Looking at the datasheet the pico is not going to work for my LEDs at their full potential. I will solder the jumpers on each channel to increase the amps to 660ma, it will work for now while looking for other boards.
the datasheet doesn’t match my real life results as I can make them brighter by using 36, 34 and 27 volts… but it’s in the ballpark see below:
running them at less than their rated current isn’t bad, and could potentially extend the life of the LEDs. If you’re getting enough light at 660mA then I wouldn’t worry about that.
If the lights are getting brighter with more voltage, it’s probably because there’s a relationship between voltage and current - adding more voltage basically gives you more current to work with, and that will make the LEDs brighter. I bet you’ll see less of a fluctuation when you change the pico to 660mA.
@robinojones
If I understand the datasheet of the AL8805 correctly, when driven by DC the range is from 25% to 100%. I think that’s what you mean by poor dimming range. That’s what I was mentioning on my previous post.
Drive with DC voltage (0.5V < VCTRL < 2.5V) to adjust output current from 20% to 100%
In the picobuck page there is also a reference to it:
Dimming can be done by an analog voltage (20%-100% of max current by varying voltage from .5V-2.5V)
I haven’t used the pico/femto but what follows seems to indicate that the full 0-100% is not available when driven by DC, only when using PWM.
or by PWM (so long as PWM minimum voltage is less than .4V and maximum voltage is more than 2.4V) for a full 0-100% range.
@friolator in an earlier post in the subject indicated that when using PWM the driver output had some residual of the PWM.
With a workaround, the arduino mega can output multiple 16 bit pwm. There are other arduinos with 12 bit pwm output
If those voltages are the Vf, what it means is that you can expect the LED forward voltage to be X-X when the current is Y-mA.
The pico/femto are switching current regulators, so as long as the Voltage you are using is higher than the LED Vf, the regulator will maintain the selected current.
In other words, all 3 may be driven with 36V, and the pico will maintain the selected current (660mA) if you go for the higher. You can measure the voltage across the LEDs with a multimeter and confirm the operating Vf.
Hope the information is helpful.
That’s a good point - the thing is that in my initial tests with the camera, it’s showing that I need very low exposure times to avoid motion blur - and to compensate I will need a lot of light especially for denser negatives. maybe this will be enough.
I will solder the jumpers now and see how it is at 660ma.
Yes these are the forward voltages.
thanks for that - I will try that code and see if I get more range.
Incredibly helpful - thank you both for everything!
I just saw this info on the Sparkfun Picobuck hookup guide. Looks like I should be able to go to 1amp. Will try that too.
Alternative Current Output
It is possible to increase the maximum current of the PicoBuck board up to 1A per channel; to do so, replace the three current sense resistors with smaller values. To calculate the new value for the resistor, use this formula:ILED = 0.1 / Rset
Thus, for a 1A current, you’d want a 0.1Ω resistor. Don’t forget to be wary of current ratings. At 1A, the sense resistor will be dissipating 1/10W, so you probably want a resistor of at least 1/8W rating. The package is a standard 0805.
I made a small prototype board and installed some potentiometers - it’s so bright it hurts to glance at it and no flicker.
Code will need to be tweaked to improve the range but at least I can continue working on camera trigger. I got a 2 channel scope today so I can troubleshoot the trigger - the camera was randomly losing perfs, this will allow me to troubleshoot that
@PM490 i looked at the 16bit pwm code you linked but I can’t get it to work with arduino mega. I will look at this again soon as I was busy doing the electronics.
Nice setup.
If you are looking for a simplified version of the 16 bit PWM with Mega, this is the code I made to test the Kinograph drivers. The output is 16 bit PWM. The input is analog 12 bit with the Mega ADC inputs.
It also converts it from linear to exponential (pow function) to make the feeling of the potentiometers-to-light output linear, taking advantage of the 16 bit range on the output.
PS. Keep in mind that only certain outputs would work with 16 bit PWM.
The capstan drive is something I’ve been thinking about for a year, but finally came up with a setup I like.
Unlike the other PTR rollers, which are free spinning on bearings, the capstan PTR has to be fixed to the motor shaft. The motor has a 3/8" shaft that’s only about 1" long, but the motor is mounted on the back of a 3/8" thick piece of aluminum, so it’s set back from the face of the scanner. This means we can’t have a set screw that holds it in place without the option to remove the fatter PTR hub as well. So, multiple parts.
Starts with some aluminum and a really crappy lathe:
This was 1" stock, and it had to be turned down to a 3/8" shaft:
Then flipped around in the lathe and a 3/8" hole bored in the base. This will slip over the motor shaft:
This is a good fit:
The PTR hub was turned from Delrin like our other ones, but this one was cut in half. It slips over the 3/8" shaft, and we’ll cut that shaft to length and thread the end for a nut. This will secure the bottom of the PTR hub to the shaft.
