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Time flies. Yesterday was day 3 of my visit to Shenzhen. I went back to SeeedStudio and worked with the engineers on improving the design we talked about during day 1. Here I will give you a sneak peak preview of what’s happening.

First, we are going to design a paper box for OpenSprinkler. The design is largely based on SeeedStudio’s classic paper box, with art work and color adapted to OpenSprinkler. The first prototype is shown below. It’s pretty simple but it looks pleasant. I should mention that we are designing the paper box mainly because we would like to make this more towards a commercial product, for example, a product that you may find on Amazon.com. So we need proper packaging to give it the professional look and feel. But don’t worry, it will remain an open-source product — no matter how commercial it may be, the open-source nature won’t be lost.

Next, we are also designing an injection molded enclosure for the OpenSprinkler Zone Expansion Board. As the circuit design of the zone expansion board has more or less finalized, I think it’s time to make a custom enclosure for it as well. I don’t have a copy of the prototype design yet, but one of the main changes is that the new expansion board will fit 16 stations per board, and yet the size is approximately the same as the current expansion board (which fits only 8 stations per board).

If you are wondering what about OpenSprinkler Pi — we won’t be designing an injection molded enclosure for OSPi yet, mainly because I heard from grape vine that there will be another round of revision for Raspberry Pi, and I have been advised not to finalize the design yet. On the flip side, I am working on a BeagleBone Black version of OpenSprinkler (for now, let me refer to it as OpenSprinkler BeagleBone). It occurs to me that BeagleBone Black is probably a more suitable platform for sprinkler applications, particularly because it has built-in ADC and RTC. This would make the extension board design really simple. The circuit should be very straightforward, but I do have to spend some time re-working the enclosure design. Will keep you updated about the progress.

With respect to hardware revision of OpenSprinkler v2.0, there will be a couple of minor changes to address some of the engineering issues. One is a circuit-side protection to prevent the users from damaging the mcu even if they plug in 24VAC into the rain sensor port. This is a rare situation, but because the rain sensor port uses the same type of terminal block as 24VAC, a careless mistake will clearly fry the mcu. The simplest solution we came up with is to add a zener diode and a resistor to the rain sensor port, such that even if you apply 24VAC to the port it won’t damage the mcu’s pin. See the circuit diagram on the right above.

This is fairly standard, so I won’t dwell much on it. But I do want to mention that the resistor RZ should be selected carefully. On the one hand, it should limit the current through zener diode D4 when 24VAC is applied to the terminal. Assuming the zener diode can sustain 10mA, and the maximum instantaneous voltage is 40V DC, RZ should be no smaller than (40-3.3)/0.01 = 3.7Kohm. On the other hand, the rain sensor port has to preserve its normal function, which is to sense the connection / disconnection of the rain sensor. Because we make use of AVR’s internal pullup resistor, which is approximately 20Kohm, in order for the mcu to detect that pin 2 on the terminal is shorted to ground (pin 1), the resistor RZ should be chosen such that 3.3 * RZ / (20K + RZ) < 0.6 where 0.6 is the maximum voltage that appears as a logic LOW to the mcu. So RZ should be smaller than 5.2Kohm. Combining the two factors together, I've chosen RZ to be 4.7Kohm, which falls in the range in between. Another minor change in the plan is to add a few components for AC current sensing, in order to detect the AC current supplied to the solenoids. This is useful for two purposes: one for detecting defective solenoids (i.e. current draw exceeds the typical current draw of a typical solenoid; the other purpose is for monitoring the number of valves that are currently open, which can be done by simply checking the total current draw. In any case, this will be a fairly simple thing to add to the revised circuit. At the end of the day, a friend of mine and I went to a traditional Beijing style hot pot restaurant. Instead of using an electric burner, the traditional style hot pot uses a pot that runs on burning charcoal. It gives the food a nice 'smoked' taste, which is what you won't find in standard hot pot. The food was delicious, and I highly recommend trying it, at least once in your life :) So much for the report today. More to follow tomorrow!

Continuing my last post about this week’s trip to ShenZhen, today’s agenda is to visit Hua Qiang Bei (HQB) — the largest electronics market in the world. HQB is located close to the center of the city. The name literally means the north of the Hua Qiang street. The area spans several big buildings, each of which is full of counters and booths selling all sorts of electronics components and gadgets.

After coming out of the metro HQB station, the first building that came into my eyes is this one:

The banner says Hua Qiang Electronics World. Sounds like a place I should be, so I headed right in. Turns out this section of the building is mostly selling laptops, mobile phones, and commercial electronics gadgets. A bit disappointing. However, I discovered that the top two floors are dedicated to LEDs, which is one of the things I am looking for. As soon as I went up, I was faced with a sea of LEDs. There are just too many of them: some are for home lighting, illumination, some are for decoration. Many of them are color LEDs, displaying fancy moving patterns. It’s really awesome. Most sellers here are only interested in customers buying in large quantity. So if you tell them you are buying less than a hundred, they tend to lose interest right away and give you the ‘go away’ look. Some sellers do have the patience of talking to you and listening to what you do. I would rather patron these sellers 🙂

One of the LEDs I was looking for is of type WS2812, which is used by Adafruit on their Neo Pixels. This type of LED has a built-in IC which allows you to set the brightness of each color separately, and most amazingly it takes only one data pin. You can also cascade multiple LEDs. The data is sent in like shift register, and it gets latched on each LED. This is really a very smart design: with only one data pin, you get to control mutiple independent LEDs, and it requires minimum mcu cycle to operate. Many sellers that carry this product sell them in the form of fully assembled tapes of LEDs; a few do sell just the LEDs, at a price of about $110 for a bag of 1000. Sounds like a competitive price. Short of a reference price, I decided to look around and come back to buy when I feel ready.

