LED Lighting systems for book scanning

[daniel_reetz]

Rusty, I'm going to be traveling for the next week and then school starts and I have harsh deadlines until the 29th. After that, I can help.

Can you please give me a little more information? Do you have all the exact same parts in front of you? If not, what have you changed? Are you going to be triggering the lighting system with a 5V pulse from Arduino, as I am, or from something else? Do you need the lights to be on dim, and then blast full-on as I have them?

The basis of this circuit is a bunch of high power LEDs, connected in series, with a single high power current limiting resistor. (I duplicate this part, so there are actually two of these assemblies, but one is fine for testing). Can you get that much of it working, and then work from that? Because that's how I started this whole thing.

[rustyhann]

Thanks for the fast response. The semester begins Monday, so I'll be under similar time constraints .

For the design I've come up with a stationary scanner with an 18' by 15' cradle using an acrylic platen that can be raised and lowered on a single drawer hinge. The cradle will be mounted on it's own set of drawer slides so that the spine of the book can move or 'sway' from right to left and will allow the spine to always be centered with the platen. I'm going to mount the camera's and lighting fixture on the platen also. It will be, basically, a cube shaped skeleton where the bottom is comprised of the plexi (glued to the frame for stability). I want to resolve some of the lighting issues before I dive in on building the rest, but would be more than happy to provide an uber detailed setup and pictorial once it's done.

The way I would like the lighting for the scanner to work would be as follows: I want the LEDs to be dimly on in between exposures. When a picture is taken, I'd like them to flare up bright, as in the demos from the 3rd generation scanner. I'd like this to be a 'one button' setup, where everything is controlled from the exposure switch that drives the cameras. I've been working on a shopping list to complete the circuit, which is where a few of my questions have come up. Here are the parts I 'think' might work :

For the transformer, http://skycraftsurplus.com/36voltcenter ... ormer.aspx - 36V center tapped transformer
Bridge rectifier, http://www.virtualvillage.com/35a-1000v ... 1-002.html - 35A 1000v
Capacitor, http://www.globalindustrial.com/p/hvac/ ... aignId=T9F - 370V 5MFD
Mosfet, http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 038wt_1165
470ohm resistor, http://www.radioshack.com/product/index ... =107599019
zener diode, http://www.opentip.com/Electronics-Comp ... 66605.html - 36V
Cree 5 LED, http://dealextreme.com/details.dx/sku.2394

I've had the most trouble with sizing the components correctly so that everything works. I am comfortable hooking up the basic circuit with transformer, rectifier, capacitor, and LEDs. I start running into trouble when you add the mosfet, 470 ohm resistor, zener diode, and pull down resistor. Here are my questions,

When you say, "These are connected to the drain of a high-power MOSFET MTW14N50E. The drain is also connected to ground through a 470 ohm resistor," I understand this as one mosfet (or is it one mosfet for each LED), for the entire circuit, which is connected to the ground coming off the capacitor, with a 470 ohm resistor in between? Is that resistor on the capacitor side of the circuit, or the LED side, (I'm assuming the mosfet in the middle)? I haven't been able to figure out where the zener diode goes in the circuit. You also mention a pulldown resistor. Where is this in the circuit, and what is it rated at? Also, what is the fuse rated at? When you say, "The source of the MOSFET is connected to ground, and the gate switches the whole thing on when fed 2-5VDC," what gate are you talking about?

You also state, "In this iteration, the 5V input is from a voltage dividing tap across the power supply, made from a couple resistors. In the final circuit, it is likely that I will drive it with Arduino." I haven't been able to find anything on arduino or how it works, and I haven't been able to figure out how to wire the lighting circuit into the capture setup, I'm totally lost on that aspect

I've been able to find all of the materials needed to make the scanner and stay within budget, especially since I already own one A590IS (a stroke of luck ), but I'm hesitant to pull the trigger and round everything up with the lighting aspect in the air. I've read that the LEDs help with capturing books with glossy pages, which will be 90% of what I'm using the book scanner for. I also like the fact that they draw high current only when capturing. The end goal is to scan the dozens of IT related books, and text books, so that I can carry that digital library around with me.

