Practicum 2A

1. Goals

The goal of this practicum is to assemble a pressure sensing circuit which will be used to monitor the depth of the ROV during operation. This will require learning how to assemble electronics on breadboards, operate power supplies and multimeters, and solder through-hole components to printed circuit boards (PCBs).

SAFETY NOTE: This is repeated below in section 6 because it is very important that everyone reviews this material before soldering.

Note on Soldering:

Before you begin soldering, watch this video on the safe soldering, then this video on proper soldering techniques. A few key things to remember when soldering:

· If the soldering iron is not in your hand, turn it off! This is important for preserving the tips of the irons, which oxidize quickly at high temperatures and lose thermal conductivity as a result.

· Use the water bottle on the central table to lightly wet the sponge for the soldering iron before you begin soldering. It should be damp but not dripping. Do not bring your sponge to a drinking fountain because the sponge contains lead from the solder. (This is one of the good reasons for us never to eat in lab!)

· Before you begin soldering, apply some solder to the tip of your iron. This is called tinning. Tinning ensures that your tip effectively transfers heat to the pad and also maintains the life the solder tip. Wipe any excess solder on the sponge. Continue to wipe excess solder on the sponge during your work.

· Keep the temperature of the soldering iron below 550°F. There is no need to have it any hotter. Turn down the temperature if you notice lots of smoke.

· Each station has a solder fan to absorb smoke. Please turn it on before soldering and do your work in front of it.

· Do not apply excessive solder to the soldering iron tip during work. The joint between the iron and the piece being soldered should get hot enough to cause the solder to flow. If you apply the solder directly to the soldering iron, you might create a large glob of solder called a cold joint which is not electrically conductive and may crack. You can identify cold joints by their rough appearances versus the normal smooth look.

· As a rule, synchronized soldering is a recipe for injury. Soldering is not a partner activity because you can only feel where your own hands are, which means you won’t have the instinctive sense to keep the tip away from your partner’s hands. Only the person holding the hot, dangerous object should have hands anywhere near where it is used.

Note on electrical safety:

Please watch this video on electrical safety. We are working with electrical components and power supplies around water. This can be a dangerous combination, and everyone should be mindful of safe practices at all time. If you have any questions, please do not hesitate to ask any member of the teaching team.

2. Deliverables

Lab Skills

Assemble electronics on breadboards
Operate power supplies and multimeters
Solder through-hole components to printed circuit boards (PCBs)

Lab Concepts

Understand the pressure sensor circuit
Resistors

Tutorial Skills

Nothing specific in this practicum

Data to Save

Depth / voltage data
Turn in Submission Sheet (found on Canvas)

3. What You'll Need

Pressure Sensor Experiment

Graduated Cylinder
Meter Stick
Tubing
1 x Absolute pressure sensor

Parts for Circuit

1 x red LED
1 x 200 Ohm resistor
1 x OpAmp (MCP6002)
1 x 47k Ohm resistor
1 x 1.8k Ohm resistor
1 x 10k Ohm resistor
1 x 0.1 uF capacitor (yellow 104)
1 x 1.0 uF capacitor (blue)
1 x 6 90-degree connector
1 x 8-DIP socket
1 x Shrouded Male Connector

4. Familiarity with Equipment

4.1 Power Supply

Power supplies are electronic devices that are used to supply electrical energy. We will use the power supply to convert the unregulated AC from the wall outlet into a constant DC output (learn more about AC vs. DC).

The power supply you will be using for this lab is the Agilent E3630A, pictured in Figure 4.1 to the right. This power supply can provide a fairly wide voltage range (±20 volts); however, we won’t need that for the purposes of this lab.

Figure 4.1: Agilent power supply with various outputs.

The small display is used to display the voltage and current being supplied. On the top right, there are three knobs, the first two adjust the output voltage. The third is the tracking ratio, which should be set to “Fixed” – the tracking ratio helps control the -20V output, which we will not be using. The power button is located on the bottom left corner. The voltage range options are to the right of the power button. Finally, the supply outputs are located on the bottom right.

