ECE 323L Basic Electronic Circuits Laboratory
Lab 9
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Do the following exercises. Report your results by editing this Word document and submitting it in WebCT.
This laboratory is the second in a series of experiences using digital electronic components. As before, you will use a 7805 voltage regulator chip to convert your 9-volt battery supply to a 5-volt supply.
- Build a “night light” using your light-sensitive resistor, a nand gate, and LED. Choose the resistor R1 to provide an appropriate switching point for your light-sensitive resistor. Demonstrate the circuit to the lab assistant.
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- Find a 7-segment display and 74LS47N chip in your tackle box. The pin diagram for these components is reproduced below.
Pin diagram for 7-Segment display.
Pin diagram for 74LS47N
Connect the cathodes of the 7-segment display (a … g) to the 74LS47N using 330-ohm resistors. Setting the lamp-test (LT) pin to low should cause all seven segments to light up equally brightly. If the brightness is uneven, the corresponding resistor may have the wrong value.
Disconnect the LT pin, and connect the input settings, A0 … A4 to ground. The digit zero should be displayed. Now systemically connect one or more of the input pins to HI and identify the combinations of high and low on pins A0 … A3 needed to display the decimal numbers. Use 0 for low inputs (connected to ground) and 1 for high values (connected to 5 V). Fill in the table below.
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A3 |
A2 |
A1 |
A0 |
Output |
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0 |
0 |
0 |
0 |
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1 |
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2 |
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3 |
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4 |
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5 |
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6 |
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7 |
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8 |
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There are an additional six combinations of inputs that do NOT produce a decimal number. Identify these combinations and fill in the table below by listing the segments (a .. g) that are lit for each combination.
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A3 |
A2 |
A1 |
A0 |
Output |
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- Build a SR-latch using two nand gates. You can use any of the unused gates from your “night light”. Connect the two outputs to separate LEDs and 330-ohm resistors, as in the night light.
Record the truth table for this circuit below, use ON if the LED is lit and OFF if the LED is dark. In this table 0 means the pin is connected to ground and 1 means the pin is connected to 5V (or disconnected).
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S |
R |
Q |
QBAR |
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0 |
0 |
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0 |
1 |
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1 |
0 |
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1 |
1 |
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Which state (row) above corresponds to a “memory” configuration where the output depends on the previous setting of the latch? Which state (row) above corresponds to an “illegal” setting, where the outputs do not always behave as expected (one LED on, one LED off)?
maintained by John Loomis, updated 4 March 2008