Lab 7: Soldering - Traffic Light Controller ReadMeFirst Lab Summary The two way traffic light controller provides you with a quick project to learn basic soldering skills. Grading for the project has been greatly simplified. The breakdown of your lab score is as follows: Lab attendance (you must be present when your final score is assessed) - 7 points You finish the soldering the traffic light controller - 1.5 points Your traffic light controller changes states - 0.5 point Your traffic light controller works perfectly! - 1.0 point Lab Background There are about a dozen different ways that you can design a traffic light controller. The method used on this board uses the charging times (time constants) of RC "filters" to set the durations for each of the states in the traffic light. The D Flip-Flop circuits control the OFF or ON State of the LEDS. When the RC step responses charge to the triggering voltage for the D Flip-Flop, the "D" value (In this case HI, or 5 Volts. LOW is 0 Volts) is "clocked" to the "Q" output. The output is just the inverted value of Q. The order that the D Flip-Flips are strung together controls the sequence of the traffic light cycle. Each state in the traffic light cycle always "clears" the previous state. This controller is also a classic State Machine design. The topic is beyond this course, but notice that there is one D-Flip-Flip for every state in the traffic light cycle. The biggest drawback to this method is that the traffic light must go through at least one or more full cycles until the times durations stabilize. When the RC circuits begin charging, they are at 0 Volts. After one cycle, they are only partially discharged, so when the charging begins again, the duration is shorter. After 2-3 cycles the beginning and end voltage stabilize on a final value and the state durations are constant.
Lab Preparation Video: Please watch the following video BEFORE attempting the lab. Soldering.mp4 Lab Supplies 1) Soldering Station 2) Safety Glasses 3) Soldering Lab Kit 4) Parts from the ECE 2020 Lab Kit (see Appendix A) There is much material to cover. Notice that the bare board has letters and numbers associated with each part. These are called Reference Designators and are matched to the symbols for the parts on the schematic as well as the Bill of Materials (Appendix A) for this particular Lab. Refer to the schematic(appendix B) and the reference designators to identify the part to be soldered in place. There are no reference designators for the 14 pin DIP sockets, however. For the D flip-flops, there are two D-flip flips for each integrated circuit (IC, or just "chip"). On the schematic each IC has 3 blocks per chip: A, B and C. The A and B blocks are flip-flops, while the C block shows the power connections to the IC. So U1 was a U1A, U1B and U1C block on the schematic. There are still 14 pins per chip and the pin numbers are not repeated. The flip-flop parts are shown inside the 14 pin Dual Inline Package (DIP) here:
Procedure: 1) Begin by soldering the 3 DIP14 sockets in place at U1, U2 and U3. Use masking tape to hold them flat. MAKE SURE TO PLACE THE D SHAPED NOTCH ON THE LEFT SIDE as discussed in the video. UNFORTUNAETLEY THE ARTWORK DID NOT INCLUDE A MATCHING NOTCH THIS TIME. NOTE THAT PIN 1 HAS A SQUARE PAD. THE NOTCH GOES ABOVE PIN 1. Solder two corners first. If the socket does not sit flush with the PC board, push on the center of the socket and reheat each corner until the socket pops flat with the PCB. DONT BURN YOURSELF.
There is a reason that the components are soldered in a certain order. It makes things easier. Please follow the procedure steps in order.
2) Next, solder all of the resistors in place. Resistor codes are included in your Bill of Materials (BOM, Appendix A). There is also a Digital Multi-meter on the bench. Why not MEASURE the resistors? ). NOTE: It just so happens that 150 Ohm and 10 MegaOhm resistor codes for a 1% 5 stripe resistor are identical and mirror images of one another. This is rare, but it happens. ANYWAY, you cannot tell the difference without measuring them on your Digital Multi- Meter.
SPECIAL NOTE: YOU MUST USE CUTTERS TO REMOVE THE LONG COMPONENT LEGS AFTER SOLDERING. EVERYONE AT THE LAB BENCH MUST BE WEARING THEIR SAFETY GLASSES WHEN ANYONE IS CUTTING OFF COMPONENTS!! 3) There are four 0.1 microfarad capacitors. These are marked 104. Using your Bill of Materials (BOM), Schematic and Reference Designators on the board. Solder them in place as shown.
4) Solder in the six 10 microfarad capacitors. Marked with 10μ and a + sign NOTE: THESE PARTS ORE POLARIZED! You must place the lead with the + and the line on the part, to the hole with the + symbol on the board. Otherwise, the factory installed smoke will be released on power-up!
5) Install the 0.33 microfasrad Capacitor, C7. The part is marked with a 334.
6) U4 is the Voltage Regulator that generates 5.00V when powered with a +9V battery. As long as the battery is above 7V, the regulator will continue to provide 5.00V. The diffference is converted to heat, but at low currents that we are consuming the part only becomes warm, not hot.
