Lab 03: Machine Status Stack Light
Application Scenario
Walk through any modern factory and you will see stack lights (tower lights) mounted on top of CNC machines, packaging lines, and robot cells. A supervisor standing across the floor can tell the status of every machine at a glance, without walking over to read a screen.
In this lab you will add a two-color status indicator to the conveyor from Lab 02:
- G1 (green): conveyor RUNNING.
- R1 (red): conveyor STOPPED, with control power present.
If both lamps are dark, the control circuit has no power. That is a useful third piece of information for a maintenance technician.
Learning Outcomes
After this lab you will be able to:
- Use the N.O. and N.C. sides of one contact set to control two loads.
- Explain why the two lamps can never be ON at the same time.
- Use indicator lamps to diagnose the state of a control circuit.
- Describe the difference between machine and utility lamp color conventions.
Required Components
| Device | Type | Role in this lab |
|---|---|---|
| PB1 | Push button, N.O. | START |
| PB2 | Push button, N.C. | STOP |
| CR1 | Control relay | Conveyor motor contactor (simulated) |
| G1 | Green lamp | Conveyor RUNNING |
| R1 | Red lamp | Conveyor STOPPED |
Background
One contact set, two loads. Contact set CR1B has three pins: common 8, N.O. 6, and N.C. 5. The common is connected to either pin 6 or pin 5, never both:
- Coil OFF: 8–5 is closed, so R1 is ON.
- Coil ON: 8–6 is closed, so G1 is ON.
Because the common switches from one side to the other, it is physically impossible for both lamps to be ON at the same time. This is called a changeover (or Form C) contact.
How it looks on the ladder. In the ladder diagram, CR1B appears twice: as an N.O. contact on Rung 3 and as an N.C. contact on Rung 4. On the relay base, both symbols are the same contact set sharing pin 8. Because both rungs start at L1, one wire from L1 to CR1:8 feeds both of them.
Lamp color conventions. Colors do not mean the same thing everywhere:
- Machine stack lights (NFPA 79 / IEC 60204-1): green = normal operation, amber = warning or abnormal condition, red = emergency or fault. Many plants also use red to mean "stopped".
- Electric utility switchgear in North America often uses the opposite convention: red = breaker closed (energized, dangerous), green = breaker open (de-energized).
Always check which convention your plant uses before you read a panel.
Ladder Diagram
- Rungs 1–2: The same three-wire start/stop circuit as Lab 02.
- Rung 3: CR1B N.O. (8–6) turns G1 ON while the conveyor is running.
- Rung 4: CR1B N.C. (8–5) turns R1 ON while the conveyor is stopped. The slash through the contact symbol marks it as N.C.
Contact Allocation
| Relay | Part | Pins | Used in |
|---|---|---|---|
| CR1 | Coil | 2–7 | Rung 1 |
| CR1 | CR1A N.O. | 1–3 | Rung 2 (seal-in) |
| CR1 | CR1B N.O. | 8–6 | Rung 3 (G1) |
| CR1 | CR1B N.C. | 8–5 | Rung 4 (R1) |
| CR1 | CR1A N.C. | 1–4 | Not used |
Wiring List
Wires 1–9 are the same as Lab 02. Only Wires 10 and 11 are new.
| ☐ | Rung# | Wire# | From | To | Purpose |
|---|---|---|---|---|---|
| ☐ | 1 | 1 | L1 | PB2:1 | +24 V to STOP button |
| ☐ | 1 | 2 | PB2:2 | PB1:1 | STOP to START (N2) |
| ☐ | 2 | 3 | PB2:2 | CR1:1 | STOP to seal-in common (N2) |
| ☐ | 1 | 4 | PB1:2 | CR1:2 | START to relay coil (N3) |
| ☐ | 2 | 5 | CR1:3 | CR1:2 | Seal-in jumper on relay base (N3) |
| ☐ | 1 | 6 | CR1:7 | L2 | Coil return to GND |
| ☐ | 3 | 7 | L1 | CR1:8 | +24 V to CR1B common (feeds rungs 3 and 4) |
| ☐ | 3 | 8 | CR1:6 | G1:1 | CR1B N.O. to green lamp (N4) |
| ☐ | 3 | 9 | G1:2 | L2 | Green lamp return to GND |
| ☐ | 4 | 10 | CR1:5 | R1:1 | CR1B N.C. to red lamp (N5) — new |
| ☐ | 4 | 11 | R1:2 | L2 | Red lamp return to GND — new |
Tip: Rung 4 does not need its own wire from L1. Pin 8 is already connected to L1 by Wire 7, and it is the common for both the N.O. side (pin 6) and the N.C. side (pin 5).
