Full Stack Electronics · Stage Four · Experiment 14

Experiment 14: The Master Schematic

The first step from the breadboard to a printed circuit board. Draw the whole 555 as one master schematic, give each part and each connection a name, list the parts, and record where the board differs from the breadboard.

Experiment 13 completed Stage Three, and with it the first milestone of the course: a 555 from twenty-three transistors on a breadboard. It ran at 192.59 ms in Experiment 12, near the 200.46 ms of the NE555 chip in Experiment 1, and RESET stops it as it stops the chip. But the prototype fills most of a full-size breadboard, and one loose jumper can stop it. Stage Four makes the same circuit into a small printed circuit board, which connects to the breadboard of Experiment 1 through the eight pins of the chip that it replaces. The second milestone is a board design that is ready to order, and this experiment is the first step to it. It draws the master schematic: the whole circuit in one document, with a name for each part and each connection, and a list of the parts.

Resources

From the breadboard to the board

Top view of a breadboard packed with black TO-92 transistors, resistors and coloured jumpers, a red LED lit at the right edge beside an electrolytic timing capacitor
Figure 14-1: The end of Stage Three: the 555 from twenty-three transistors, with the LED caught mid-blink.
The finished black surface-mount board, its silkscreen naming every block and transistor of the discrete 555, wired by eight jumpers in different colours into the breadboard beside it, where a red LED glows next to the timing capacitor
Figure 14-2: The end of Stage Four: the board in place of the NE555, wired into the breadboard of Experiment 1 through the same eight pins.

The table gives the eight steps from Figure 14-1 to Figure 14-2.

ExperimentStepResult
14 · The Master Schematic (this experiment)Draw the whole circuit on paper, with names and a parts listThe master schematic
15 · Schematic CaptureEnter the master schematic in KiCadA KiCad schematic with zero errors from the electrical rules check
16 · The Smoke TestSimulate the KiCad schematic in SPICEThe timing of the design, before a board exists
17 · Going Surface-MountGive each part a packageA footprint for each designator
18 · The BoardPlace the parts and route the copperThe board layout
19 · Before You OrderCheck the design, then order the boardMilestone: the board design is complete
The factory makes the board and sends itSome weeks of waiting
20 · Assembly DaySolder the parts on the boardAn assembled board
21 · First Power-UpApply power with a current limit, then measureMilestone: the board blinks the LED

Each step after this one starts from the master schematic. KiCad copies it, SPICE simulates the copy, and the layout draws copper for its nets. The parts list becomes the order. So an error here goes through each step into the copper. On the breadboard, a wrong wire takes a minute to move. On a board, it takes a cut track and an added wire, or a new order.

The board is not an exact copy of the breadboard. This experiment reads the master schematic sheet by sheet, then gives the five places where the board differs, with the reason for each. It needs no power and no instruments.

You will need

Names for parts and connections

A breadboard figure names holes, such as 43d, and the names change when a part moves. A schematic names the parts and the connections, and those names stay the same in KiCad and on the board. Each part gets a reference designator: a letter for the type of part and a number. Q is a transistor, R a resistor, C a capacitor, J a connector and TP a test pad. The board has each designator printed beside its part.

What a net is

A net is a set of terminals that are connected together, so they are at one voltage. A strip of five holes on a breadboard is one net. On a schematic, a net can have a name. Two wires with the same name are connected, even if no line joins them. So the master schematic draws each block on a sheet of its own and joins the sheets by net names, as KiCad does in Experiment 15.

The top sheet

With names for parts and nets, the whole timer fits on one top sheet. The pale outline in Figure 14-3 is the board. Its eight pins are J1, a header with the pin numbers of the NE555. Follow the figure from left to right.

