ASSEMBLING the ECEbot

Printed Circuit Board: Part Three

Due Date

The Part Three assembly steps must be completed prior to:

Prepared by R.C. Maher October 2005

Copyright © Department of Electrical and Computer Engineering, Montana State University, 2005 All Rights Reserved

20051018RCM PCB Assembly Part Three 2

3.1. Soldering Step 7: Light Emitting Diodes

3.1.1. Display

Figure 3-1 shows the location of the 4-digit LED Display (U1) to be mounted at the top center of the PCB. 4-digit Display (decimal points: lower right)

Figure 3-1: Position of 4-digit LED Display

First, take the 4-digit display and insert its 12 pins with the same orientation as the silk screen: decimal points to the bottom right of each digit.

→ As before, use a piece of tape or some other technique to keep the display flush with the PCB while soldering the display's pins on the back side.

3.1.2. Discrete LEDs

Four individual light emitting diodes need to be installed (LED2, LED3, LED4 and LED5).

Remember: The LEDs (light emitting diodes) are directional. The long wire is the anode and the short wire is the cathode. Make sure the LED is inserted so the short wire goes through the square pad

cathode: short wire anode: long wire

Short wire… …square pad

PCB Assembly Part Three 3

Figure 3-2: Discrete LED Position and Orientation

→ Solder each LED in place and carefully snip the excess wire.

3.2. Soldering Step 8: DIP Switches and Decoder Chips

3.2.1. DIP Switches

→ Next, solder in place the 16 pins of the DIP switch (S1), making sure that O 1 2 3 4 5 6 7 8 the switch numbers and ‘ON’ label read right-side up before you start soldering. N By now you are probably getting pretty comfortable soldering, but be sure to only use just enough heat and time to make good joints: overheating can cause damage to the components. 74LS47

3.2.2. Decoder Integrated Circuits

The final electronic components are two integrated circuit (IC) packages. 74LS138 These are black, 16-pin, dual in-line packages. Locate the two ICs in your parts kit, and notice that they are labeled with different part numbers: they perform two different functions. PCB Assembly Part Three 4

Hint: The integrated circuits can be damaged easily by mishandling. The major issues are static electricity and overheating during soldering. It is possible for a static discharge to occur even if you don’t feel or see a spark! To reduce the chance of an inadvertent static zap, be sure to discharge yourself frequently when working at the lab bench by touching a grounded panel or chassis. If possible, use an approved grounding wrist strap and antistatic mat.

→ The device placed in location U8 (left spot) is the 74LS47 (“BCD to 7-segment decoder”). Turn the device so that the notch on the package is to the left, and gently align the 16 pins with the U8 silk screen and holes. Keep the package flush with the board while you solder the pins on the back side. Don’t overheat the pins!

→ The device placed in location U6 (right spot) is the 74LS138 (“1-of-8 decoder”). Orient the package so the notch is on the left, align the pins with the holes, and hold the package flush with the board while you solder the pins on the back side.

74LS138 74LS47

Figure 3-3: Placement of the Decoder ICs.

3.3. Soldering Step 9: Computer Serial Port

→ The 9-pin serial communications jack mounts on the UNDERSIDE of the printed circuit board. Thus, you need to align the connector pins with the holes on the back side of the board and do the soldering on the front side. Be sure to attach the mounting screws and nuts before soldering: this prevents any unwanted stress on the solder joints. PCB Assembly Part Three 5

Secure with screws and nuts before soldering Solder on front side

Insert from underneath (back side) Figure 3-4: DB9 Connector Mounts on the BACK SIDE of the PCB.

3.4. Soldering Step 10: Board Fixes

Ever heard of Murphy’s Law? Yes, when something can go wrong, it sometimes does go wrong. The printed circuit board revision for Fall 2005 has three fabrication errors that must be fixed.

The first fixes require installing two short jumper wires. One wire needs to go from pin 2 on the header J23 to the middle hole of VR2 in the “optional power” area of the board, and the other jumper wire needs to go from pin 8 on IC U6 and also to the middle hole of VR2.

→ Cut a 6-7 cm length of solid wire and strip a few millimeters of insulation from each end. Using needle nose pliers, form one stripped end into a small hook shape. Flip the PCB over and look at the back side (solder side) of the board. Locate J23 pin 2 and the center hole of VR2, as shown in Figure 3-5. Hold the hooked end of the wire over solder joint for J23 pin 2 and run the other end of the wire through the center hole in VR2. Carefully solder the hooked end to J23 pin 2 and solder the other end to the center hole in VR2.

→ Next, cut a 4 cm length of jumper wire and strip a few millimeters of each end. Make a hook on one end and leave the other end straight. Again on the back side of the board, locate pin 8 on U6 and the nearby pin 2 hole of J12, as shown in Figure 3-5. Carefully solder the hooked end to U6 pin 8, and then solder the other end to the indicated hole of J12. PCB Assembly Part Three 6

S1 pin 16

J6 pin 3

U6 Pin 8

J12 Pin 2

VR2 middle

J23 Pin 2

Figure 3-5: BACK side of PCB indicating three jumper wire locations.

→ The third jumper wire needs to go from pin 16 of the DIP switch (S1) to the nearby pin 3 (ground) of header J6, as shown in Figure 3-5. Cut a 2 cm length of wire and strip a few millimeters from each end. Locate the proper pins on the back side of the board and carefully solder the jumper wire in place.

→ Use a multimeter to verify that the jumper wires are connected properly by testing the continuity between the ground pin (J24) and both ends of each jumper wire: all of the affected areas should now be connected to the board’s ground reference. PCB Assembly Part Three 7

3.5. Final PCB Visual Inspection

Take a moment to examine the board carefully and critically.

ƒ Are there any missing components—other than the unused areas of the board? ƒ Are all the pins soldered completely? ƒ Are there any "whiskers" or "solder bridges" between the pads? ƒ Is there any loose debris (like solder crumbs or metal flakes) caught anywhere on the board? Look closely. ƒ Are the jumper wires securely attached and routed appropriately? ƒ Do you see any damage to the board or its components, like bent pins, cracked housings, or crushed LEDs?

Be sure to ask your instructor or lab TA to examine any questionable areas of your PCB. You will perform electrical tests during the next lab session.

You must complete the entire PCB assembly to this point by the due date.