There are 5 outputs, listed to the right.

Table 4.1: Various outputs of a common power supply.

Often times, you will connect COM to ⏚ (Earth ground) for safety reasons (learn more about common vs earth ground).

Voltage is measured relative to other voltages. “Common” is an arbitrary baseline voltage that is assumed to be equal to 0 volts in the circuit. All of the red outputs in the power supply are measured relative to COM (common), so it is often used as “ground” in circuits.

4.2 Multimeter

Multimeters are electronic instruments that give you the ability to measure many electrical attributes. Most multimeters are able to measure resistance, voltage, capacitance (and sometimes inductance) between two electrical nodes. Figure 4.2 illustrates an example of a multimeter.

Figure 4.2: (a) Greenlee DM-210A Multimeter set to measure voltage; (b) Multimeter probes used for measurements.

4.3 Measuring Voltage

Turn on your multimeter and set it to the voltage measurement setting. This mode lets you measure the potential difference between two nodes in an electrical circuit. Connect a black probe to COM and a red probe connected to the V terminal. Important: Make sure that the multimeter is in DC mode (indicated in the top left of the display).

Turn on your power supply. Ensure that it is in the 6V mode by pressing the +6V button. Use the 6V voltage adjustment knob to set the voltage to a value between 0V and 4V. See Figure 4.3.

Figure 4.3: Power supply set to approximately 2 volts using 6 volt adjustment knob.

As illustrated in Figure 4.4, place the red multimeter probe into the +6V terminal in the power supply. Place the black probe into the COM terminal in the power supply. You should see the same (or very close) voltage reading on the multimeter.

Figure 4.4: Using the multimeter to measure the potential difference between the +6V terminal and COM terminal in a power supply supplying 4V DC.

4.4 Resistors

Resistors are passive electrical components that dissipate power in a circuit. The dissipative effect of a resistor is referred to as resistance and measured in a unit called ohms (Ω). Resistance is a value that can vary from 0Ω (an ideal wire, often referred to as a short circuit) to infinity (a break in the circuit, sometimes called an open circuit). Explicit resistors which are added to a circuit fill the gap in between these two extremes.

If you look closely at a conventional resistor, you will notice a series of colored bands. These bands indicate the resistance of a resistor. Figure 4.5 illustrates how to read these bands. For most resistors, the first band represents the first digit, the second represents the second digit, the third represents the multiplier, and the final strip represent the resistor’s tolerance. For example, if you see a brown, black, yellow, gold resistor, its resistance is 10*10KΩ +/- 5%, or 100K Ω +/-5%.

Some of the resistors you may encounter are 5 band resistors. Feel free to look up the color coding for those.

Figure 4.5: Resistance color codes.

1. Turn on your multimeter and set it to the resistance mode.

2. Have one partner get a random resistor from the central supply without telling the second partner the value. The second partner should use the chart above to decipher the resistance.

3. Now, verify the value by touching the probes of the multimeter to each side of the resistor. If the resistor says OL, you will need to increase the range of your multimeter. If the resistor reads 0.00, you will need to decrease the range of your multimeter.

How accurate is the actual to the expected resistance? Is this within the acceptable tolerance as indicated by the silver or gold band?

4.5 Equipment and Techniques

Locate the large grey box in your station, and look for a double banana plug to BNC female adapter. See Figure 4.6 below. BNC is a type of coaxial cable. The outside enclosure is grounded, while the inside carries the signal.

Figure 4.6: (a) Box containing various electrical adapters; (b) BNC male to Double Banana Plug.

Locate the side of the banana plug that says GND on a tab – this indicates the ground connection of the adapter, so it must be connected to COM or you might damage your circuit. Plug the adapter into the COM and +6V terminal in the power supply. IMPORTANT: Make sure the tab is on the right-hand side when plugged in. See Figure 4.7 below.

Figure 4.7: Adapter plugged into the power supply (+6V and COM).