The top sheet of the master schematic. A pale rounded outline is the board, with eight numbered pin badges. On the left edge from top to bottom: 5 CONTROL, 6 THRESHOLD, 2 TRIGGER, 4 RESET. On the right edge: 3 OUTPUT and 7 DISCHARGE. Pin 8, +5 V, at the top left feeds a red line along the top. Pin 1, GND, at the bottom left feeds a blue line along the bottom. Near the left edge, R1, R2 and R3, each 4.7 kilohms, run down from the +5 V line to the ground line. The junction of R1 and R2 joins the CONTROL wire from pin 5. The junction of R2 and R3 is labelled REF13, with test pad TP5. The THRESHOLD, TRIGGER and RESET wires cross the divider without a connection. Four boxes: Threshold comparator Q1 to Q7, R4, R5 takes CONTROL and THRESHOLD; Trigger comparator Q8 to Q14, R6, R7 takes REF13 and TRIGGER; Reset Q21 to Q23, R16 to R20 takes RESET; Latch Q15 to Q18, R8 to R13 in the middle. R runs from the threshold comparator to the latch, with test pad TP2. S runs from the trigger comparator to the latch, with test pad TP1. From the reset box a wire labelled Q23 joins S, and a wire labelled Q22 joins Q. Q leaves the latch to test pad TP3. Q-bar leaves the latch, with test pad TP4, and splits: through R14, 10 kilohms, to the base of Q19, and through R15, 10 kilohms, to the base of Q20. Q19's collector goes to pin 3, OUTPUT, and through R21, 4.7 kilohms, to the +5 V line. Q20's collector goes to pin 7, DISCHARGE. Both emitters go to the ground line. At the right, C1, 0.1 microfarad, runs from the +5 V line to the ground line.
Figure 14-3: The top sheet. Each box has a sheet of its own, Figures 14-4 to 14-7, which also gives its +5 V and ground. Click to enlarge.
  1. R1, R2 and R3 make the references. The 2/3 tap is CONTROL, pin 5. The 1/3 tap is REF13, which has no pin.
  2. The threshold comparator compares THRESHOLD, pin 6, with CONTROL and makes R. The trigger comparator compares TRIGGER, pin 2, with REF13 and makes S.
  3. The latch takes R and S and makes Q and Q̄.
  4. Q̄ drives Q19 through R14 and Q20 through R15. Q19's collector is OUTPUT, pin 3, with R21 as its pull-up. Q20's collector is DISCHARGE, pin 7.
  5. The reset block takes RESET, pin 4. During a reset, Q22 holds Q at 0 and Q23 holds S at 0.
  6. C1 connects +5 V to ground. TP1 to TP5 are test pads on S, R, Q, Q̄ and REF13.

The four block sheets

Each box on the top sheet has a sheet of its own, drawn with the layout of the experiment that built the block. The circuits are the same as on the breadboard. In the comparators, four pairs of transistors have each other's names on the board. This page uses the board's names, because they are printed on the board. The table after Figure 14-5 gives both names.

