Testing & Fault-Finding
Why Test a Circuit?
Building a prototype circuit is only worthwhile if the circuit is then tested methodically to confirm it behaves as designed. Testing reveals faults — incorrect voltages, broken connections, components that have failed — before the product is manufactured in quantity. The OCR specification lists five items of test equipment: the multimeter, the continuity tester, the oscilloscope, the signal generator, and the logic probe. Each instrument has specific uses, benefits, limitations, and safety requirements.
Multimeter
A multimeter measures DC voltage (in volts), DC current (in amps or milliamps), and resistance (in ohms). To measure voltage, the probes are placed across (in parallel with) the component. To measure current, the circuit must be broken and the multimeter inserted in series — the current flows through the meter. To measure resistance, the component must be disconnected from the circuit and no power applied. Safe use requires selecting the correct measurement function and range before connecting probes; measuring current without breaking the circuit or selecting the wrong range can damage the meter or cause a short circuit. Digital multimeters display a numerical reading and are more accurate and easier to read than analogue models. A multimeter cannot measure rapidly changing or high-frequency AC signals — an oscilloscope is needed for those.
A multimeter measures DC voltage (probes in parallel — across the component), current (probes in series — circuit broken, meter inserted), and resistance (component disconnected, no power). Always select the correct function and range before connecting. Never measure current without breaking the circuit first — this risks a dangerous short circuit through the meter.
Continuity Tester
A continuity tester checks whether a continuous electrical path exists between two points. When the probes are touched to either end of a wire, track, or connection, the tester beeps (or lights an LED) if current can flow. It is the quickest way to identify a broken wire, a lifted PCB track, or a faulty switch. Safety requirement: the circuit must be powered off before testing — applying continuity probes to a live circuit risks damaging the tester and causing a short. A continuity tester cannot measure voltage or resistance values; it only confirms presence or absence of a path.
Oscilloscope
An oscilloscope displays voltage plotted against time, showing the actual waveform of a signal on its screen. Unlike a multimeter, it can capture rapidly changing signals — both AC sine waves and digital square waves — and measure their amplitude (peak voltage), frequency, and periodic time (the time for one complete cycle). This makes it essential for testing audio circuits, timing circuits, and any system where signal shape matters. Probes are connected in parallel across the point to be measured. An oscilloscope is more complex to set up than a multimeter and more expensive; it does not directly measure resistance or DC current. Safety: use probes with the correct voltage rating for the signal being measured.
The oscilloscope screen is divided into a grid. The vertical axis (Y) represents voltage — each division represents a set number of volts (the volts/div setting). The horizontal axis (X) represents time — each division represents a set time period (the time/div setting). To find the peak-to-peak voltage, count the vertical divisions from the lowest to highest point of the wave and multiply by the volts/div. For a centred sine wave, the amplitude is half this value. To find the period (time for one cycle), count the horizontal divisions for one complete wave cycle and multiply by the time/div. Frequency equals one divided by the period (f = 1/T). In the OCR exam you may be asked to read values from an oscilloscope display — practise counting grid squares carefully.
Signal Generator and Logic Probe
A signal generator is used alongside the oscilloscope: it produces a controllable electrical signal — sine, square, or other waveform — at a chosen frequency and amplitude, which is fed into the circuit under test. This allows the engineer to check how the circuit responds to specific input signals without needing a live sensor. A logic probe is a simple pen-shaped tool used specifically on digital circuits. Touching its tip to a point in the circuit lights a high LED (logic 1) or a low LED (logic 0), instantly revealing the state of a digital signal. It cannot measure analogue voltage levels but is faster and cheaper than an oscilloscope for basic digital circuit checking.
Five test instruments: multimeter (DC voltage, current, resistance — general purpose); continuity tester (checks for unbroken electrical path — must be used on a powered-off circuit); oscilloscope (displays waveform, measures amplitude and frequency); signal generator (produces a controlled test signal to feed into a circuit); logic probe (shows logic high or low on digital circuits — simple and fast).
A systematic approach to fault-finding follows a logical order. First, visually inspect the circuit for obvious problems: solder bridges, components in the wrong orientation, or damaged tracks. Second, check the power supply with a multimeter — confirm correct voltage is reaching the circuit. Third, use a continuity tester to verify all intended connections are complete. Fourth, work through the circuit stage by stage: measure the voltage at the output of each section (input stage, processing stage, output stage) and compare it with the expected value. Fifth, if signals are involved, use an oscilloscope to verify waveform shapes at key nodes. Replacing a suspected faulty component is the last step after the fault has been narrowed down logically, not the first step.