short answer is I think you are correct. The long answer is, well longer:

You only need the sum of the forward voltages of the LEDs in series to drive them. A constant current driver will put out enough voltage to produce a current at it's set value. If you add other LEDs in parallel the current is divided between the parallel branches. So limiting yourself to 12V with a 1000mA driver and 3.8V LEDs means you can have at most at 3 in series to not go over 12V. So to get 12 LEDs on one driver you would need 4 sets of 3 in parallel. This would cut your current to 250mA per series of 3. This will be much dimmer than the LEDs can safely run at. So you can choose LEDs that work with a lower current, a driver that can put out more than 1000mA, or a combination of the two. I didn't find a combination of LEDs and drivers that would let me run at 12V and still put 12 high power LEDs in the head driven near their max brightness. However two years have passed since I last looked at the available components, so maybe the right combo is available now (or was before and I didn't find it).

The FlexBlock Documentation goes into the series/parallel wiring.

Also, I'm not an electrical engineer, so I could very well be wrong on this.

[ic-racer]

There are certainly many ways to get a LED to illuminate, but after looking at the link what stands out as pure genius is Larry's clever integration of his LEDs into the existing DURST L138 structure. Fantastic work!