Lesson 3: The 555 monostable

Timers & comparators

In this lesson

  • Say what a monostable does.
  • Name the 555 pins in the circuit.
  • Describe how the pulse starts and ends.
  • Calculate the pulse time.
  • Simulate it in Yenka.
  • Build it on breadboard and test it.

Booklet and slides

Lesson 3 is the third lesson in the booklet. Tasks 1 to 6 and your exam questions are there.

Block 6 booklet (PDF) Lesson slides
Concept 1

What a monostable does

Key words

Monostable
a circuit with one stable state. A trigger gives one pulse, then it goes back.
Trigger
the signal that starts the pulse.
Pulse
the time the output stays high.

A monostable waits with its output low. That is its one stable state.

  • Slide the switch to 0 V and back. This is the trigger.
  • The output goes high. LED1 comes on.
  • After a set time, the output goes low again by itself.

One trigger gives one pulse. The pulse time is set by two parts, R2 and C1. It does not depend on how long you press.

You meet monostables in a stair light that stays on for two minutes. A hand dryer that runs for a set time uses one too.

pin 2, trigger SW1 slid to 0 V pin 3, output pulse time T time →
A short trigger on pin 2 gives one long pulse on pin 3.
Practice 1

A monostable gives a 10 s pulse. SW1 is slid to 0 V and back within 1 s.

State how long LED1 stays on. Give a reason.

Answer
10 s. R2 and C1 set the pulse time. A short trigger only starts the pulse.

You can now…

  • say what a monostable does.
  • say what starts the pulse and what sets its length.
Concept 2

The 555 timer's pins

Key words

555
an 8-pin timer chip.
Reset
pin 4. It must go to +5 V, or the chip will not run.
Threshold
pin 6. It ends the pulse when C1 is ⅔ full.
Discharge
pin 7. It empties C1 when the pulse ends.

Hold the chip with the notch at the top. Pin 1 is top left. The pins count down the left side, then up the right side.

  • Pins 8 and 1 power the chip: +5 V and 0 V.
  • Pin 4, reset, goes to +5 V too.
  • Pin 2, trigger: low starts the pulse.
  • Pin 3, output: high during the pulse. It drives LED1.
  • Pins 6 and 7 are joined. R2 and C1 connect here.
  • Pin 5 is not used.
555 1234 8765 0 Vtriggeroutputreset +Vdischargethresholdcontrol (not used)
The 555 from above. Notch at the top, pin 1 top left.
Practice 2

State the pin number for each job.

  • (a) starts the pulse
  • (b) drives LED1
  • (c) must go to +5 V, as well as pin 8
Answer
(a) Pin 2, trigger. (b) Pin 3, output. (c) Pin 4, reset.

You can now…

  • find pin 1 on a 555.
  • name the job of each pin in the monostable.
Concept 3

Read the circuit diagram

Key words

Changeover switch
a slide switch with three pins. The middle pin joins one side or the other.
Pull-up resistor
holds a pin high until something pulls it low. Here, R1 on pin 2.
Electrolytic capacitor
C1. It has a + leg, so it must go the right way round.

This is the circuit in your booklet.

  • SW1 joins pin 2 to +5 V, or to 0 V when you slide it.
  • R1 holds pin 2 high while SW1 is moving and touches neither side.
  • R2 and C1 set the pulse time. They join at pins 6 and 7.
  • R3 protects LED1 by limiting the current.

Test points T1 to T4 are where you put the meter and the probe.

+5 V0 V T1 T2 SW1 R110 k + R2100 k C1100 µF 762 48 15 3 555 IC1 R3270 LED1 T3 T4 5 V 0 V low
The booklet circuit, with test points T1 to T4. Where the pin 2 wire crosses C1's wire there is no dot, so they are not joined.
Test pointWhat it measuresWaitingDuring the pulse
T1the +5 V supply5 V5 V
T20 V0 V0 V
T3pin 2, the trigger5 V0 V while SW1 is at 0 V
T4pin 3, the outputabout 0 V3.5 V to 5 V
Practice 3

State the job of R3.

Answer
R3 limits the current through LED1, so LED1 is not damaged.

You can now…

  • say what each part of the circuit does.
  • say what each test point measures.
Concept 4

How the pulse starts and ends

Key words

Charge
C1 fills up through R2. The voltage across it rises.
⅔ of the supply
3.3 V on a 5 V supply. When C1 reaches it, the pulse ends.
Describe
give the steps in order. Each step is worth one mark.

Follow the chain from the switch to LED1:

  1. SW1 is slid to 0 V, so pin 2 goes low.
  2. Pin 3 goes high. LED1 comes on.
  3. C1 charges through R2. The voltage across C1 rises.
  4. When C1 reaches ⅔ of the supply, pin 3 goes low. LED1 goes off.
  5. Pin 7 empties C1, ready for the next trigger.