Next there are rubber O-rings (not pictured) that slide over the delrin and sit between its flange and the metal ring of the PTR roller. The other half of the hub slides into the PTR roller, and is secured to the base half with two 90mm M4 screws. This is an approximation, because the motor shaft hasn’t been cut yet, and because the 90mm screws are in the mail.
Binding posts for the bottom side will be hot glued in place so they don’t slip out.
This took all day but I think it’s a design I’m happy with.
A while back we ditched the Schneider MDrive stepper motors that we were going to use for the camera/lens assembly. They were a bit too long, and as NEMA17 motors, required a step-up adapter to fit the NEMA24 linear stages we’re using. So we replaced them with simple, cheap low profile steppers. More than powerful enough, but they require external drivers, unlike the Schneider motors, which were integrated.
Those external drivers happen to require a 5V PULSE and DIRECTION signal from the controller. If you’re using an Arduino, that’s fine. But our ClearCore outputs 24V, which is common for industrial automation tools. The ClearCore has an internal 10k pullup resistor for each of its signal outputs. So geting it into the <5V range is as simple as wiring in an inline 2k resistor. Our LED driver board has these built in for the PWM inputs, but for the stepper motors, we were using temporary cables, which have a thru-hole resistor on a thin piece of breadboard, with the wires soldered to it, and heatshrink tubing to hold it all together. It’s ugly.
So, the solution is to make another DIN-rail mounted breakout. This will have 8 input/output pairs. It’s wired through a $4 resistor array. Total cost for board manufacturing (not including shipping) was $2 from JLCPCB (for 5 of them). each board gets about $8 in parts. Should be here next week.
So this board didn’t work out properly. Let this be a lesson to double check things…
Easy EDA, the tool I use for PCB schematics and layout, has a vast user-contributed library of parts. But that doesn’t mean they’re accurate. The first board came back with completely the wrong footprint for the resistor array I purchased.
Also, I didn’t realize that the DIN rail mount I bought was vertically oriented until it arrived, so that would have meant the board would be rotated 90 degrees. Functionally no difference, but would have driven me nuts.
So, new board is here, with the proper resistor array footprint, and a new vertical orientation.
Back to some mechanical stuff today. Finalized the mounting system for the free-spinning PTR rollers:
These are generic shaft flanges you can buy on Amazon with 24mm spacing between the holes (across the flange). There’s one of these on the face going into the deck plate and another on the back, with set screws that hold the shaft in place:
They’re held in place with low profile M3 bolts and lock nuts. The total depth of these two flanges combined is about 30mm, which seems to be pretty solid, with no flexing so far. The shafts themselves are 10mm diameter and are a pretty snug fit inside these flanges, so they seem to be nicely perpendicular to the deck plate. We have an alternative mounting method if need be (some spacers that match these holes, which we had made previously), which would give us an additional 15mm of purchase on the shaft, but I don’t think that’s going to be necessary
The Capstan motor is now mounted with proper lock nuts on the back as well, and as you can see from the picture above, the powder coating done by the place we used locally is pretty crappy. Spent a lot of money on that, but they didn’t properly prepare the metal and there are lots of places where it’s peeling off, especially near mounting points. Black sharpie? have to figure out how to conceal that.
Finally, all 5 PTR rollers are installed, free spinning:
And they have spring plungers to mate with the inside of the PTR. These keep it from riding off the hub, And were significantly more work to install than I had anticipated:
Once everything was assembled, it became apparent that there’s a fair bit of play in the roller shafts, which causes them to not be perfectly perpendicular with the deck plate. This causes the film to want to drift off the end of the PTR. This is bad.
So, this afternoon I designed some new blocks that we’ll have machined. These go on the back of the deck plate and have a very snug fit around the roller shaft, with about 65mm worth of sleeve for the shaft to sit in. This will be bolted from the front, and the flat front surface will sit perpendicular to the deck. This should eliminate that problem…
And then I started work on the capstan, but quickly realized the previous design has some problems as well. So that has now been completely redesigned and will also be manufactured at the same time as the other parts. There are three pieces, the shaft, the inner and the outer hub.
The shaft sits over the motor shaft to extend it through the hub, and has two set screws to lock it in place.
There’s a “paddle” that that registers with the inner part of the hub. The shaft continues up until it gets to the outer part of the hub, where an M8 thumb screw will lock it together.
