Ray's Hobby Projects

Blog moved to rayshobby.net

2011-08-03T20:58:00.000-07:00

I have decided to move my blog to http://rayshobby.net/blog/, which is based on WordPress. All previous posts and comments have been moved to the new site.

A MOSFET based fix to the Makerbot extruder motor problem

2011-02-01T18:49:00.002-08:00

As described in my previous blog about the first-time experience with the Makerbot, we encountered a problem where the extruder motor would stop moving after a while. Apparently there have been many discussions about this issue, such as here, here, and here. It seems to be generally agreed that the problem originated from the suboptimal performance of the extruder motor, particularly that its resistance can drop to close to 0 during operation, causing an overload current that fails the H-bridge (A3949) on the extruder controller board. I tried connecting a small power resistor in series with the motor and it still failed after a while.

The most commonly used solution seems to be the relay fix by rwensley. This allows the extruder motor to be powered directly from the +12V supply line, avoiding the failure of A3949. But the downside of a relay fix is that PWM is disabled, thus the feed speed cannot be fully controlled. So I started thinking about a MOSFET based fix. The first version is to use a single MOSFET (IRF510) in the place of relay, and this allows the motor to only move forward (which often suffices). The schematic is shown below:

The 1K resistor R1 is a pull-down resistor (1/4W is sufficient). Its value turns out to be important, because the on-board A3949 H-bridge seems to have a pull-up resistor of about 70K. Thus even when the H-bridge is inactive, the motor ports (1A and 1B) are pulled up to +12V, which must be pulled down to well below 2V in order for Q1 (IRF510) to remain off. The 1K value works perfectly (if the value is too large, it won't be able to pull the voltage low enough). The reason IRF510 works is because it allows a high current (20A pulsed) to pass through it, which is much larger than what the A3949 chip can handle.

Using MOSFET, the main benefit is that the PWM works, hence the motor feed speed can be controlled and tuned in order to print out fine details of a 3D model. I was able to print several models with great details that I wasn't able to achieve when PWM had to be fixed to 255 all the time.

After the single MOSFET solution worked, I started thinking how to make the motor move backward as well. It looked like the simplest way is to reproduce an H-bridge using 4 MOSFETs. The schematic is shown below:

It's basically one of the standard H-bridge implementations. Q1 and Q4 are N-channel MOSFETs and should use IRF510 or any alternative(STP16NF06, RFP30N06LE etc.) that supports at least 10A drain-source current; Q2 and Q5 are similar but P-channel, and should use IRF9530 or any suitable alternative (STP12PF06, FQP27P06 etc.)

Note that there is no special mechanism here to handle shoot-through, which is a known issue for H-bridge design. But in this case, even if shoot-through happens, it will be over a very short period of time, and the MOSFETs are able to handle a high impulse current if that happens. I've tested the circuit in continuously printing for several hours and have not encountered any problem. Also note that the kickback protection diodes are ignored as the MOSFETs already have built-in diodes. Here is the small protoboard with all components soldered on it:

With this fix, the extruder motor can now move forward and backward, and PWM works in both directions! I've seen in several places that eventually the extruder motor should use a stepper as it provides much better control. But before that happens, I am sticking with my fix for now.

A little angel for the Makerbot

2011-01-21T17:06:00.000-08:00

I tried to print out a scanned angel model. The details are a bit weak given the Makerbot's resolution. But it makes a nice decoration for the bot.

Two additional photos:

First-time experience with the MakerBot

2011-01-20T23:01:00.000-08:00

At the MakerFaire two years ago, I saw Bre Pettis demonstrating the first generation of Makerbot. I got fascinated, and since then I've always hoped to find a reason to buy one. Fortunately, a few weeks ago one of our projects needed 3D printed objects, and we had a chance to place an order of the Cupcake CNC Ultimate kit before it went out of stock. I heard many orders were placed, and we would like to thank Ethan at Makerbot Industry for his great help in assisting us with the ordering.

Assembling the Makerbot is an enjoyable task but requires skills and persistence. It took us two days to put everything together and get the device running. Here is a nice picture of the bot:

It really looks adorable. Now, that being said, keeping it to running stably turned out to be a quite challenging task, and there were several engineering issues we encountered. I am documenting these issues and the ways we solved them, in case they may help other Makerbot owners.

