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Using the test board, one 74138 Decoder chip and two 7404 Inverter chips, connect the circuit shown in Fig-3. The inputs A0, A1, A2 & E must be connected to switches to implement the various cases of the inputs. The outputs must be connected to LEDs. Connect the circuit to a power supply then complete the logic truth table shown in Table-3A, based on the knowledge that when the light is ON then the output is logic (1), and when it is OFF the output is logic (0). Disconnect the LEDs from the circuit and use the DMM to measure the output voltages, then complete the voltage table shown in Table-3B. How many enables inside this chip ? What is the function of this circuit ? Compare the circuits in Fig-2 & Fig-3.

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posted a classic decoder lab and was right to ask for the missing figures; pointed toward the datasheet. Practical steps and checkpoints that aren’t in the thread will make the lab reproducible years later: confirm the IC’s VCC and GND pins and use a regulated 5 V supply, tie any unused inputs to a defined logic level (don’t leave them floating), and always use series resistors with LEDs (typically 330–1k ohm at 5 V).

When you run the experiment, be aware of the enable/input polarities and how the LEDs were wired. The device has enable inputs with different polarities — read the silkscreen on the IC or the part label for which are active‑high vs active‑low, and assert them correctly before testing. Many decoder outputs are active‑low; in that case either:

  • connect each LED (with resistor) from VCC to the output so it lights when the output goes low, or
  • feed the decoder outputs into a 7404 inverter and connect LEDs from the inverter outputs to ground so the LED indicates a logical “1.”

For the voltage table: disconnect LEDs (as your lab asks), set the meter to DC volts and measure each output to ground for every input combination. For TTL parts expect lows below about 0.8 V and highs above about 2.0 V (with a 5 V supply). Avoid tying outputs together, don’t exceed pin current limits, and double‑check enable wiring — those differences (enable polarities and whether outputs are inverted before the LEDs) are typically what distinguishes the two figures you were asked to compare.

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What is in figures 2 and 3? What is truth table 3a? What do inputs A0, A1, etc. look like? You’re only showing us 10% of the question being asked.

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