[daniel_reetz]

I may have made mistakes when writing it up, and the schematic is a little incomplete. So if you spot something inconsistent, it might just be inconsistent.

I have another copy of the schematic taped to the wall of my workshop, and I photographed it for you. Hope it helps. The component values are written in the European style, because a German drew it all.

[myenhdl]

I'm trying now to figure this out. Let me start by posting the drawings in one piece:



If I come up with anything, I'll be sharing it in this thread. At the moment, I'm understanding not a whole lot from the drawn circuits- as they do seem somewhat incomplete and I'm still only starting to study this stuff in school.

Initially, I bought a whole bunch of these Cree Q5 XRE LEDs about almost two years ago to convert my car interior to super-bright white lighting. Needless to say, I was inexperienced and lacked the knowledge and ended up burning 5-6 of these expensive lights. But since then- I learned a few things about LEDs and driving them, so let's see where this takes us.

[daniel_reetz]

Thanks for doing that! I appreciate you taking the initiative to get things together in a way that helps everyone, including me.

The schematic is complete, except for the power supply. I appreciate the work you guys are putting into understanding this , and I would love to see more people with LED-lit scanners. I'm not really sure what's wrong with it though, so if you can please explain in more detail what parts don't make sense, that would help me help you. Right now, it feels to me like either I have the benefit of having already built this thing and so it all seems elementary, or that you might be making things a little harder than they have to be. Please don't take that the wrong way -- I only mean to say that once you get the principle of operation it will probably seem pretty simple to you, too.

I'm attaching an annotated schematic. At its core, this circuit has two basic concepts.

1. Using a transistor as a switch. The MOSFET transistor is given a signal, and it connects the ground to the LED array. (low-side switching). This section is indicated by the red box.
2. Lighting an LED with a current-limiting resistor. This section is indicated by the green box.

When you have time, please review the annotated schematic and let me know what appears incomplete, and especially let me know if you have ideas on how to communicate this circuit more clearly. We threw this together over beers one night, so it's not like it was meant as a tutorial. You can see that a lot of the circuit is there just to be "safe" and keep it working forever and ever.

[daniel_reetz]

I wrote "resistor for undervoltage" on the input of the MOSFET, but that's obviously a mistake. It's to hold the input of the MOSFET low.

[myenhdl]

Thanks Dan for being so promptly about answering answering everything from questions to mere thoughts here. You and your friend has no doubt, a better understanding about electronics than myself and probably most of the users on this forum. I figured that what we needed was something simpler- like a "this wire here and next wire here" diagram like rusty said.

From the picture, this is what I'm getting: We have AC source (from the mains) to the transformer (step down...to 36v?) --> bridge rectifier to flip the negative signals to positive (on the sine wave) --> a capacitor & bleed resister in parallel to "smooth" out the new DC voltage. It then gets passed through the LT317T setup to regulate the voltage (to ~20v?- since 5x Crees in series take about 18.5v max). The Crees are then powered by this converted power source, followed by the MOSFET to trigger full power for the LEDs when activated...when not activated, the bleed resister keeps the LEDs dim but bright enough to see what we're doing. Now...

I made a few marks here on the diagram labeling what I understand or think I understand- and things I'm having a little bit of trouble piecing together.



I'm not really understanding the "paranoid" components- I understand that this is just probably something simple and is used as added insurance, but I'm having a little trouble understanding how it actually works. Looks to me like the diode isn't really doing anything and the resistor is actually for overvoltage...somehow.

I'm a little confused about the voltage divider, but I feel that'll make sense once I have everything else figured out. Let's leave that for now.

Simple questions:
- What is the yellow component, or link(?)
- The white components (3) circled...are those resistors as well?

A bit more complicated questions:
- What are the qualities of a bleed resistor? Are they normal resistors used for this purpose and labeled as so?
- Are the current limiting resistors any special type of resistors? I have yet to make any calculations...they could very well be normal resistors. I'm looking into that.

I'm sure there are more to my madness...but I'll try to not explode a single post.

[daniel_reetz]

I figured that what we needed was something simpler- like a "this wire here and next wire here" diagram like rusty said.