7) Install the RESET switch, SW1. NOTE: THIS IS NOT AN ON / OFF SWITCH. This switch holds the circuit in RESET or allows it to run. The circuit will continue to draw current and drain the battery unless you unsnap the +9V battery from the clip.
8) Solder in the LEDS. BUT BEFORE YOU DO, you will have to figure out which ones are red, green and yellow. Using a 4.99K Ohm resistor, and the bench top power supply test each LED. a) Set the supply for 5.00V b) Connect ground to the short leg of the LED (Cathode). c) Connect 5.00V to the 4.99 KOhm RESISTOR. d) Briefly touch THE OTHER END OF THE RESISTOR to the LONG leg of the LED (Anode). e) Sort by red, yellow, green. NOTE: THE SQUARE PAD ON THE PCB IS THE CATHODE. Put the SHORT LEG of the LED in the hole with the SQUARE pad, next to the FLAT side of the LED outline on the PCB. (See next figure).
9) Last part. Solder the 9V Battery Clip in place. The wires are already pre-tinned so you don't have to worry about that. The RED wire MUST go in the hole marked +. The BLACK wire must go in the hole marked -.
10) Once the board is completely soldered. take it to the wash station to remove the flux using a brush and 99% alcohol. 11) Insert the SN74HC74N dual D Flip-Flops into the sockets. This is far easier to do if you place each side of the chip on the bench top and bend the legs to a right angle relative to the plastic body of the chip. Circuit Operation: Attach the +9V Battery. Change the switch from RESET to RUN. When you do you, the green light on the left should illuminate. After 63 seconds, the light will cjange to yellow. After 3 more secnds the traffic light should enter the proper sequence. The cycles will become slightly shorter after a number of iterations. The final cycle is about 25 seconds per side, or 50 seconds overall. Anything more realistic was somewhat boring to watch. Run At 63 Seconds Operational YOU MUST DEMONSTRATE YOUR OPERATING CIRRCUIT TO THE LAB MONITOR OR TA BEFORE LEAVING LAB IN ORDER TO RECEIVE A GRADE.
A FEW WORDS ON TROUBLESHOOTING Background: Troubleshooting is very much dependent on the system that is under test. Three questions always consistent however: 1) What is this device supposed to do when performing correctly? 2) What is this device NOT doing correctly? 3) What parameters can be measured that are essential to the proper function of the device? There are also two general methods that engineers believe lead them to the problem with the least amount of trouble: 1) Final Output to Input, 2) Input to Output, an 3) Test half way each time The first method begins with the missing or incorrect outputs and examines what subsystem is generating, or not generating the errant result. Once identified, the subdevice is examined for correct or in-correct input. If the input is incorrect, it is temporarily assumed that the problem recedes the current subdevice. The Errant input is the new incorrect output and the subdevice that is generating this signal or result is examined. The process continues until the offending subdevice is identified. The second method begins with the classic questions of "Is it plugged in?" and "Is it turned on?". If so, each of the power sources is examined for proper operation. Once proper power is verified at all points in the device under test, each subsystem is tested for proper input and output. In this method, the least processed input is always examined first. The third method is believed by some to be the fasted way to arrive at the problem. In reality, it very much depends on the system. This method is very similar to the SAR (successive approximation register) method of analog to digital conversion. In that method the number half way between the last guess and either the smallest or largest number that is possible is tested. Is the result smaller or larger than this value? Each iteration, gets you halfway to your answer. You continue until the accuracy desired is achieved. For troubleshooting, Look at the resulting signal half way between the input and final output of a nonfunctioning system. If the system is not working at this point, look halfway between the input and this subsystem for proper function again. Repeat until the offending subsystem is identified.
Troubleshooting the Traffic Light Controller: In the traffic light controller, you have a number of fundamental "subsystems": 1) The 5 Volt Power Supply 2) RC circuits 3) D-Flip-Flops 4) LED- resistor circuits Ask whether or not each RC is charging; whether or not the D-flip flips get triggered, and if the LED s work when they are supposed to be ON. Notice that the LEDs are controlled by LOW TRUE signals. They are always powered, but the GROUND end is switched in at the of a D-flip-flop in order to "sink" current through the LED. When the LED is ON, the is LOW. Notice that the corresponding Q begins the charging for the next stage at the same time the LED is illuminated. According the TI datasheet for the SN74HC74AN, The D flip-flop is clocked when the CLK line crosses 3.15 Volts (min) if the power supply is 4.5 Volts (min). You can assume approximately 3.5V for a clocking transition voltage if you are looking at the CLK signal.
Appendix A: Bill of Materials Appendix B: Schematics, page 1 of 2
Appendix B: Schematics, page 2 of 2
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