Lab 13: Warning Beacon Flasher

Application Scenario

A chemical storage tank has a high-level alarm. When the level switch trips, a red beacon on top of the control panel must flash so that it catches the eye of anyone in the area — a steady light is easily ignored, but a flashing one is not. The alarm stays active (latched) until an operator presses RESET, even if the level drops again. When everything is normal, a green NORMAL lamp is ON.

A flashing output needs a circuit that turns ON and OFF by itself, over and over. None of the timers you have used can do this alone:

• An on-delay timer turns ON once, after a delay.
• An off-delay timer turns OFF once, after a delay.
• An interval timer gives one pulse, then stays OFF.

By connecting two on-delay timers so that each one controls the other, you build an oscillator (also called a flasher or recycling timer). In this lab, PB1 simulates the high-level switch, PB2 is RESET, R1 is the flashing ALARM beacon, and G1 is the NORMAL lamp.

Learning Outcomes

After this lab you will be able to:

• Build a two-timer oscillator from on-delay timers.
• Explain, step by step, why the circuit keeps switching by itself.
• Set the ON time and OFF time of a flashing output independently.
• Use a multi-function timer (TR4) as an on-delay timer in a larger circuit.

Required Components

Background

How the two-timer oscillator works. Both timers are fed from the alarm latch (node N3). Each timer's coil passes through a contact of the other timer:

• TR1's coil is fed through TR4A N.C.T.O. (1–4).
• TR4's coil is fed through TR1A N.O.T.C. (1–3).

Follow one cycle:

1. The alarm latches. TR4 is not timing, so TR4A (N.C.) is closed, and TR1 starts timing. The beacon is OFF.
2. After TR1's time (1 s), TR1 times out: TR1A closes and starts TR4; TR1B closes and turns the beacon ON.
3. After TR4's time (1 s), TR4 times out: TR4A (N.C.) opens and removes power from TR1.
4. TR1 resets at once: TR1B opens (beacon OFF) and TR1A opens, removing power from TR4.
5. TR4 resets at once: TR4A closes again, and TR1 starts timing again. The cycle repeats from step 1.

So: TR1's setting is the OFF time and TR4's setting is the ON time of the beacon. Pressing RESET removes power from N3, which stops everything.

Timing Diagram

Notice the very short gaps in the TR1 coil line. Each time TR4 times out, TR1 loses power for only a moment — just long enough to reset — and then starts timing again.

Functional Requirements

1. With no alarm, G1 (NORMAL) is ON and R1 is OFF.
2. A momentary press of PB1 latches the alarm: G1 turns OFF and R1 starts flashing, about 1 s ON and 1 s OFF.
3. R1 keeps flashing until PB2 (RESET) is pressed. Then R1 stops (OFF) and G1 turns ON.
4. While PB2 is held, PB1 cannot start the alarm.

Ladder Diagram — Step-by-Step Drawing Guide

Draw on the Ladder Diagram Worksheet. Use pencil. This design uses 6 grid lines.

Step 1 — Rungs 1–2: alarm latch

1. Grid line 1: PB2 (N.C.), PB1 (N.O.), CR1 coil (2–7). Mark N2 and N3.
2. Grid line 2: CR1A N.O. seal-in (1–3) from N2 to N3.

Step 2 — Rung 3: TR1 (OFF-time timer)

1. From N3, draw a line down to grid line 3.
2. Draw a TR4A N.C.T.O. contact (pins 1 and 4), then the TR1 coil (2–7). Connect TR1:7 to L2.
3. Mark the node between TR4A and the TR1 coil as N4.

Step 3 — Rung 4: TR4 (ON-time timer)

1. From N3, continue the line down to grid line 4.
2. Draw a TR1A N.O.T.C. contact (pins 1 and 3), then the TR4 coil (2–10). Connect TR4:10 to L2.
3. Mark the node between TR1A and the TR4 coil as N5.

Step 4 — Rung 5: ALARM beacon

1. Starting from L1, draw a TR1B N.O.T.C. contact (8–6), then R1. Connect R1 to L2. Mark the node as N6.

Step 5 — Rung 6: NORMAL lamp

1. Starting from L1, draw a CR1B N.C. contact (8–5), then G1. Connect G1 to L2. Mark the node as N7.

Step 6 — Trace your drawing with a pencil

Use the five steps in the Background section. For each step, write which coils are energized and which of TR1A, TR1B, and TR4A are closed. Then answer:

1. What would happen if TR4A were wired on pins 1–3 (N.O.) instead of 1–4?
2. What would happen if TR1 and TR4 were both fed from L1 instead of N3?
3. You want the beacon ON for 0.5 s and OFF for 1.5 s. Which timer gets which setting?

Step 7 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

List every terminal on each node (with device names), chain them (k − 1 wires per node, no more than two wires per terminal), and fill in the wiring table on the worksheet. The N4 row is done for you.

Get instructor approval before wiring.

Key Wiring Points

1. Cross-connect the timers: each timer's coil goes through the other timer's contact.
2. One N.C. contact breaks the loop. TR4A must be the N.C.T.O. side (1–4). With two N.O. contacts, nothing would ever start.
3. Feed both timers from the latched node (N3), so RESET stops the flashing.
4. ON time = TR4 setting; OFF time = TR1 setting.
5. TR4 uses TR2-style pins (coil 2–10, contacts 1-3-4 and 11-9-8), and its trigger is not used in on-delay mode.

Wiring Procedure

1. Power OFF. Set TR1 to 1 s. Set TR4 to on-delay mode, 1 s.
2. Wire the circuit from your approved wiring table, one node at a time.
3. 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.

   • L1 to CR1:1 should read near 0 Ω, and high while PB2 is pressed.
   • CR1:2 to TR4:1 and CR1:2 to TR1:1 should read near 0 Ω (all on N3).
   • TR4:1 to TR1:2 should read near 0 Ω (TR4A N.C.T.O. closed).
   • TR1:1 to TR4:2 should read high (TR1A N.O.T.C. open).
   • CR1:8 to G1:1 should read near 0 Ω (CR1B N.C. closed).
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below.

Test & Acceptance

Troubleshooting

Review Questions

1. Explain in your own words why this circuit keeps switching by itself. Which contact "breaks" the cycle and restarts it?
2. Why can't a single interval timer (TR3) make a light flash continuously?
3. Why does TR1 set the OFF time and TR4 set the ON time? What would change if R1 were driven by TR4B N.O. (11–9) instead of TR1B?
4. Why is a flashing light more effective as an alarm than a steady light? Name two other ways industrial alarms get attention.
5. Most PLCs have a built-in "clock pulse" or you can build a flasher with two timers. Describe how you would do this with two TON timers in a PLC, based on this lab.

Applying This Circuit Pattern

The two-timer oscillator produces a repeating ON/OFF cycle for as long as it is enabled. Each timer sets one half of the cycle.

Design question for the future: Lab 11's machine runs all day but gets only one lube shot. Using this lab's circuit, how would you give it a 5-second lube shot every 60 seconds while the machine is running? Which node would enable the oscillator, and which contact would drive the lube pump?

Extension: Alternating Flasher

Railroad crossings and some machine beacons use two lamps that flash alternately. TR1B is a changeover contact (common 8, N.O. 6, N.C. 5). Without adding any relay, change the NORMAL lamp G1 so that:

• with no alarm, G1 is ON (NORMAL), and
• during the alarm, R1 and G1 flash alternately.

Which pin does G1 move to? What happens to CR1B? Draw it, wire it, and test it.