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Lab 3

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Goals: In this lab you will carry out soldering required to prepare the printed-circuit board on which you will build your ultrasonic velocity sensor.

1) Introduction

In this lab you will solder sockets and some electronic components into the printed-circuit board that will become your 6.002 ultrasonic velocity sensor. Future labs will focus on completing and testing different sections of the sensor, and ultimately demonstrating full functionality.

While soldering instructions and techniques will be offered at the beginning of the lab session, one piece of advice is offered here. Generally speaking, the sockets and components you will solder to the printed-circuit board, and the board itself, can withstand the heat of soldering reasonably well beyond the time it takes to make a good solder joint. Therefore, make sure to heat each solder joint well enough to melt all the solder allowing it to fully wet the metals being joined. Not doing so tends to create cold solder joints that are essentially open circuits, and difficult to see and debug. They sometimes crack over time, making debugging doubly hard. Use enough solder to make a good connection, but not so much that it spills over to the neighboring connections.

Shown below is the blank printed-circuit board. Note that the boards this term are ever-so-slightly different than the layout shown here. The left-hand photograph is a top view of the board. The sockets and components will be mounted on this side. The right-hand photograph is a bottom view of the board. Most, though not all, of the soldering will be done on this side. Note the soldering pads on this side of the board. Note also that the board is well labeled on the top and bottom to show where various sockets and components go. Test points and power supply connections are also labeled.

2) Solder!

Now you are ready to solder. To make soldering easiest we suggest generally adopting the following plan which allows for assembly-line style soldering.

  1. Insert the pins of the sockets, and the leads of the components, into their respective holes on the printed-circuit board from above. The pins and leads should all stick out from the bottom side where soldering takes place.

  2. Put a piece of cardboard or something similar on top of your board to sandwich the sockets and components in-between your board and the cardboard.

  3. Turn the sandwich over, and lay it flat on the soldering table. The cardboard should keep the parts from falling out in the process.

  4. Press down very lightly on the board to make sure the sockets and components are snug against the board.

  5. Now solder all socket pins and component leads to their respective pads on the board, one by one in succession.

The soldering plan outlined above works best if all sockets and components have the same height. Unfortunately, they do not. So, we recommend soldering in several passes, ten in all, beginning with soldering the lowest-profile parts during the first pass, and then moving on to solder successively taller parts and components on successive passes. In this way, the cardboard sandwich trick can keep all sockets and components snug against the board during soldering.

###Part 1

The lowest-profile parts are the integrated-circuit sockets. These should all be soldered during the first pass. Your board requires one 4-pin, seven 8-pin, and one 14-pin sockets as shown below in the left-hand image. They are soldered to the board as shown below in the right-hand image.

Now is a good time to show your socket soldering to a staff member. The expectation here is that you have good solder joints that wet the pads and the socket pins while not spilling over to neighboring joints.

###Part 2

The next lowest-profile parts are the discrete component sockets, and they should all be soldered during the second pass. Your board requires four 4-pin and two 6-pin sockets as shown below in the left-hand image. They are soldered to the board as shown below in the right-hand image. Note that these sockets are single strips each. The discrete components such as resistors and capacitors will be inserted into two opposing sockets as per the labeling on the top side of the board. This will be done in future labs.

###Part 3

The Teensy and display sockets should be soldered during the third pass. Collectively they are made from one 5-pin, one 7-pin, and two 14-pin single row sockets as shown below in the left-hand image. They are soldered to the board as shown below in the right-hand image.

###Part 4

Like most complex analog and digital circuits, your ultrasonic velocity sensor is organized into different sections, or stages. Each stage performs a different function. In order to make debugging each stage as simple as possible, the stages are not all connected to each other by the printed-circuit board. This allows you to input known test signals into each stage and view its operation separately and in isolation. Inserting and measuring various signals is done by connecting test equipment to the various test points laid out around the board. However, in the end, all stages must be connected together so that the sensor works as a whole. Connecting the stages is accomplished with headers. Headers are like sockets, but instead they have pins that stick out from the board on the top side. Connections between neighboring pins are made by slipping a jumper over the neighboring pins. The headers are soldered during the fourth pass. Your board requires four 2-pin and one 3-pin header as shown below in the left-hand image. They are soldered to the board as shown below in the right-hand image.

###Part 5

Next comes the power edge connector, which is soldered during the fifth pass. Your board requires one 5-pin edge connector as shown below in the left-hand image. It is soldered to the board as shown below in the right-hand image.

