ENG101 GBA Marker

Front bench setup

Students bring their board to the front and hook it up to the bench function generator, oscilloscope and power supply. Watch them make these connections: it is part of the 10 equipment marks.

Function generator → signal to A, ground to F. Square wave, 2 Hz, 10 Vpp (±5 V), 0 V offset. Confirm on the scope that A really is ±5 V; a generator set for a 50 Ω load can show double the amplitude.
Power supply → 9 V. The + terminal goes to circuit ground and the − terminal goes to the top rail (R4/R7). This makes the −9 V negative supply the PNP stage needs. Set a current limit of about 100 mA if the supply has one.
Oscilloscope → both probe ground clips on circuit ground (F) only. DC coupling on both channels (AC coupling hides the clamp shift). About 5 V/div and 100–200 ms/div, trigger on CH1.
One common ground for generator, supply and scope. Switch on the supply first, then enable the generator output.

Figure 1 schematic: Part 1 clamper and voltage multiplier (A to G), Part 2 PNP switching stage with LEDs (H and I) — Figure 1. Test points A–I. B and C are the same node.

Inspection checklist

Do this with the board unpowered. Compare every position with Figure 1 and the PCB silkscreen.

Ref	Part	How to identify	Orientation / placement
R4, R7	820 Ω	Grey–red–brown (gold)	Not polarised. In series with each LED to the −9 V rail.
R5, R6	100 kΩ	Brown–black–yellow (gold)	Not polarised. R5 from B to the Q1 base; R6 from H to the Q2 base. If swapped with R4/R7, both LEDs are very dim.
C1–C4	47 µF 25 V electrolytic	Stripe with “−” marks the negative (shorter) lead	C1 + toward A · C3 + toward C · C2 + toward F (ground) · C4 + toward the C2–C4 junction.
D1–D4	1N4001	Band marks the cathode	D1 and D3: band toward the bottom rail. D2 and D4: band toward the top rail.
LED1	Green LED	Long leg = anode; flat rim = cathode	Anode to Q1 collector (H), cathode toward R4.
LED2	Red LED	Long leg = anode; flat rim = cathode	Anode to Q2 collector (I), cathode toward R7.
Q1, Q2	BC327 PNP	TO-92; flat face toward you, legs down: C–B–E (confirm on datasheet)	Flat face matches the silkscreen outline. Emitters to ground.

Solder joints

Good: shiny, smooth, concave cone; lead outline visible; pad fully wetted; lead trimmed short.
Excess solder: blobs or balls that hide the lead.
Short: a bridge between neighbouring pads. Check the three transistor legs closely.
Dry joint: dull, grainy or cracked; solder sitting on the lead without wetting the pad.

Expected: LED behaviour

Green LED1 and red LED2 flash alternately, each about twice a second (2 Hz input, 500 ms period, about 250 ms on each). They should never be on together.

Input A in its negative half → B ≈ −9 V → Q1 on → green on; H ≈ 0 V so Q2 is off.
Input A in its positive half → B ≈ +0.7 V → Q1 off → H goes negative → Q2 on → red on.

Expected: waveforms

Scope on DC coupling, 5 V/div, 100–200 ms/div. Reference captures are from the Marking Guide; click to enlarge.

A and B

A: 10 Vpp square wave, ±5 V, period 500 ms (2 Hz).
B: same shape and size, shifted down: top ≈ +0.7 V, bottom ≈ −9 V (guide Vmin −9.2 V).
This is the negative clamper at work.

C and D

C: identical to B (same node).
D: same 10 V swing shifted further down, bottom about −17.6 V.

B and H

H (Q1 collector) switches between ≈ 0 V and ≈ −7 to −9 V (guide Vmin −8.8 V).
H is inverted relative to B: H sits at 0 V while B is low.

H and I

H and I are opposite square waves (180° apart).
I (Q2 collector) switches between ≈ 0 V and ≈ −6.6 V, in step with B.

Expected: Multisim

Demonstrated on the students’ own laptop. Accept the Figure 1 circuit or their Q7 modification (diodes reversed for a positive shift, NPN transistors, +9 V supply, LEDs reversed).

8 marks: Simulate ▸ Run (F5). The two LED indicators blink alternately.
6 marks: Tektronix virtual scope(s) correctly connected and showing A & B, and H & I.

Multisim modified circuit with NPN transistors and Tektronix scope — Q7 reference design (Marking Guide).

Multisim virtual scope showing A and B — Virtual scope: A and B (B shifted up in the modified circuit).

Multisim virtual scope showing H and I — Virtual scope: H and I, opposite square waves.

Fault finder

Use this if a board does not work at power-on, to see what went wrong.

Symptom	Likely causes
Neither LED lights	Supply off or reversed (+ must go to ground) · no common ground · LED1 reversed or R4 open (Q2’s base current also flows through LED1 and R4, so both stop) · transistor legs wrong
Red stays on, green never lights	No signal at A (generator output off) · C1 open or dry joint · D1 reversed, shorted, or open/dry joint (B then shifts up instead of down) · R5 open · Q1 wrong way or damaged
Green flashes, red never lights	LED2 reversed · R7 or R6 open · Q2 wrong way or damaged
B shifted up instead of down	D1 reversed, or D1 open/dry joint
Both LEDs very dim	100 kΩ and 820 Ω resistors swapped
B looks the same as A (no shift)	Scope on AC coupling · probe on the wrong point
Flashing too fast or too slow	Generator not at 2 Hz

Viva question bank

Fundamental questions with model answers. Each viva opens with one of your model questions, then adds questions drawn at random from the other topics. Questions are not repeated within a group, and the draw favours questions earlier groups have not heard.

Mark scheme

The circuit is graded out of 60, using the criteria on the Marking Guide’s “Total marks: 60” page. Every criterion, including the viva, is a group mark, so both students receive the same total.

One adjustment. The Marking Guide gives “Award 3 marks” for each soldering-quality item, which adds to 6, but the section is worth 4. This tool uses 2 + 2 so the total is exactly 60.

Front bench setup

Inspection checklist

Solder joints

Expected: LED behaviour

Expected: waveforms

A and B

C and D

B and H

H and I

Expected: Multisim

Fault finder

Viva question bank

Mark scheme

Results