Screw a BNC cable onto a BNC to test-hooks probe adapter. See Figure 4.8.

Next, connect the BNC cable to the female BNC-plug adapter on the power supply.

(c)

Figure 4.8: (a) Female BNC to test hooks; (b) Test hooks attached to BNC cable; (c) Test hooks + cable assembly connected to female BNC adapter on power supply.

Make sure that the power supply is turned off even if it was on before. You will come back to this setup in Section 5 when using your breadboard.

Wire Stripping

Next, you will practice how to strip wires. Wire stripping is the process of removing the outer insulation/casing in order to expose the internal, conductive wiring. This lets you connect the wire to other things, like the inside of a MyDAQ or even other wires.

The grey box at your station should have a pair of wire strippers. See Figure 4.9.

Figure 4.9: Different kinds of wire strippers.

Grab some scrap wire and have both partners practice until you begin feeling comfortable stripping wires. Place the end of a wire in the notch of the wire strippers. Apply gentle pressure as you squeeze down on the wire (with the simple wire strippers like the ones pictured on the left, you will have to feel for a change in resistance, and this takes practice to perfect). While cutting through, gently pull away from the wire until the insulation is removed. Watch this video on how to strip a wire for more information.

Now that you know how to strip wires, you will be inserting the wires into breadboards in the following section of the practicum.

5. Breadboard Assembly and Pressure Test

In electrical engineering, breadboards are not used for slicing bread (although they were originally made with bread boards), instead, they are used as a means of quickly prototyping electrical circuits. They are reusable and make it very easy to connect multiple wires without soldering or twisting them together. Figure 5.1 shows what an ordinary breadboard looks like.

Figure 5.1: (a) Common, 30-row breadboard with connection diagram; (b) Back of breadboard revealing internal wiring.

In a breadboard, each hole in a row which spans across letters a-e or f-j (letters are on the top and bottom) is electrically connected. Note that the dashed line in Figure 5.1a marks a break in the connection between rows. Each column headed with a “+” or “—“ is also electrically connected. These connections allow you to prototype circuits. For example, Figure 5.2 shows how you can make a simple LED circuit.

Figure 5.2: Simple LED circuit on breadboard.

Switch to continuity mode on your multimeter . Continuity mode helps you check to see if two nodes are electrically connected. It is often used as a means of debugging a circuit; in this practicum, you will be using this mode to learn about and verify the connections in a breadboard.

The multimeter should beep when you touch the two probes together.

Take a wire that is stripped on both ends and place it somewhere in a column with a minus sign at the top in your breadboard. Place another wire somewhere else on the same column. Use the probes to touch the other ends of both wires. The multimeter should beep, indicating that the entire column, sometimes called a rail, is electrically connected.

Make sure that the probes are making a good contact with the wires sticking out of the breadboard.

Connect the positive and negative rails of one side of your breadboard to the positive and negative terminals of the power supply. Attach short wires (Red for +6V and black for ground) to your breadboard in the power and ground strips (the “+” and “—“ columns respectively) and grab them with the hooks connected to your power supply from step 4.5.4.

Assemble the circuit, shown in Figure 5.2, on your breadboard. Use an LED (light emitting diode) and a 200 ohm resistor from the central supply to assemble the circuit. If your LED doesn’t light up, switch which pin is plugged into the resistor and which is plugged into ground.

Leave your power supply off. Turn it on only after you have reviewed your circuit and are ready to measure an output. If wired incorrectly, the LED will permanently break. Be sure to only use the 6V supply.

Try to follow good breadboarding practice. Do your best to keep wires as short as possible by measuring and trimming them before placing them on your breadboard. Don’t worry about cutting up wires: they’re cheap and it is good practice to cut them to length responsibly. Think of your breadboard like a function in Python; the neater it is, the easier it is to debug and show others.

If you run into any issues, consult with a professor or proctor. They are there to help!

