Programming Siemens LOGO! Weekly Timers for Workshop Shift Bells

A Siemens LOGO! 8 logic module is well suited to drive a workshop sounder on a recurring shift schedule because the controller ships with an integrated real-time clock and a Weekly Timer function block that supports up to three on/off cams per instance with one-minute resolution. This reference walks through hardware selection, output wiring, ladder-logic construction in LOGO! Soft Comfort, pulse-width handling for a fixed 60-second bell, and the RTC battery and daylight-saving pitfalls that catch first-time installers.

1. Application Overview

The reference application drives an audible sounder at the start of work, lunch, mid-afternoon break, and end of day in a workshop with the following recurring pattern:

Each bell event is a single 60-second pulse. A Weekly Timer cam directly keyed to the bell ON and OFF times is the most compact implementation; a separate pulse generator is required only if the bell duration must be variable from the LOGO! display or an attached HMI.

2. Prerequisites

• Siemens LOGO! 8 base module with relay or transistor outputs. Order codes: 6ED1052-1HB08-0BA2 (230RCE), 6ED1052-1MD08-0BA2 (24RCE), 6ED1052-2MD08-0BA2 (24CE transistor), or 6ED1052-1CC08-0BA2 (12/24RCE).
• LOGO! Soft Comfort V8.x or V9.x, downloadable from the Siemens LOGO! 8 system manual (ID 109751784).
• Ethernet (RJ45) or micro-USB programming cable compatible with the LOGO! base module.
• Workshop sounder or bell rated for the LOGO! output voltage and within the contact rating (see Section 5).
• Backup battery module (LOGO! Battery Card 6ED1057-1BA00-0AB0) if the application must survive a power outage longer than the integrated supercap retention window of about 80 hours (see Section 10).

3. Shift-Bell Schedule Specification

Before writing code, freeze the schedule. Each event is described by the tuple (Day mask, ON time, OFF time). The day mask is a 7-bit field where bit 0 = Monday through bit 6 = Sunday. The Weekly Timer cam resolution is one minute.

Cams 1-4 occupy two Weekly Timer instances (3 cams per FB), while Friday, Saturday, and Sunday each require their own instance. The full schedule therefore needs five Weekly Timer blocks - well within the program memory of any LOGO! 8 base module.

4. LOGO! Hardware Selection

Pick the base module to match the sounder voltage. The most common reference designs use the parts listed below; for full catalog reference see the LOGO! 8 system manual.

The relay variants handle the majority of 110 V AC and 230 V AC industrial bells directly. The transistor variants are reserved for 24 V DC sounders drawing under 300 mA continuous. If the sounder inrush exceeds the LOGO! contact rating (electromechanical bells commonly pull 1.5 - 2 A inrush at 230 V AC), wire an interposing contactor sized to the sounder and let the LOGO! relay only switch the contactor coil.

Reserve one digital input (for example I1) as a manual-override pushbutton on the operator panel and another as a silence/reset input. These are not required by the original specification but are good commissioning practice and allow on-demand bells and an emergency silence.

5. Wiring the Sounder to the LOGO! Output

Connect the sounder to Q1 of the LOGO! base module. The schematic below covers a 230 V AC bell on the 230RCE variant.

L (230 V AC) --- [F1 6 A] --- COM of Q1 relay contact --- Bell --- N
                 |
                 +--- LOGO! 230RCE L terminal
N (230 V AC) ----------- LOGO! 230RCE N terminal
PE --------------------- LOGO! 230RCE PE terminal and bell chassis

For a 24 V DC sounder on the 24RCE variant, wire as follows:

+24 V DC PSU (+) --- Bell (+) --- Q1 contact --- PSU (-)
                     |
LOGO! supply:       +24 V --- 24 V terminal
                    0 V   --- 0 V terminal
GND common between PSU and sounder required.

The 24 V DC sounder must share a common ground reference with the LOGO! supply, or the transistor output (if used) will not switch the sounder. Use a separate DC PSU for the sounder if the LOGO! internal 24 V sensor supply (max 200 mA on the 230RCE and 300 mA on the 24RCE) cannot deliver the required sounder current.

6. Weekly Timer Function Block

In LOGO! Soft Comfort the Weekly Timer is found in the Special Functions toolbox. Each block exposes three cams. Each cam has the following parameters:

Because the resolution is one minute, a 60-second bell is configured by entering ON = 07:00 and OFF = 07:01. Cross-midnight cams (where ON time is later than OFF time) span midnight and are supported natively by the firmware.