Operating Sequence
| Step | Operator action | CR1 coil | CR1B 8–6 | CR1B 8–5 | G1 | R1 |
|---|---|---|---|---|---|---|
| 1 | Power ON, no button pressed | OFF | Open | Closed | OFF | ON |
| 2 | Press and release PB1 (START) | ON | Closed | Open | ON | OFF |
| 3 | Press and release PB2 (STOP) | OFF | Open | Closed | OFF | ON |
| 4 | Control power lost | OFF | Open | Closed | OFF | OFF |
| 5 | Control power restored | OFF | Open | Closed | OFF | ON |
Procedure
- Power OFF the trainer (or unplug the power cord).
- If your Lab 02 circuit is still wired, verify it against Wires 1–9. Otherwise, wire Wires 1–9 first.
- Add Rung 4: Wires 10 and 11.
- Check your work against the wiring list, one wire at a time. Mark each wire as checked.
- Meter check, power still OFF:
Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.
- CR1:8 to CR1:5 should read near 0 Ω.
- CR1:8 to CR1:6 should read high (the meter reads through G1 and R1 in series).
- L1 to CR1:8 should read near 0 Ω (Wire 7).
- Ask your lab partner to do a buddy check, then show the instructor.
- Power ON and complete the test table below.
Test & Acceptance
| # | Test | Expected result | ✓ |
|---|---|---|---|
| 1 | Power ON, do nothing | R1 ON, G1 OFF | ☐ |
| 2 | Press and release PB1 | G1 ON, R1 OFF | ☐ |
| 3 | Start and stop several times while watching both lamps | The two lamps are never ON at the same time | ☐ |
| 4 | Press and release PB2 | R1 ON, G1 OFF | ☐ |
| 5 | Hold PB2, then press PB1 | R1 stays ON; G1 stays OFF | ☐ |
| 6 | Start the conveyor, then switch power OFF | Both lamps dark (no control power) | ☐ |
| 7 | Switch power back ON | R1 ON, G1 OFF (conveyor does not restart) | ☐ |
Troubleshooting
| Symptom | Likely cause | What to check |
|---|---|---|
| Lamps are reversed (red while running) | Wires 8 and 10 swapped | Wire 8 must land on CR1:6; Wire 10 on CR1:5 |
| R1 stays ON all the time | R1 bypasses the relay contact | Wire 10 must come from CR1:5, not from L1 |
| R1 goes OFF while PB2 is held down | R1 is wired to CR1A instead of CR1B | CR1A's common is N2 (after STOP). Move Wire 10 to CR1:5 |
| R1 never turns ON | Rung 4 open | Wire 10 at CR1:5; Wire 11 from R1:2 to L2 |
| G1 and R1 both OFF with power ON | No power at CR1B common | Wire 7 from L1 to CR1:8 |
Review Questions
- Explain, using the pin numbers of CR1B, why G1 and R1 can never be ON at the same time.
- A student connects R1 to CR1:4 instead of CR1:5. At first the circuit seems to work: R1 is ON when the conveyor is stopped. What does R1 do while PB2 is held down? Why? (Hint: trace where CR1A's common gets its power.)
- A technician walks up to a machine and sees that both stack lights are dark. What does this tell them? What would they check first?
- You transfer from a manufacturing plant to an electric utility substation. A panel lamp is red. Why must you find out which color convention the site uses before you touch anything?
Extension: Lamp Test
Real control panels have a LAMP TEST button that lights every indicator at once, so a burned-out bulb is not mistaken for a "stopped" or "running" signal.
Power OFF. Wire PB3 (N.O.) so that PB3:1 connects to L1, and PB3:2 connects to both G1:1 and R1:1. Power ON and press PB3: both lamps should light. Release PB3: normal status returns.
Think about it: while PB3 is pressed, nodes N4 and N5 are tied together. Why does this cause no problem in our circuit? (Hint: what voltage is on CR1:8?) Industrial panels usually isolate each lamp with a diode or a dedicated lamp-test relay so circuits cannot feed back into each other.