You will be using a multimeter to measure and display the current passing through the LED/resistor assembly you have created. Turn the power supply off while make changes to your circuit. Disconnect the red plug on the multimeter from the V/ohm terminal and plug it into to the mA terminal. Disconnect the red hook from your breadboard and connect it to the red probe of the multimeter. Turn the power supply on and set it to 6V. Set the multimeter to measure current in mA and touch the black multimeter probe to the red power supply wire on the board. Record the measured current. Repeat the process with the power supply set at 5V, 4V, 3V, 2.5V, 2V and 0.1V increments down to the level at which current does not flow through the LED. A circuit with a multimeter configured to measure current appears in Figure 5.3.

Figure 5.3: (a) Multimeter with probe plugged into current terminal and measurement mode; (b) Multimeter probes attached to resistor/diode circuit to measure current.

Notice that the multimeter is hooked up in a very different way for this measurement. Before, our black probe was always attached to ground and our red probe poked at points in the circuit to measure their voltage. To measure current, we’ve inserted the multimeter into the path of the circuit so that current must flow through it. (We’ll learn later that we have put the multimeter in series with the other elements in the circuit).

Plot your measurements. Is the current through a resistor/diode (LED) linear in voltage or non-linear in voltage?

Answer Question 1 and 2 on the submission sheet.

Return your red multimeter probe to the V/ohm multimeter plug when you’re done with this measurement.

You should be temporarily finished using your power supply at this point. Be sure to turn it off.

6. Circuit Board Assembly and Test

For permanent circuits, it is common to design and fabricate a circuit board. Figure 6.1 shows you the inside of the E79 Motor Control Board (MCB) that was manufactured just for the purpose of this course. Think of a circuit board like a custom, permanent breadboard. The inside of the board has copper traces that act as wires and connect components together. Admittedly, it might look very intimidating to try to understand the circuit board at first. However, as you spend more time with the circuit, you will find that it is just a combination of many simple circuits, each with their own purpose.

With your partner, review the circuit board layout below. Compare it to the printed circuit board (PCB) that you received for the practicum. If you have some time, use a multimeter on continuity mode to investigate which components are connected together.

Figure 6.1: E79 circuit board layout.

You are about to solder resistors, capacitors, and sockets for an op-amp and pressure sensor to the board. These components are pictured below in Figure 6.2. Find these elements, which are specified in the parts list in Section 3, in the central supply for the room.

Figure 6.2: Parts to be soldered to PCB.

If neither partner at your station has soldered before, find an instructor or proctor for a brief soldering tutorial before you start on your own. If one partner has soldered, they should advise the other partner.

Note on Soldering:

Before you begin soldering, watch this video on the safe soldering, then this video on proper soldering techniques. A few key things to remember when soldering:

· Keep the temperature of the soldering iron below 550°F. There is no need to have it any hotter. Turn down the temperature if you notice lots of smoke.

· Each station has a solder fan to absorb smoke. Please turn it on before soldering and do your work in front of it.

You will start off with an unpopulated printed circuit board (PCB). The PCB has through-holes for the sockets and resistors that you will be mounting. Figures 6.3 and 6.4 give you some information about how to solder components to the board. Figure 6.3 shows how to bend the leads of a resistor so that it is secured to a board before soldering it. Figure 6.4 shows several common types of bad solder joints; try not to make them. If you see a joint you think is suspicious, reheat it without adding more solder; this process often causes solder to reflow and fix itself.

Figure 6.4e is a particularly pernicious mistake called solder bridging. It shows accidentally connecting two adjacent pins with solder. If you make this mistake, then apply heat to the center of the solder bridge and apply pressure between the pins to try to “cut through” the center of the bridge. If this doesn’t work, summon a proctor or instructor. Bridging can be avoided in the first place by striving to not use too much solder.

Figure 6.3. Side view of resistor mounted onto PCB.

Figure 6.4: An example of a proper solder joint compared to many improper solder joints.