The first issue we encountered was that the extruder motor was not operating properly. Sometimes it would run for a few seconds then stopped rotating. I was very puzzled by this at first, but after looking around for a while, I noticed that when it stops, the motor's resistance drops to 2.6 ohm, which is way below normal (should be around 45-50 ohm normally). I suspect that it may be due to the motor brush running across a specific point that corresponds to a low resistance state, and normally due to the inertia it would pass that point immediately. But in this case, it seems the h-bridge that is driving the motor is not capable at handling the instant large current, causing the motor to stop eventually. After searching online for a bit, I found the extruder relay fix by rwensley, and we made a simpler version of the fix, assuming that the motor will only move forward and never backward. The fix involved an extra 5V relay, and 2 resistors that serve as voltage dividers to adapt the h-bridge's 12V to 5V for driving the relay. This has nicely fixed the extruder problem. I think a better option should be to use a more powerful h-bridge, or use a separate MOSFET driver. In any case, for now the quick and dirty fix works (see the picture below).

The second issue we encountered was that the heating unit of the automated build platform (abp) did not function -- it was simply not turning on the heating power. We thought of many potential causes of it, but none of them was correct. Eventually we noticed that the LED corresponding to the heating unit output never lit up, and this allowed us to quickly nail down the problem to the NIF5003 MOSFET. After testing, we found it's defective. This came as quite a surprise: a component on the extruder driver board is dead on arrival?! This was a hard find! I didn't have an NIF5003 replacement, but a quick look at the schematic reveals that this MOSFET is only used to drive a 12V relay, so it should be replaceable by a standard MOSFET like IRF510 (which I have tons of). Once we identified this, the fix was easy. The IRF510 is largely pin compatible with NIF5003 (even though the latter is a SMD component). After replacement as shown in the picture below, the heating unit worked like a charm.

The last issue that got really annoying was the z-axis motor making a gigantic noise and skipping steps. We found that this was due to the overly tightened screws and one solution I found online seemed to suggest that one platform screw should be released and ignored just to remove the over-constrained tension force. I was doubtful about this fix, but to our great surprise, this actually worked! No more loud noise and no more z skipping!

At this point the device is basically functioning properly. There are still a couple of small issues we need to fix, such as the heated abp not being able to stick the objects onto the platform. But overall we are able to print some nice 3D objects and are enjoying playing with them. So much for our first-time experience with the Makerbot!

Update on 'How to control Orbit 62035 valve'

2011-01-20T22:39:00.000-08:00

It has been a while since I updated my blog. Things have been quite crazy the past few months, but now I am back alive writing more blogs sparingly.

The first thing I want to share about is an update to my previous post that talked about how to control the Orbit 62035 valve. There have been a couple of missing pieces there which I would like to clarify. First, I found that a MOSFET cannot reliably control that valve. I am not sure why, but it may have to do with the on-state drain to source resistance. But using a MPSA14 (NPN darlington) works, and it requires a base current limiting resistor, so I've updated the schematic as below. Second, I was reminded that two kickback protecting diodes are needed to protect the transistor from the inductive current from the solenoid, so those are also added. These are the two main changes. The circuit below has been tested to work. Feel feel to leave comments.

Eagle schematic can be downloaded from here.

Tesla's image on Serbian money, and Tesla coils

2010-10-03T17:50:00.002-07:00

I was listening to the 'Ask an Engineer' show on Saturday night, and they mentioned that the $100 Serbian paper bill has Tesla's image on it. A friend of mine happened to be traveling in Serbia, and so very luckily, I requested one from him :) It seems this is equivalent to 1.29 US dollars.

And speaking of that, I am excited to post this video I recorded at the Make Faire New York 2010. It's the ArcAttack guys performing in the middle of a bunch of Tesla coils. Totally awesome!

Mini Maker Faire and the WaterFire event at Providence, RI (08/28/2010)

2010-08-29T19:54:00.000-07:00

Meet the Arduino WaterValveShield

2010-08-21T14:11:00.000-07:00

After finishing the previous minty water valve controller, I decided to make it an Arduino shield. This way, I can easily stack it onto other shields and extend its capability. I also added a few input buttons, and a DS1337 real-time clock, so that it can keep up with accurate time. Now the circuit has become much smaller, so I can't produce it with home-made PCB any more(sadly...). Instead, I ordered professionally made PCBs from Laen, and here you are, meet the Arduino WaterValveShield!

PCB board:

Components soldered:

Close-up view:

Connected to a serial LCD display

The schematic:

You can download Eagle schematic and PCB design here. Feel free to use it and/or modify it, but be kind to give me some credit for it :)

Parts list with Mouser/Digi-key links: valve_shield_parts.zip

As for sketch code, refer to my previous posts for code to control the valve and read input buttons. To interface with DS1337 RTC, I use this excellent RTC library.

Next steps:

Try to make the circuit more power efficient, and run on batteries for a long time. This should be possible by periodically putting Arduino to sleep.
Modify the circuit to control the Orbit 62035 valve.
Use Arduino PWM voltaget booster to eliminate the need for the LT1303 chip.