Schematic of the threshold comparator with the board's names. A +5 volt line along the top feeds the emitters of the PNP load Q6 on the left and Q5 on the right, the emitter of Q7, and R4, 4.7 kilohms. Q6's collector is joined to its base. Below the load is the NPN pair: Q2 on the left with CONTROL straight into its base, and Q1 on the right with THRESHOLD coming in from the left on a wire that hops Q2's collector wire. The emitters of Q1 and Q2 are joined and go to the collector of Q4. Q4 and Q3 form the tail mirror on the ground line, with Q3 on the right fed by R4 and its collector joined to its base. Q1's collector runs to Q7's base. Q7's collector goes to the output terminal R and through R5, 4.7 kilohms, to ground.
Figure 14-4: The threshold comparator, Q1 to Q7. R4 feeds the tail mirror, and R5 is the pull-down on Q7's collector. Click to enlarge.
Schematic of the trigger comparator with the board's names. A +5 volt line along the top feeds the PNP tail mirror, Q11 above the pair and Q10 to its right, and R7, 4.7 kilohms, at the far right. Q10's collector is joined to its base and goes through R6, 4.7 kilohms, to the ground line. Q11's collector feeds the joined emitters of the PNP pair: Q9 on the left with TRIGGER straight into its base, and Q8 on the right with REF13 coming in from the left on a wire that hops Q9's emitter wire. Below the pair is the NPN load on the ground line: Q13 on the left with its collector joined to its base, and Q12 on the right. Q8's collector runs to the base of Q14, which hops R6's lead. Q14's emitter is on ground, and its collector goes to the output terminal S and through R7 to +5 volts.
Figure 14-5: The trigger comparator, Q8 to Q14. R6 sets the tail current, and R7 is the pull-up on Q14's collector. Click to enlarge.
TransistorName on the breadboardName on the board
Threshold pair, base on the 2/3 tapQ1Q2
Threshold pair, base on THRESHOLDQ2Q1
PNP load, diode sideQ5Q6
PNP load, side that drives Q7Q6Q5
Trigger pair, base on TRIGGERQ8Q9
Trigger pair, base on the 1/3 tapQ9Q8
NPN load, diode sideQ12Q13
NPN load, side that drives Q14Q13Q12
Schematic of the latch with the board's names. Gate 1 on top: R comes in from the left through R8, 10 kilohms, to the base of Q15. Q16 stands to its right, turned round, with its base on the right. The collectors of Q15 and Q16 join under R12, 1 kilohm, from +5 volts, and that line is Q, which leaves on the right. Gate 2 below: S comes in through R11, 10 kilohms, to the base of Q18, with Q17 turned round to its right. Their collectors join under R13, 1 kilohm, and that line is Q-bar, which leaves on the right. Q-bar rises to R9, 10 kilohms, into Q16's base. Q falls to R10, 10 kilohms, into Q17's base. All four emitters go to one ground.
Figure 14-6: The latch, Q15 to Q18. R8 and R11 take R and S in, R9 and R10 are the feedback resistors, and R12 and R13 are the 1 kΩ pull-ups on Q and Q̄. Click to enlarge.
Schematic of the reset block with the board's names. A +5 volt line along the top. RESET comes in from the left. R16, 100 kilohms, runs from the +5 volt line down to the RESET wire. RESET continues through R18, 10 kilohms, to the base of Q21. R17, 10 kilohms, runs from the +5 volt line to Q21's collector, labelled RST. From RST, R19, 10 kilohms, goes to the base of Q22, and R20, 10 kilohms, goes to the base of Q23; the RST wire to R20 hops Q22's collector wire. Q22's collector goes to the output terminal Q. Q23's collector goes to the output terminal S. All three emitters go to one ground line.
Figure 14-7: The reset block, Q21 to Q23. R16 and the S terminal are new on the board. Click to enlarge.

Where the board differs from the breadboard

Figure 14-7 has two of the five changes from the breadboard. The top sheet has the other three. The table lists them.

ChangeBreadboardBoard
Q19's baseQ, through 10 kΩQ̄, through R14
Pull-up on OUTPUTThe LED and its 220 ΩR21, 4.7 kΩ
Q23's collectorDISS
Pull-up on RESETNone: the RESET lead goes in the +5 V railR16, 100 kΩ
DividerThree 10 kΩThree 4.7 kΩ

OUTPUT the same way up as pin 3

On the breadboard, Q19 takes Q, so its collector is low while C charges, and the LED lights. Pin 3 of the NE555 is high while C charges. On the board, Q19 takes Q̄, so OUTPUT is high while C charges, as on pin 3.

Q19 is an open collector output. It can pull OUTPUT down to 0 V, and it cannot pull it up. On the breadboard, the LED did the pull-up. The board has no LED, so R21 does it. Through R21 the board can supply about 1 mA. The NE555 can supply up to 200 mA from pin 3, as the TI datasheet gives.

The reset on the board

With Q19 on Q̄, the reset of Experiment 13 is not sufficient. During a reset, C empties and CAP falls below 1/3, so S rises to 1 and makes Q̄ 0. Q19 turns off, and OUTPUT goes high. The NE555 holds its output low during a reset. So on the board, Q23 holds S at 0. Follow Figure 14-8.

The latch drawn as two NOR gates, Gate 1 on top making Q from R and Q-bar, Gate 2 below making Q-bar from Q and S, with the feedback wires crossing between them. Q22 hangs from the Q line, its emitter to ground, its base wire ending in the name RST. Q23 hangs from the S input line, turned round, its base wire ending in the name RST. Q-bar runs right, hops Q22's emitter wire, and splits: through R14, 10 kilohms, to Q19, whose collector is OUTPUT with R21, 4.7 kilohms, to +5 volts; and through R15, 10 kilohms, to Q20, whose collector is DISCHARGE. Green markers: 1 beside Q22, 2 beside Q23, 3 under the Q-bar line, 4 at the split.
Figure 14-8: The reset on the board. Both bases end in the name RST, so both connect to Q21's collector. The numbers match the steps below.
  1. RESET goes low, and RST rises. Q22 turns on and holds Q at 0.
  2. Q23 turns on and holds S at 0. The trigger comparator can no longer change S.
  3. Gate 2 has 0 on both inputs, so Q̄ is 1.
  4. Q̄ turns on Q19 and Q20. OUTPUT is low, as on the NE555, and Q20 empties C to 0 V.