The first thing you will solder onto the board is the socket for the op-amp chip. Instead of directly soldering the op-amp to the board, the socket allows you to easily replace a defective op-amp. In Figure 6.5, you see the socket soldered onto the board. It is necessary to hold the socket in place during soldering. This can be accomplished with electrical tape on the side NOT being soldered or by flipping the board over after the socket has been placed and applying pressure to it to hold the board down on top of the socket. Be mindful of the direction of the notch on the socket.

(a)

(c)

(b)

Figure 6.5: (a) Printed circuit board (PCB) with chip socket slot in red (b) Chip socket inserted into the board; (c) Back of PCB with solder joints boxed in red.

Next, you will solder the resistors for the pressure sensor circuit to the board. There are other circuits on the board, many of which contain resistors, which you will assemble in future practicums. Do not assemble those other circuits.

Figure 6.6 shows where each resistor should be placed. The board also has helpful annotations, called silkscreens, which tell you where each resistor should go. Resistors are non-directional, so their orientation does not matter (see Figure 6.6a).

DOUBLE CHECK YOUR RESISTOR VALUES BEFORE YOU SOLDER.

Mixing up the resistor values is the most common reason that student boards don’t work. Note that the board has the proper resistor values written on it, so you can check your values (using the colored bands or your multimeter) against those written on the board.

Once you have placed the resistors into the correct places turn the PCB over and solder them all in place. After soldering, use diagonal cutters to snip off any long leads. Be sure the wire ends are not pointing at your face before you cut. See Figure 6.6 for illustrations.

Fig 6.6(a)

Fig 6.6(b)

Fig 6.6(c)

Figure 6.6: (a) Resistors placed around the op-amp socket; (b) Resistor leads protruding from the underside of the PCB. Bend the leads to hold them in place; (c) Resistors soldered and snipped; (d) Diagonal cutters.

(d)

Switch which partner is doing the soldering at this point: we ask this to ensure that each partner solders about half of the components. Next you will solder the capacitors as pictured in Figure 6.7. You may ignore the orientation of the capacitors. Though many capacitors are directional, the ones we selected are non-directional. Remember that the .1 µF capacitors are the yellow 104 ones. 1µ are the blue or larger yellow ones.

Figure 6.7: Bypass capacitors soldered to the board.

As shown in Figure 6.8, find and solder the pressure sensor connector to the board. The proper location to connect it is labeled with the word “P_SENSE” and shown in Figure 6.8. The connector has a 90-degree bend. Place the pins into the holes on the board (but do not plug in the pressure sensor!) and solder them in place. This part is tricky, but avoid having your partner touching the board as you solder. Use a heat resistant object to hold the connector in place as you solder it.

Figure 6.8: (top) 90-degree connector for pressure sensor, which should be soldered to the board; (bottom) Pressure sensor connected to 90-degree connector. Do not solder the connector with the sensor attached. Do not solder the sensor to the board.

Add the op-amp chip to the socket by placing its pins in the socket holes and pushing down. Be sure the notch on the top of the op-amp chip aligns with the notch in the socket to make sure you have the chip right-side up. You may need to delicately bend the pins to get them to fit into the socket. Figure 6.8 shows a close up view of the op-amp so you can identify it (notice that it says MCP6002 on the top) and a picture of the op-amp in its socket.

Figure 6.9: (left) An MCP6002 operational amplifier; (right) An MCP6002 in its socket.

In order to collect measurements and eventually, run the motors on your robot, you will need to have a shrouded male connector soldered on to your board. See Figure 6.10 for location.

Make sure you solder the male connector so that the notch is facing away from the board (in Figure 6.10, the notch is on the left of the red box enclosing the male connector). This is very important, as it will reverse the pin connections to your breakout board and is very difficult to desolder.

Answer Question 3 on the Submission sheet.

Figure 6.10: Board with shrouded male connector soldered on.

There will be a breakout board with another shrouded male connector already soldered on. You will use a ribbon cable to connect your board to the breakout board, and then that breakout board will be plugged into a breadboard. This way you can plug separate wires securely into the breadboard to take your measurements.