Multiple button inputs using Arduino analog pin

2010-08-08T08:06:00.001-07:00

At times I feel short of digital pins on the Arduino to handle multiple button inputs. Here is an easy way to use 1 analog pin to handle many input buttons. The way it works is very straightforward: use a resistor network as voltage dividers, and then let each button feed a different voltage to the analog pin. Thus by detecting the voltage we can tell which button has been pressed.

Schematic:

Download the corresponding Arduino sketch code.

As a downside, it cannot handle simultaneous button presses. To do that, one could potentially use resistors at doubly increasing resistance (1K, 2K, 4K, 8K...). Hence by checking the detected voltage, we should be able to tell which buttons are pressed simultaneously.

How to control Orbit 62035 valve

2010-08-02T21:07:00.000-07:00

As an update to my previous post, I took a look at the Orbit 62035 valve, which works with the older Orbit's yard watering system 62032. This valve is white colored, and has a standard 3-pin 3.5mm stereo audio plug.

To figure out how to control the valve, my initial guess is that the valve contains two coils, one opens the solenoid and one closes it. To verify this, I measured the resistance between the 3 pins of the plug. It turns out that pin 1 and 2 have a 4.5 ohm resistance, while pin 1 and 3 have a 0.9 ohm resistance. The 4.5 ohm resistance is reasonable, as it's roughly the same with the Orbit 91592 valve that I used previously. But the 0.9 ohm resistance is strange -- it clearly indicates a coil but the resistance seems too lower.

Having no other reference, I went ahead to apply voltage on the pins to see what would happen. Interestingly, applying +24v on pin 1 and 2 successfully opens the valve, but doing the same on pin 1 and 3 fails to close the valve. I tried everything I could to figure out what went wrong, but nothing came up. Out of luck, I decided to buy the full kit (62032) and reverse engineer the control unit a little bit. When I opened the control unit, I found that the entire circuit board is covered by a thick layer of water-resistant paste. This didn't look good. However, I did notice several big resistors, each reading about 3.9 ohm. The size of the resistors seems to suggest that they are rated at 2W.

Given this finding, my suspicion is that applying +24v directly across pin 1 and 3 discharges the voltage too quickly, thus cannot close the solenoid properly. In fact, given the 0.9 ohm resistance, a momentary current of 26 Amp is produced, which sounded quite scary. Adding a 3.9 ohm resistor is probably used to limit the current, slowing down the voltage discharge. This actually helps to build the electromagnetic field in the solenoid, allowing it to close properly. The idea turns out to work like a breeze: I connected a 3.9 ohm resistor between pin 3 and ground, and this time the valve nicely closed. At this point, I'm pretty sure I've figured out how it works.

You might wonder what the differences are between this valve with the Orbit 91592 valve. Here are my two cents:

Pros:
- 3.5mm stereo audio jack makes it easy to connect (in comparison, the 91592 valve requires custom connector)
- Pin 1 can remain connected to +24v, while grounding pin 2 or 3 is used to control the opening/closing of the valve. This simplifies the circuit design a lot. In fact, only two low-side drivers are needed to ground pin 2 or 3, which is much simper than h-bridge driver required by the 91592 valve.

Cons:
- Seems to be of its own kind on the market (my impression is that this is a discontinued product). Most other latching solenoids available on the market are similar to the 91592 valve. Fortunately Walmart still carries this product currently, but I don't know how long it will last.

Below is a sketched schematic when using this valve to replace the 91592 valve. As you can see, the circuit is much simpler than before. The driver can use either a darlington transistor (such as MPSA14), or an N-type MOSFET (such as IRF510).

Minty Water Valve Controller

2010-06-14T21:19:00.001-07:00

I've always been fascinated by minty projects -- circuits that fit neatly into a mint tin. There is an entire webpage on Make that documents such projects. For a long time I've been thinking of my own minty project: what can I build in a small mint tin?

Recently an idea came up: I had a new lawn installed in my backyard a couple of weeks ago, and I needed to start watering the lawn regularly everyday. I was looking into some automatic watering option like this one, but it provides limited functionality and does not suit my need. So I thought that perhaps I can build a water valve controller myself; and best of all, I can fit the circuits entirely in a mint tin. Voila, here comes my minty water valve controller!

Before describing how to build it, let me highlight some features of it:

A single li-poly rechargeable battery drives the circuit and a 24v latch solenoid
An Arduino pro mini programs the valve control
An RF module enables wireless control
Above all, it fits neatly into a mint tin

Here is a video demonstrating the controller in action:

(music by i am robot and proud)

The Design

For the water valve, I picked the Orbit yard watering valve. It's widely available in home improvement stores, and it is cheap. It has two pins: applying +24v opens the valve, and -24v closes the valve. It uses a latch solenoid, drawing power only when you open or close it. This makes it very power efficient.