A bigger R2 or a bigger C1 makes C1 fill more slowly. The pulse is longer.

03.35 ⅔ of 5 V pin 3 output C1 T time (s) voltage (V)
C1 charges until it reaches ⅔ of the supply. Then the pulse ends.

Another way to trigger it

Exam papers often use a push-to-make switch from pin 2 to 0 V. R1 holds pin 2 high. Pressing the switch pulls pin 2 low and starts the pulse.

The rest of the circuit stays the same.

+5 V0 V R110 k to pin 2 SW1push to make
The push-to-make version of the trigger.
Practice 4

Describe what happens after SW1 is slid to 0 V and back. (3)

Answer
  • Pin 2 goes low, so pin 3 goes high and LED1 comes on (1).
  • C1 charges through R2 (1).
  • When C1 reaches ⅔ of the supply, pin 3 goes low and LED1 goes off (1).

You can now…

  • describe how the pulse starts and ends, one step for each mark.
  • say how R2 and C1 change the pulse.
Concept 5

Calculate the pulse time

Key words

kΩ to Ω
multiply by 1000. 100 kΩ = 100 000 Ω.
µF to F
divide by 1 000 000. 100 µF = 0.000 1 F.
Pulse time of a 555 monostable
T = 1.1 R C
T = pulse time (s) · R = resistance of R2 (Ω) · C = capacitance of C1 (F)
Worked example 1 Our board has R2 = 100 kΩ and C1 = 100 µF. Calculate the pulse time.
T=1.1 R C
T=1.1 × 100 000 × 0.000 1Ω and F
T=11 s
Worked example 2 C1 = 10 µF. A pulse of 1.1 s is needed. Calculate the value of R2.
T=1.1 R C
1.1=1.1 × R × 0.000 0110 µF in F
R=1.1 ÷ (1.1 × 0.000 01)
R=100 000 Ω = 100 kΩ
Practice 5

A monostable has:

  • R2 = 68 kΩ
  • C1 = 47 µF

(a) Calculate the pulse time.

(b) State the new pulse time if C1 is changed to 470 µF.

Answer

(a) T = 1.1 × 68 000 × 0.000 047 = 3.5 s

(b) C1 is 10 times bigger, so the pulse is 10 times longer: 35 s.

You can now…

  • calculate the pulse time from R2 and C1.
  • change kΩ to Ω and µF to F.

Monostable bench

The booklet circuit, live

Hold the button to slide SW1 to 0 V. Watch LED1, C1 and the graph.

Predict what happens to the pulse if R2 is changed from 100 kΩ to 200 kΩ.

LED1 glows while pin 3 is high.

Let go to slide it back.

Pulse time, 1.1 R C–
LED1 on for–
Voltage across C1–
Pin 3 (T4)–
Shown at–
Presets
Solid: C1. Dashed: pin 3. The pulse ends where C1 meets ⅔ of 5 V.
Concept 6

Simulate it in Yenka

Key words

Voltmeter
measures the voltage across C1. Connect it from C1's + leg to 0 V.
Logic indicator
shows the logic level. In Yenka, red = high and white = low.

This is booklet Task 1. Build the circuit in Yenka first, then add:

  • a voltmeter across C1
  • a logic indicator on pin 3

Slide SW1 to 0 V and straight back. Time how long the logic indicator stays red. Watch the voltmeter climb while it is red.

+5 V0 V SW1 R1 + R2C1 V 762 48 15 3 555 logic indicator R3 LED1
The Yenka set-up: a voltmeter across C1 and a logic indicator on pin 3.
Practice 6

In Yenka, the logic indicator is red. The voltmeter across C1 reads 1.2 V.

State whether the pulse has ended. Give a reason.

Answer
No. C1 has not reached ⅔ of the supply (3.3 V) yet, so pin 3 is still high.

You can now…

  • add a voltmeter and a logic indicator to a Yenka circuit.
  • time a pulse in Yenka.
Concept 7

Build it on breadboard

This is booklet Tasks 2 to 4. The worked guide takes you from the diagram to the breadboard, one net at a time. A net is a set of legs that are joined. Then find the mistake on three faulty boards.

Pre-power-up check

Do this before you connect the supply. Tick each one in booklet Task 4.

  1. No beep between T1 and T2 on continuity: no short across the supply.
  2. IC1 straddles the centre gap, notch and pin 1 on the left.
  3. C1 the right way round: + leg to pins 6 and 7, − leg to 0 V.
  4. LED1 the right way round: long leg to R3, short leg to 0 V.
  5. Every wire matches the wiring schedule.
  6. The supply is set to 5 V before it is connected.
long leg +short leg − flat edge − from above:flat = − − stripe = − leg 1845 notch at the top,pin 1 top left
Three parts that must go the right way round.
Practice 7

Give two checks you would make on C1 before you connect the power.