When RESET goes high, Q22 and Q23 turn off, S sets the latch, and C charges from 0 V. That is the long first charge of Experiment 13. Q23 now takes R7's 1 mA, which needs a current gain of about 8. The onsemi datasheet gives at least 100 for the 2N3904.

A pull-up on RESET

On the breadboard, the RESET lead goes in the +5 V rail, and a loose lead holds the board in reset. On the board, R16, 100 kΩ, holds pin 4 high. With nothing on pin 4, R16 and R18 give Q21 about 0.04 mA. That turns Q21 on, so the timer runs. A wire from pin 4 to ground resets the timer.

4.7 kΩ in the divider

Three equal resistors put the taps at 1/3 and 2/3 for any value, as Experiment 9 showed. The board uses 4.7 kΩ, the value of R4 to R7, so there is one value fewer to buy. The divider current increases from 0.17 mA to 0.35 mA, so the input current of a comparator moves its tap about half as much.

Parts only on the board

J1 is the only connection to the outside. RA, RB, C and the LED stay on the breadboard, as they stood around the NE555 in Experiment 1. C1 and the five test pads are the other parts that the breadboard did not have.

Why C1 is there

Each time a block switches, it takes a short pulse of current from +5 V. The supply leads cannot deliver a fast pulse without a short drop on +5 V, and that drop also moves the references of the divider. C1, 0.1 µF, supplies the pulse from beside the circuit. This is called decoupling. Experiment 18 puts C1 beside pin 8.

The test pads

On the breadboard, each signal between the blocks was on a strip of holes. In Experiments 10 to 13 you put an oscilloscope probe on S, R, Q and Q̄ there. On the board, these nets are copper tracks between SOT-23 legs that are 0.95 mm apart, and J1 has no pin for them. A test pad gives each one a place for the probe tip. The table gives the five pads.

PadNetWhat it shows
TP1SThe trigger comparator's output. It is 1 while TRIGGER is below REF13.
TP2RThe threshold comparator's output. It is 1 while THRESHOLD is above CONTROL.
TP3QThe latch output of Gate 1.
TP4Q̄The latch output of Gate 2, which drives Q19 and Q20.
TP5REF13The 1/3 tap of the divider. Pin 5 gives the 2/3 tap, but the 1/3 tap has no pin.

The pads are on these five nets because the nets are the signals between the blocks. A fault on the assembled board is usually in one block. Compare each block's input with its output, in the order of Figure 14-3, and the first output that is wrong shows the block with the fault.

  1. TP5 is not at 1/3 of the supply: the fault is in the divider.
  2. THRESHOLD goes above CONTROL, and TP2 does not go to 1: the fault is in the threshold comparator.
  3. TRIGGER goes below TP5, and TP1 does not go to 1: the fault is in the trigger comparator.
  4. TP1 and TP2 change, and TP3 and TP4 do not: the fault is in the latch.
  5. TP4 changes, and OUTPUT or DISCHARGE does not: the fault is in Q19 and its resistors, or in Q20 and R15.

Each test pad is a round copper pad, 1.0 mm across, on the top of the board. It has no solder mask and no part on it, so it costs nothing and needs no hole. TP1 to TP4 are in one row, so the probe can move from one pad to the next. To use a pad, take the hook off the probe, put the ground clip on pin 1, and hold the probe tip on the pad. A 10× probe puts about 10 MΩ on the net. The resistors that set these nets are 4.7 kΩ or less, so the probe changes the voltage by less than 0.05%.

The nets

The test pads are five of the terminals on the named nets. The table lists each named net and all its terminals. A net inside a block has no name on the sheets. KiCad names it from one of its pins, such as Net-(Q1-C).