To use a single li-poly battery to drive the valve, I needed a voltage booster to raise the 3.6v provided by the battery to 24v momentarily before connecting to the valve solenoid. For this I chose an LT1303 DC/DC step-up converter, but any similar converter will do as well. Switching between applying +24v or -24v to the solenoid is achieved by using some MOSFETs. I can't use small BJT transistors because they won't handle the large impulse current through the solenoid (as high as 5A). Darlington transistor would work but I prefer MOSFETS for their power efficiency.

To program the valve, I use an Arduino pro mini. It's adorably tiny and perfect for a mint tin project. I initially wanted to use the 3.3v/8Mhz version, as it can be directly powered by the 3.6v battery. But later I found that the wireless RF module only works with 5v anyways, so in the end I went with the 5v/16Mhz pro mini. This requires another voltage booster to raise the 3.6v battery to 5v. Fortunately I didn't have to build another voltage booster for it; instead, I reused an existing minty boost which I soldered a while ago. I took off the tiny circuit board from it. Again, it fits cutely inside the space-limited mint tin.

The wireless module I used is an RF link 434MHz transmitter and receiver from SparkFun. They are small and easy to use. In particular, the receiver is quite thin and can sit comfortably along one side of the mint tin.

The schematic of the circuit is included below:

Parts List

LT1303 DC/DC step-up converter
AOP605 complementary MOSFETs (each contains 1 N-channel and 1 P-channel)
2N2222 BJT transistor
1N5817 and 1N4001 diodes
RF link 434MHz transmitter and receiver
Li-poly rechargeable battery
Various resistors, capacitors, and inductor as specified in the schematic.
(note that the 2200uF capacitor C2 must be rated 25v or above)

The circuit directly draws power from the 3.6v battery. I use the Arduino pin 6 to control the shutdown pin of LT1303. This way, I turn on the voltage booster only when I need to open or close the valve. The voltage booster outputs roughly 24.4v.

Arduino pin 8 and 9 are used to control opening or closing of the solenoid. Both pins are set to low at start. Next, setting pin 8 to high causes +24v to apply on the solenoid, opening the valve; on the contrary, setting pin 9 to high causes -24v to apply, closing the valve. Don't try to set both pins to high at the same time, as it may short the circuit and cause damage.

Both the Arduino and the RF receiver are powered by the 5v output from minty boost. The data pin of the RF receiver is connected to Arduino pin 9 (which supports PWM).

The PCB

I soldered the initial prototype on a perf board. I made a mistake in connecting the MOSFET IC pins. This produced a spark and instantly fried the IC. Well, careful playing with 24v. After fixing the issue, the circuit worked like a charm.

But the perf board looks a bit ugly and is too bulky for the mint tin, so I decided to design a custom PCB using Eagle CAD. This is the first PCB I've ever designed and made, so I felt quite a bit excited. I used the toner transfer method to produce the PCB. Playing with etching chemicals was not very pleasant. Here are two snapshots of the PCB:

Assembly

I picked some mint tin cans from Whole Foods. They have beautiful cover images. Assembling everything to the tin proved to be tricky than I thought: it's not that I can't fit everything, but because working with a bunch of wires and fixing buttons to the side of the can in such a small space made me feel like sowing embroidery. Tweezers are absolutely must-have tools. Also, I puts lots of electric tapes inside the tin and on various circuit parts to cover exposed area. You don't want to accidentally short wires and cause trouble. Finally, I used hot glue sparingly to fix parts together. Below are snapshots of the parts before and after they are assembled into the tin:

Additionally, I added a power switch on the front and two buttons on the right to allow for manual control of the valve. Everything packs neatly!

Here is an annotated snapshot showing where each part is located:

Testing

As the tin is not water-proof, I use a zipper plastic bag together with a paper clamp to seal it. I built a simple RF transmitter circuit on a breadboard to test the wireless control. It did work, but the range is currently limited to about 5 meters. I attribute this to the low voltage (3.6v battery) I used to power the transmitter. I am sure using 12v will increase the range a lot.

The whole circuit is reasonably power efficient. I've run it for two days and it is still working. The battery I use is a 900mAh rechargeable battery. A nice feature I would love to have in the future is to have it solar powered. This will completely eliminate the need to recharge the battery manually.

There are currently plenty of pins left on the Arduino unused. This provides some space to possibly control more valves using the same Arduino. But I am unsure if everything can still fit neatly in a mint tin anymore. Perhaps surface mounts are the way to go.

I haven't programmed the Arduino to timer control the valve yet, but this should be straightforward. Some more advanced features can be included, such as installing a rain sensor to delay watering when it rains; or even better, use weather reports from online websites for intelligent watering! The wireless feature of the controller makes many options possible.

Code and Schematic Files

The Arduino code, Eagle CAD schematic/board files are attached below. Enjoy!