What is sequence control?
In sequence control, a machine does its jobs in a fixed order. Each step starts when the one before it is finished, or when a time has passed, or when a switch is pressed.
Example: a bottle-filling machine. Bottle arrives, then the tap opens, then the tap closes, then the belt moves on. The order never changes.
This is different from feedback control, which keeps one value (like temperature) near a target. Sequence control asks: what happens next?
Two kinds: time-based (the next step starts after some seconds) and condition-based (the next step starts when a switch or sensor says so).
Sequence control devices
Every sequence circuit is built from three groups of parts.
- Input devices tell the circuit what is happening: push buttons (START, STOP), selector switches, limit switches (pressed when a part reaches a place), proximity and photo sensors, float switches.
- Control devices decide: relays, timers, counters. A relay is a switch worked by an electromagnet: current in its coil closes or opens its contacts.
- Output devices do the work: motors, lamps, buzzers, solenoid valves. A big motor is switched by a contactor (a heavy relay).
Contact types: NO (normally open: closes when the coil is on) and NC (normally closed: opens when the coil is on). The STOP button is NC, the START button is NO.
Sequence control circuits
We draw these circuits as a ladder diagram: two upright rails (power) and horizontal rungs, like a ladder. Each rung is one small job.
Self-holding (seal-in) circuit: START (NO) is in parallel with the relay's own NO contact K1. Press START: coil on, K1 closes. Release START: K1 still feeds the coil. STOP (NC) in series cuts everything.
Interlock: two motors, forward and reverse. Each one's NC contact sits in the other's rung, so both can never be on together. This stops a short circuit.
ON-delay timer: when the coil gets current, the timer waits (say 5 s) and then closes its contact. Use it to start motor 2 five seconds after motor 1.
Logic ideas: contacts in series = AND, contacts in parallel = OR, an NC contact = NOT.
Using programmable controllers (PLC)
Wiring many relays is slow to build and hard to change. A PLC (programmable logic controller) is a small industrial computer that does the same job with a program. To change the sequence you change the program, not the wires.
Parts: input module (takes signals from buttons and sensors), CPU with memory (holds the program), output module (drives contactors, valves, lamps), power supply.
Scan cycle (repeats thousands of times a second): 1) read all inputs, 2) run the program from top to bottom, 3) write all outputs.
Most PLCs are programmed in ladder logic, the same rung picture you saw here, so electricians can read it. The START/STOP rung is usually the first program a learner writes.
Good habits: wire STOP as NC so a broken wire stops the machine; add an emergency-stop button; test with the motor disconnected first.
Try it: your own sequence
Without any parts: write the steps of a street-crossing signal in order: (1) press the button, (2) wait 5 s, (3) cars get red, (4) walkers get green for 10 s, (5) back to start. Which steps are time-based and which are condition-based? Then, in the 3D above, press START, release, and predict what happens before pressing STOP.
Key formulas and definitions
- Series contacts = AND (all must be closed)
- Parallel contacts = OR (any one closed is enough)
- Self-holding: Coil = (START OR Coil) AND NOT STOP
- PLC scan: read inputs → run program → write outputs → repeat
- NO contact: closes when energised; NC contact: opens when energised
Worked examples
1. In the START/STOP circuit, the motor runs after START is released. Which part keeps it running?
The relay's own hold contact K1 (in parallel with START). It keeps current flowing to the coil.
2. A lamp must glow only when switch A AND switch B are both ON. How do you wire the contacts?
Put the two contacts in series. Current can reach the lamp only if both are closed.
3. A buzzer must sound if the front door OR the back door sensor is triggered. How do you wire it?
Put the two contacts in parallel. Either one closed is enough.
4. Motor 2 must start 4 s after motor 1. What device do you add?
An ON-delay timer set to 4 s, started with motor 1. Its contact then switches motor 2 on.
5. Write the logic of the self-holding rung.
Coil = (START OR Coil) AND NOT STOP. START or the coil's own contact feeds it, and STOP (NC) breaks it.
6. A PLC scan takes 10 ms. About how many scans does it do in 1 second?
1 s / 10 ms = 100 scans per second.
Common mistakes
- Wiring STOP as a normally-open button. A broken wire would then not stop the machine. Use NC.
- Forgetting the hold contact, so the motor runs only while START is pressed.
- Mixing up the coil with its contacts. The coil is the electromagnet; the contacts are the switches it moves.
- Thinking a PLC is magic. It only reads inputs, runs your rungs and sets outputs, again and again.