NetTerminals on the netPin
+5VJ1 pin 8, the emitters of the PNP transistors Q5, Q6, Q7, Q10 and Q11, the resistors R1, R4, R7, R12, R13, R16, R17 and R21, C18
GNDJ1 pin 1, R3, R5, R6, the emitters of all sixteen NPN transistors, C11
THRESHOLDJ1 pin 6, Q1's base6
TRIGGERJ1 pin 2, Q9's base2
CONTROLJ1 pin 5, the 2/3 tap between R1 and R2, Q2's base5
REF13The 1/3 tap between R2 and R3, Q8's base, TP5–
RQ7's collector, R5, R8, TP2–
SQ14's collector, R7, R11, Q23's collector, TP1–
QThe collectors of Q15, Q16 and Q22, R12, R10, TP3–
Q̄ (QB in KiCad)The collectors of Q17 and Q18, R13, R9, R14, R15, TP4–
OUTPUTJ1 pin 3, Q19's collector, R213
DISCHARGEJ1 pin 7, Q20's collector7
RESETJ1 pin 4, R16, R184
RSTQ21's collector, R17, R19, R20. KiCad does not name it.–

THRESHOLD and TRIGGER are separate pins. In an astable, they meet outside the board at C, as on the NE555 in Experiment 1.

The parts list

The nets give the connections, and the parts list gives what to buy. The MMBT3904 and MMBT3906 are the 2N3904 and 2N3906 in the surface-mount SOT-23 package. 0805 is a resistor or capacitor of 2.0 mm by 1.25 mm. Experiment 17 gives each part its footprint.

PartQtyDesignatorsPackage
MMBT3904, NPN16Q1–Q4, Q12–Q23SOT-23
MMBT3906, PNP7Q5–Q11SOT-23
4.7 kΩ resistor8R1–R7, R210805
10 kΩ resistor10R8–R11, R14, R15, R17–R200805
1 kΩ resistor2R12, R130805
100 kΩ resistor1R160805
0.1 µF ceramic capacitor1C10805
Pin header, 1×8, 2.54 mm pitch1J1Through-hole
Test pad, 1.0 mm5TP1–TP5Copper on the board

Read the master schematic

Before Experiment 15 enters the schematic in KiCad, follow it on paper with a pencil. Print the PDF in the box “Resources” at the top of this page. Each sheet has its figure number in the title block.

  1. Follow R. Start at Q7's collector in Figure 14-4. Find R on the top sheet, Figure 14-3, then R8 and Q15 in Figure 14-6.
  2. Follow S. Start at Q14's collector in Figure 14-5. Find R11 and Q18 in Figure 14-6, TP1 in Figure 14-3 and Q23's collector in Figure 14-7.
  3. Name the pins. Cover the Pin column of the net table. For each pin from 1 to 8, find it in Figure 14-3 and write its net name. Then compare your list with the table.
  4. Find the parts on your breadboard. Find Q15 to Q18 of Figure 14-6 on the breadboard of Experiment 13. Then find the comparator transistors of Figures 14-4 and 14-5, with the name table after Figure 14-5.

Quick check

Three questions on this experiment. They ask how a net name connects, why Q23 moved to S, and what R16 does.

1. Two wires on two different sheets end in the name S, and no line joins them. They are…

A net is the set of terminals with one name. Every terminal with the name S is on the same net, on each sheet. The board layout then makes the copper for that net.

2. On the board, Q19's base takes Q̄. If Q23 held DIS, as on the breadboard, OUTPUT during a reset would…

C empties during a reset, so CAP falls below 1/3 and the trigger comparator sets S. S makes Q̄ 0, which turns Q19 off. With Q23 on S, S stays 0, Q̄ stays 1 and OUTPUT stays low.

3. Nothing is connected to pin 4 of the board. The timer…

R16 connects pin 4 to +5 V. Current through R16 and R18 turns Q21 on, so RST is near 0 V and Q22 and Q23 are off.

Before moving on

The master schematic gives each part a designator and each connection between the blocks a net name. It lists the parts and records the five changes from the breadboard. That is the first step from the breadboard to the board. The second step is Experiment 15: Schematic Capture, which enters the same sheets in KiCad and checks each connection.

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