The two halves of the hub interlock with one another, and rubber o-rings against the PTR roller will keep that from slipping. This is ultimately a bit simpler to use than before, but of course, more complicated to make.
And it’s a perfect fit on the deck plate, with the spacing just right.
Hopefully we’ll have these parts back in a week or so.
Fantastic documentation. Jealous of your machining skills! I liked the idea of using those flange bearings as opposed to the pillow block ones I’ve been using. @Jeffcrowl mentioned that to me today as well.
Jeff was able to find this traction sleeve material for use as a PTR sleeve, which sounds similar to what you are doing. What material are you using?
So are you suggesting that that slides over the roller hub and is the capstan material itself, or that it’s used to hold the removable capstan PTR in place?
My setup used a standard PTR and the hub for it is delrin in two halves. Because the inside of the PTR is aluminum, it slips against the delrin hub. So the solution was to machine the delrin hub to be pretty tight, then put a rubber o-ring on each of the flanges of the Delrin hub.
So basically the PTR exactly fits the Delrin hub, and the O-Rings go between the hub flanges and the ends of the aluminum tube at the center of the PTR. This does two things:
In theory, there should be no slippage with this setup and it should transfer the power from the capstan motor directly to the film
That being said, if this traction sleeve material isn’t too sticky, I could almost see it as the outer surface of a Delrin hub, the part the film actually comes in contact with. But I think I’d need to see it first to know. Interesting stuff.
These new parts are on their way from China now, and I should have them in a couple days. Can’t wait to get them all installed so I can move forward!
New parts shipment has arrived!
From left to right: The new shaft sleeves for the PTR Rollers, A bar that holds down the front end of the camera/lens sled inside its cabinet, the two halves of the Capstan PTR hub, and the capstan motor shaft extension those hubs lock to.
The PTR shaft sleeves came out really nice, perhaps a bit too nice. The fit is very snug, so I think I’ll be spending the afternoon trying to make it a bit easier to slide the shafts in.
This is the shaft extension that goes on the capstan motor. There are set screws on the other side of the base of this, which lock into a channel on the motor shaft.
The lower half of the PTR hub slides onto this:
and then the upper half onto that, with a thumb screw to hold it all closed.
On the underside of the outer hub half will be the o-ring mentioned above. The purpose of this is to create a squishy layer between the hub and the metal core of the roller. This will provide the friction needed to keep the PTR from slipping on the core, hopefully.
I’ll post some more photos when everything is on the scanner, later today hopefully.
All in all I’d call this a successful upgrade to the scanner. With the new shaft sleeves installed there is zero play, and the film rolls over each roller perfectly smoothly and without telescoping on the takeup reel.
Motor with shaft extension installed:
Motor with shaft extension and complete capstan hub:
Shaft Sleeves installed:
This is a nice shot because you can see the whole thing from the side. The shaft goes almost all the way into the sleeve, which is held perfectly perpendicular to the deck plate by the flat end of the sleeve.
capstan motor installed. This is the motor, the bottom half of the capstan hub, and a PTR. The bottom half is attached to the shaft with some set screws, so it stays attached, but the outer half and the PTR capstan are removeable:
@matthewepler - this is the o-ring mentioned above, on the outer half of the capstan hub:
and with the PTR roller installed. You just need to get it thumb tight with the nut at the end and it seems to be completely slip-free:
Complete capstan assembly:
It’s keeping the film perfectly in line with the gate:
The film can be threaded A or B wind. In an eventual firmware upgrade, we’ll allow for A->B and B->A wind as well, like the ScanStation. That can be handy sometimes when you get film that’s incorrectly wound.
This is great. Love the simple film path. I will definitely steal that o-ring idea from you! Thanks for showing me that.
I totally agree with supporting multiple winding strategies. You never know what you’re going to get!
Did you say those PTR hubs are machined delrin? That’s what I think I see but wanted to ask. Are you willing to share the design files for those? I’d like to try them against the pressure plate design I’m using to see how they compare. And I want to feel that satisfying click of the roller mounting!
Thanks!
They are delrin. Do you mean the regular PTR rollers or the capstan roller? I don’t know if I have a cad file for the regular rollers but it would be easy enough to whip one up. I made the regular rollers by hand on the lathe (not CNC). The capstan roller is cad and was CNC machined.
The capstan roller would be great.
UPDATE
@friolator I think I get it now. I was confused about the threaded part of the capstan shaft. I thought it was an off-the-shelf metal part and couldn’t figure out how you fixed it to the hub. Now I’m seeing it’s all one plastic piece. You had that CNC’d, yes? Even the threads? That’d have to be one spiffy CNC at least 5 axis, no?