###Part 6

Hooray! You are now done soldering sockets and connectors. But, there are still a few electronic components that must be soldered. The majority of these components are decoupling capacitors, all to be soldered during the sixth pass. A separate decoupling capacitor goes between each power supply pin of every integrated circuit and the ground pin of that integrated circuit. The decoupling capacitors remove noise from the power supply voltage to keep that voltage as constant as possible, so that the integrated circuit works properly. To do their job best, they should be located as close to the integrated-circuit pins as is physically possible/reasonable; proximity minimizes parasitic inductance. So, the decoupling capacitors are soldered to the board as opposed to being placed in the discrete-component sockets.

Your board requires 17 0.1 uF decoupling capacitors. 16 are shown below in the left-hand image, while the 17th is put in position C14. They are soldered to the board as shown below in the right-hand image. At this point you can dispense with the sandwiching trick. Once you insert a capacitor from above, hold it snugly against the board and then bend its leads slightly apart in a “V” shape. It will now stay in place when you turn the board over for soldering.

###Part 7

Next come the 14 test points with their clever color coding, all to be soldered during the seventh pass. We suggest the following test point color pattern:

  • TP1 – brown; TP2 – red; TP3 – orange; TP4 – yellow; TP5 – green; TP6 – blue; TP7 – violet; TP8 – gray; TP9 – white; TP10 – brown; TP11 through TP14 – black.

Do you recognize the color pattern for test points TP1 through TP9? Test points TP10 through TP13 are all ground, hence they are colored black.

One extra test point is one of the only two differences between your board and the one in the images.

The test points are inserted into the board as shown below in the left-hand image. They are soldered to the board as shown below in the right-hand image. Note that once inserted into the board, the test points should stay in place as you turn the board over for soldering. You might wish to rotationally align the test-point pins as shown, with the holes horizontal side-to-side, so that it is easy to connect a scope probe to the test points.

###Part 8

The ultrasonic transducers are soldered to the board during the eighth pass. Here, the soldering is done on the top side of the board as shown below in the left image. The long leads of the transducers lay flat on top of their respective soldering pads, and are soldered directly to those pads. No soldering to the back of the board is carried out during this pass as shown below in the right image.

Note that the transmitting and receiving transducers are different. The transmitting transducer has a black back side, and is soldered on the left; see the “XMIT” label on the board (Note: When you take the transmitter transducer from the component box, make sure you don't take a piezo speaker by mistake. They look VERY similar and the both have a black back side). The receiving transducer has a metal back side, and is soldered on the right; see the “RECV” label on the board. Additionally, one lead from the receiving transducer is connected to the metal back side. That lead should be soldered to the right hand pad on the board as the board as shown in the image below. This orientation grounds the metal package around the transducer allowing it to act as a shield.

Soldering the ultrasonic transducers can be tricky. One way to do so is to first melt a little solder on one of the solder pads, and also on the corresponding transducer lead. Then, while holding the transducer with one hand and the soldering iron with the other, hold the transducer in place on the board and melt the solder on the lead and pad simultaneously with the soldering iron so as to tentatively solder the transducer to the board. The transducer should now remain in the desired position. This allows you to next solder the second lead firmly to its pad, and then return to the first lead and firmly solder it to its pad. Alternatively, you could try using an alligator clip to hold the transducer in place while soldering its leads to the pads.

Finally, note that the transducers are wider than the board is thick. Therefore, it is necessary to prop up the board off the table in order for the transducer leads to lay flat on the board. If you care to jump ahead in this document to learn about them, probably the best way to prop up the board is with the permanent standoffs described below. On the other hand, the permanent standoffs may make it too crowded to use alligator clips to hold the transducers.

Part 9

During the ninth pass you will insert a 1N5338B Zener diode into the board as shown below in the left-hand image. It is soldered to the board as shown below in the right-hand image. Note that the diode has a polarity marked by its white band. This polarity should match the marking on the board. The Zener diode serves to protect the Teensy and other semiconductor components in the event that you accidentally connect the wrong power supply to the board. The Teensy and the other components require only an external 5-V supply, yet we use a 30-V supply to power the ultrasonic transmitter. Accidentally connecting the 30-V supply to the Teensy and the other components will have sad consequences, necessitating the protection.

Hooray! Hooray! All soldering is done for the time being. A little more soldering of the discrete parts that make up the boost-converter power supply is in your future, but that soldering can wait.

While the soldering is done for a while, there is a little more assembly that can be performed at this point, namely the insertion of spacers to support the display, and to lift the printed-circuit board off the table so that its bottom-side solder joints do not become inadvertently shorted by metallic trash on the table. Lifting the board is carried out using four ¼”-diameter by ½”-long corner supports. Supporting the display is carried out using four 3/16”-diameter by 7/8”-long supports. All supports are held in place using eight 4-40 by 3/8” pan-head screws. The display supports go on the top side of the board as shown below in the left-hand image. The lifting supports go on the bottom side of the board as shown below in the right-hand image.