Answer
Any two: the value is 100 µF; the + leg goes to pins 6 and 7; the − leg (the stripe) goes to 0 V; its legs are in the right columns.

You can now…

  • plan the monostable on breadboard, one net at a time.
  • do a pre-power-up check.
Concept 8

Test your board

This is booklet Tasks 5 and 6. Pick a tool, a moment and a test point. See what your board should read.

Meter and probe on the monostable

The meter is on DCV 20 with the black probe on T2. The probe is clipped to +5 V and 0 V.

The test point you pick is shown in green.
Tool
When
Test point

Waiting

Fault from readings

Compare each reading with the one you expect. The first one that is wrong tells you where to look.

  • T1 not 5 V: look at the supply leads.
  • T3 never goes to 0 V: look at SW1 and its wires to pin 2 and 0 V.
  • T4 goes high but LED1 stays off: look at R3 and LED1.
  • T4 never goes high: look at pins 4 and 8, and at IC1 itself.
T1 = 5 V? T3 goes to 0 V? T4 goes high? yesyesyes nonono check supplycheck SW1check pins 4, 8and IC1 check R3 and LED1
Test in order, from the supply to the output.
Practice 8

LED1 never lights. T1 reads 5.0 V. T3 goes to 0 V when SW1 is slid. T4 reads 3.6 V for 11 s.

Suggest the fault.

Answer
The 555 is working: pin 3 goes high. The fault is after pin 3. LED1 is the wrong way round, or R3 or LED1 is not joined.

You can now…

  • say what the meter and the probe should show at each test point.
  • find a fault from the readings.
Check

Check your understanding

Pick an answer to see feedback on it. You can then try the others. Your score counts first tries only.

Fill the gaps

The pulse starts when pin 2 goes . C1 charges through . The pulse ends when C1 reaches of the supply. A bigger C1 gives a pulse.

RAG self-check

Tap Red, Amber or Green for how sure you feel.

Booklet answers: Lesson 3

Try the task in your booklet first. Then tap to check.

Task 1: Yenka
  • R2 = 100 kΩ: pulse about 11 s. C1 reads about 3.3 V (⅔ of 5 V) when it ends.
  • R2 = 47 kΩ: pulse about 5.2 s. C1 still reads about 3.3 V when it ends.
Task 2: Plan it
Check your plan against Part 2 of the worked guide above. Every net needs its own column or rail, and IC1 straddles the centre gap.
Task 5: Test it
Why your average might not be exactly 11 s: capacitor tolerance (often ±20 %), resistor tolerance, or your reaction time.
Task 6: Fault from readings
  • (a) Pin 2 never goes low. SW1 is wired wrongly or not joined to pin 2, or its 0 V pin is not connected.
  • (b) Any two: SW1 left at 0 V. A wire or a leg joins pin 2's column to 0 V. SW1 is one column out, so its 0 V pin sits in pin 2's column.
Exam question 1 (2025 Q11)
  • Rails labelled, with nodes (1).
  • Pins 4 and 8 to +6 V, pin 1 to 0 V (1).
  • SW1 and R1 in series between the rails (1).
  • R2 and C1 in series, C1 the right way round (1).
  • Pins 6, 7 and 2 joined, pin 5 not connected, Voutput from pin 3 with a node (1).
  • Symbols and labels (1).
Exam question 2 (2025 Q7)
  • (a)(i) Red wire to the +V supply, black wire to 0 V.
  • (a)(ii) Place the probe tip on the point (1). The red LED lights (1).
  • (b)(i) T3 and T4.
  • (b)(ii) Pin 7 is connected to 0 V (ground).
Exam question 3: three errors
  • C1 is the wrong way round.
  • The LED is the wrong way round.
  • Pin 4 (reset) is connected to 0 V. It must go to +5 V.
Key words
1. monostable · 2. trigger · 3. low · 4. length · 5. longer · 6. pre-power-up

Picture glossary

Monostable

One trigger gives one pulse of a set length.

Trigger

Pin 2 going low. It starts the pulse.

555

555 timer

An 8-pin timer chip. Notch at the top, pin 1 top left.

+

Electrolytic capacitor

C1. Its + leg must go to the higher voltage.

Pull-up resistor

Holds a pin high until something pulls it low.

Changeover switch

Three pins. The middle pin joins one side or the other.

⅔ of the supply

3.3 V on 5 V. The pulse ends when C1 reaches it.

Sources & credits: booklet answers to 2025 Q7 and Q11 follow the National 5 Practical Electronics marking instructions, © Qualifications Scotland (SQA). Text, drawings, simulator, build guide and booklet by R Stewart. R Stewart - October 2026.