Digital Circuit Measurements, Troubleshooting and Laboratory Practice
The laboratory domain rewards candidates who can choose the right instrument, read it correctly, and localise a fault systematically instead of guessing. Expect questions on oscilloscope and meter use, logic levels, power and grounding, and standard bench procedures.
Instruments and measurement technique
- The DMM measures voltage, resistance and continuity; resistance and diode tests must be made with the circuit unpowered and, where possible, the component isolated.
- The oscilloscope shows amplitude and time together: read volts/division for amplitude and time/division for period, then f = 1/T. A 1 kHz square wave has a period of 1 ms.
- A x10 probe loads the circuit far less than x1 but the reading must be scaled by ten; compensate the probe against the calibrator square wave before measuring.
- The ground clip defines the reference; connecting it to a live node shorts that node. Use a logic probe for single lines and a logic analyzer for buses and timing relationships.
Systematic troubleshooting
- Classify the fault: open circuit, short or solder bridge, stuck-at-0, stuck-at-1, or a floating input.
- TTL inputs float high; CMOS inputs float to an undefined level and may oscillate, so every unused CMOS input must be tied.
- Trace the signal from input to output, compare each node with the expected truth table, and split the circuit in half each time.
- Check supply, ground, decoupling capacitors and current limit before suspecting the logic.
Worked example
An LED is to run from 5 V with a forward voltage of 2 V at 15 mA. R = (5 - 2)/0.015 = 200 ohm, so choose the nearest standard value of 220 ohm, giving 3/220 = 13.6 mA, which is safe. Resistor power is 3 V x 0.0136 A = 41 mW, so a 0.25 W part is adequate. Note the LED orientation and that the resistor may sit on either side of the LED.
Traps that cost marks
- Measuring resistance in a powered circuit, which gives meaningless readings.
- Forgetting the x10 attenuation factor and reporting a value ten times too small.
- Leaving the scope ground clip off and trusting a floating waveform.
- Assuming a floating CMOS input is at logic 0.
- Exceeding the bench supply current limit or omitting the series resistor for an LED.