The next stop is the ‘Q’ section of the building, which is dedicated to all electronics components. When I walked into this section, it feels absolutely like a heaven for electronics lovers. There are endless counters that sell all sorts of components that you can think of.

When I say ‘endless’, there is no exaggeration: standing at any point of the floor, you simply cannot see the end. It’s very easy to get disoriented. Also, there are multiple floors, each of which has a slightly different focus. The picture above on the right shows a counter that sells switches. Thinking of those times you spent on trying to find a Digikey part that fits your need? Here you can take out a real sample right at the counter and check it out in your hand.

I am curious how to choose which seller to buy from — since there are so many of them, the choice almost feels random. One thing I learned is that you shouldn’t feel rushed to buy: go around and compare different sellers before you make up your mind. Even at this low profit margin, there are still noticeable differences in price. Of course quality is also very important — often you can get a good idea by talking to the seller, and see if they are knowledgeable and professional. I tend to patron sellers who are willing to engage in a conversation and interested in knowing what I do.

It’s very easy to get lost and spend hours and hours here in the market. Moving to a different section in the building, I came across a floor which sells all sorts of cables, connectors, power adapters, terminal blocks etc. If you intend to buy a large enough quantity, you can also ask the seller to customize the product. Most things can be tailored to order. For example, I asked about 24VAC power transformers, and one seller told me depending on my need, they can customize the winding of the transformer and change the plug to any type of my choice. Wow, this is simply awesome 🙂 I was about to head to my next stop, which is the Sai Ge Electronics Market, and I ran out of time. So I will have to come back and visit again.

At the end of the day, I didn’t buy a whole lot: my goal was to do some initial sight-seeing in order to get an idea of where to focus my attention when I come back a couple of days later. Below is what I bought though: two bags of 1K self-flashing RGB leds, one bag of 1K SMD slide switches, two bags of 200 testing probes, and a few mini relays. With a total spending of less than $100, it’s a real bargain. Can’t complain at all 🙂

After I was done with HQB, my friend brought me to the swimming pool at the ShenZhen Bay Sports Center. This is undoubtedly the largest pool I’ve ever swimmed in, and a good way to end today. So long for today’s report. Will keep blogging tomorrow!

This week I am on a business trip to Shenzhen — the city with the largest electronics markets in the world. My plan is to visit several studios / factories and the of course the electronic markets here, and also some of my old friends. I will try to document my trip as much as I can.

Shenzhen is located at the very south of China — it’s right above Hong Kong. The weather at this season is usually very hot and humid. Fortunately it has been raining for the past couple of days so it’s not as toasting as it normally is. The image above on the right is the look of the city from my hotel window. There is an electronics market right across the street on the other side 🙂

One of the challenges of traveling in China is that a lot of external websites are blocked by the Chinese government. For example, Facebook, Twitter, Gmail, NYTimes. I was able to use a private VPN to get around with it, but the data transfer speed is really slow. On the plus side, you have access to a wide variety of inexpensive food, which is hugely attractive to a gourmet traveler like me 🙂

On the first day, I visited SeeedStudio, the company that is currently in charge of manufacturing the fully assembled OpenSprinkler 2.0. SeeedStudio is well known in the open-source hardware community, and they have been growing at amazing speed in the past couple of years. I have been looking forward to this visit for a while.

The building where SeeedStudio is located at looks very, short of finding the right word, non-technical, which made me feel that I’ve walked into the wrong place. But as soon as I found their main lobby, I was reassured. On close look, the back wall is made of hundreds of circuit boards painted in white. Very interesting.

Here is a picture of the ‘innovation corner’ and also the marketing / PR section of the studio.

The main agenda today was to discuss the design of a paper box and a printed user manual for OpenSprinkler, making a mold for the OpenSprinkler Zone Expansion board, and designing a laser cut enclosure for OpenSprinkler Pi. The meeting was quite productive: I talked with the product manager, an enclosure engineer, and an electronics engineer for several hours. Things got started moving right away. Among the electronics discussions, we covered several potential improvements to the current OpenSprinkler circuit, including preventing users from damaging the circuit by inserting the power to the wrong terminal port, automatic detection of defective solenoids, adding built-in WiFi etc. I felt very excited to start prototyping these ideas right away.

After the meeting, the product manager gave me a short tour of the studio. The top floor is where the offices and meeting rooms are located. The bottom floor is where the production pipeline is.

Here are pictures of the soldering / assembly area, and the SMT pick and place machine.

Next, the laser cutter and a testing area where engineers were testing a brain wave (EGG) controlled drones. Hmm, very impressive!

Finally, here are two bins of assembled and tested OpenSprinklers. It really pleases my mind to see these lovely OpenSprinklers 🙂

So that’s all for my report of Day 1. My next stop is Hua Qiang Bei (???), where you can find almost any electronic component you can think of right at the counters. Looking forward to it!