Figure 6.11: Breakout board.

Get a meter stick from near the door. It should already have tubing taped to it and a pressure sensor at the end. This will allow you to measure the submerged depth in the water.

Connect the pressure sensor to your board in the right orientation (valve should point up in the same direction as the board, see Figures 6.10).

Figure 6.12: One side of the tubing is connected to the pressure sensor valve, the other is taped to a ruler, which will be used to measure the submerged depth.

Use a ribbon cable to connect your breakout board to your main board. Plug the breakout board into a breadboard. See Figure 6.13.

Figure 6.13: Board setup with ribbon cable and breakout board.

Fill the large graduated cylinder with water from the sink near the robot storage cabinets to a depth of at least 30 cm. Place the graduated cylinder on the ground, away from anyone’s path.

To power the circuit, connect the +6V terminal of the power supply to the pin labeled “5V.” Connect the ground terminal of the power supply to the pin labeled "GND."

Before turning on your power supply and supplying the required 5V to the circuit, have an instructor or proctor quickly review your circuit.

The PRES node is connected to the output of the op-amp. To measure the pressure output with a multimeter, plug a wire into the row corresponding to PRES and connect it to a multimeter using an alligator clip. The negative probe/alligator clip must be connected to the circuit’s analog ground at the GND connection or the power supply COM plug. Figure 6.11 shows you where 5V, GND and PRES are located.

Once approved, turn on the power supply and set it to supply 5V. Measure the output voltage when the tube is sitting in air.

At this point, your setup should look similar to Figure 6.12. The figure does not show the board connected to the power supply, but yours should be. Be sure the power supply is on! (Don't worry about the label on the ribbon cable).

Figure 6.14: Set up and ready to measure the voltage output from the pressure sensor, using a digital multimeter (DMM).

You will do a calibration curve later on in Practicum 2D, but first it's important to check that your board is soldered correctly.

To do a quick test, simply dunk your meter stick into the graduated cylinder full of water. The voltage should increase as you plunge the meter stick deeper. If it does, then you have a working board. If not, you will need to debug your board.

Answer Question 4.

If you have time, measure the voltage at seven evenly-spaced depths and record your data. This data will be used later to determine the depth of your robot based on the pressure sensor voltage reading!

Tips on Debugging:

Check all of the solder joints.
Use a multimeter on the connectivity setting to make sure your solder joints are done correctly. Make sure the power to the circuit is off in order to get consistent results. Touch a probe to either side of the joint, and listen for the beep.
Make sure wires that are plugged into a terminal or breadboard are plugged in all the way.

Figure 6.15: Meter stick with clear rubber tubing inserted into graduated cylinder.

7. Optional (for now) data analysis

These are steps that you do not need to complete now. They will be part of the Week 2D practicum. If you do them now, be sure to save your work to use later.

· Using Excel, find a linear fit for the data you collected. We can assume the data is linear within this range since this is a linear sensor and you are using it well below its maximum operation range (take a quick look at the datasheet if you’re interested).

· We are using water to test the pressure sensor because the pressure in water (neglecting atmospheric pressure) varies linearly with depth:

Use 9.81 m/s2 for the gravitational constant and 1000 kg/m3 for the density of water. The pressure you calculate using Equation 1 is the relative pressure difference between the surface of the water and the pressure at some submerged distance, .

Note: Be careful about the units for pressure. For consistency, use the SI system (Pascals (Pa) for pressure, meters for distance, etc.).

The linear fit to your data can be used to relate measured voltage to depth below the surface. Your fit should be of the form of Equation 2.

The constants a and b should be extracted from your linear fit.

The total pressure at any submerged depth is measured relative to the pressure on the surface of the water (atmospheric pressure). So the total pressure as a function of the output is expressed by Equation 3.

You may check the current atmospheric pressure online.

· You now have an equation that calculates the pressure as measured by your pressure sensor. You will use this equation in LabVIEW later in the course.