Maybe between the three of us, we can get it to that point.

From the picture, this is what I'm getting: We have AC source (from the mains) to the transformer (step down...to 36v?) --> bridge rectifier to flip the negative signals to positive (on the sine wave) --> a capacitor & bleed resister in parallel to "smooth" out the new DC voltage. It then gets passed through the LT317T setup to regulate the voltage (to ~20v?- since 5x Crees in series take about 18.5v max). The Crees are then powered by this converted power source, followed by the MOSFET to trigger full power for the LEDs when activated...when not activated, the bleed resister keeps the LEDs dim but bright enough to see what we're doing. Now...

Correct. Although I didn't actually use that power supply, that is the ideal power supply.

I'm not really understanding the "paranoid" components- I understand that this is just probably something simple and is used as added insurance, but I'm having a little trouble understanding how it actually works. Looks to me like the diode isn't really doing anything and the resistor is actually for overvoltage...somehow.

The Zener will shunt a too-high signal to ground. A Zener actually allows voltage to flow in two directions. Under normal operation, the Zener is like any other diode, like a one-way valve. But if the voltage is too high (over 5.1V) it will shunt that to ground. That is the purpose of the zener.

The resistor is there to provide a path to ground when there is no input signal. You'll see when you start playing with MOSFETs that the inputs don't just sit there. You must that resistor (a "pull down" resistor) to keep the input of the MOSFET truly low when you are not giving it a signal. If you don't put that resistor there, it will operate very erratically, turning itself on and off.

I'm a little confused about the voltage divider, but I feel that'll make sense once I have everything else figured out. Let's leave that for now.

It's just there to split 5V off the power supply. Could also be accomplished with any 5V voltage regulator. You don't need it if you are using a microcontroller to give the 5V high signal (the "turn the lights on signal). You don't need it if you are building a battery box.

Simple questions:
- What is the yellow component, or link(?)

A switch.

- The white components (3) circled...are those resistors as well?

Capacitors. See the ratings next to some of them (10uf, 16v)?

A bit more complicated questions:
- What are the qualities of a bleed resistor? Are they normal resistors used for this purpose and labeled as so?

You need a resistor that can handle a few amps. I used very large white ceramic resistors.

- Are the current limiting resistors any special type of resistors? I have yet to make any calculations...they could very well be normal resistors. I'm looking into that.

Same here. I'm using resistors that look like this.

[myenhdl]

Great. You hammered out all of my questions!

Since I'm only so new to AC circuits (this starting semester), I will stick to a different power supply for now. I have in the recent past played with alternate power supplies for desktop computers...PicoPSUs and different AC adapters, etc. I found the AC adapter for one of the Dell Optiplex desktops to be very efficient and sufficient. The model # is DA-2. This brick has a single 12v rail, rated at 18A for a total 216W of power.

As for voltage regulating (though I'm pretty sure this PSU is stable enough on its own), I'm thinking about the Sharp PQ12RD21- which is a LDO (low-dropout) Voltage Regulator rated at 2A. Since the PSU brick is already at 12V, I'm estimating about 0.4-0.5V drop after regulation. I was introduced to this V-Reg on the HIDPlanet forum...which has a subforum just for LEDs. This V-reg runs pretty cool compared to similar regulators at the same ratings.

Since the Cree Q5 XREs run at (I think...haven't looked at the data sheet for more than a year) 3.7v and 700mA max, I think a power source of 11.5-11.6v and 2A will be more than safe enough to run 3 LEDs in series.

I have about 15 Crees left, but have a feeling a handful of them are damaged. Currently planning to run 3 strings of 3x Crees for a total of 9 LEDs. Maybe 12 if I'm lucky...and if needed. Now I'll be doing some more reading on MOFSETs and I'll be testing this out sometime during the week. I'm pretty sure there's nothing hard but I never played with a MOFSET component before.

[daniel_reetz]

Geez, 12V 18A.. that's beefy.

Do you have any experience with board layout software? That's one thing I've been thinking would simplify this for everyone. A PCB and a parts list would probably solve most problems.

Everything else sounds great...