If the bell pulse width must be changed at runtime - for example a 'short bell' mode on Saturdays - the cam ON and OFF times can be written through the LOGO! display, the integrated Web Server (LOGO! 8.3 and later), or via Modbus/TCP from an attached HMI. The cam parameters map into the variable memory (VM) at offsets that depend on the block instance number assigned by LOGO! Soft Comfort.

7. Step-by-Step Programming in LOGO! Soft Comfort

1. Launch LOGO! Soft Comfort V8.3 or V9.0. Create a new project and select the matching base module from the device catalog.
2. Open Tools > Select Hardware and pick the 0BA8 or 0BA9 series base module. Verify the IP address of the LOGO! is reachable via Ethernet (default 192.168.0.10 with subnet 255.255.255.0; the LOGO! 0BA8 default gateway is 192.168.0.1).
3. Drag five Weekly Timer function blocks onto the FBD canvas:
   • WT1: Cam1 = Mo|Tu|We|Th, 07:00 - 07:01; Cam2 = Mo|Tu|We|Th, 12:00 - 12:01; Cam3 = Mo|Tu|We|Th, 15:00 - 15:01.
   • WT2: Cam1 = Mo|Tu|We|Th, 18:00 - 18:01; Cam2 and Cam3 spare.
   • WT3: Friday schedule (cams programmed per site timetable).
   • WT4: Saturday schedule (cams programmed per site timetable).
   • WT5: Sunday schedule (cams programmed per site timetable, if any).
4. Wire each cam output through a multi-input OR block to digital output Q1. A four-input OR is sufficient for WT1 + WT2; chain additional OR blocks as needed to combine all five Weekly Timer instances.
5. Assign a digital input (for example I1) as a manual bell trigger. Feed it through a Pulse Generator set to 60 seconds so the operator button produces the same one-minute tone. OR the pulse output with the Weekly Timer chain.
6. Configure the LOGO! real-time clock via File > Properties > Time or via the on-board menu. Set the correct date, time, time zone, and DST rule (Auto DST, EU rule, US rule, or none).
7. Save the program and use Tools > Transfer > PC -> LOGO! to download the program into the LOGO! flash memory. Online test mode does not persist the program across a power cycle.

8. Alternative: Weekly Timer with Pulse Generator for Variable Width

If the bell pulse width is variable, for example operator-adjustable between 5 s and 120 s, replace each cam's OFF time with a Pulse Generator:

[Weekly Timer Cam  (1 min window)] -- [Pulse Generator  edge-triggered, single-shot] -- [OR] -- Q1
                                       Pulse time = 60 s (parameter VM)

The Weekly Timer becomes a one-minute trigger window and the Pulse Generator stretches the active state for the desired bell duration. Set the Pulse Generator to non-retriggerable (edge-triggered, single-shot) to avoid extension if the input stays high for longer than the pulse time. The pulse time can be parameterised at runtime by writing to the VM address that backs the Pulse Generator's TH (pulse time) input.

9. Alternative: Real-Time Clock Block with Comparator Cascade

For installations where the schedule changes frequently, or where hundreds of events must be supported, a different topology can be used:

LOGO! RTC block --[day-of-week BCD]--> Compare (B015) --> Q1
                  [hour BCD]         --- Compare ---
                  [minute BCD]       --- Compare ---

The RTC block provides the current hour, minute, second, and day-of-week as integer outputs. A cascade of compare blocks evaluates the day mask and time-of-day against the active schedule. This consumes more program memory than Weekly Timer cams and is harder to maintain, but it allows arbitrary schedules to be pushed from a higher-level SCADA or HMI via parameter VM writes without re-flashing the LOGO!.

10. Real-Time Clock, Battery Backup, and Daylight Saving

The LOGO! 8 has an internal supercapacitor that retains the RTC for approximately 80 hours after a power loss. For longer retention, install:

Without battery backup, an extended power outage causes the LOGO! to lose its RTC and time-stamped events. When power is restored the LOGO! clock starts from 00:00:00 on the date it boots, and shift bells will ring at the wrong times until the operator manually resets the clock through the on-board menu or the Web Server.

Daylight saving time: The Weekly Timer compares the on-board clock time (including any DST offset) to the configured ON and OFF times. If DST is enabled (EU rule or US rule), the clock shifts by one hour on the appropriate date. A bell scheduled at 07:00 will ring at 07:00 local time year-round, including across the DST transition. For installations where bell times must remain anchored to solar time (rare in industrial settings), disable DST and use two separate Weekly Timer instances - one for summer, one for winter.

11. Verification and Commissioning

1. Connect the LOGO! to the programming PC, open the online monitor, and confirm each Weekly Timer block shows the correct cam state at the scheduled times.
2. Force the LOGO! clock to 06:59:50 and watch Q1 transition at 07:00:00 and drop at 07:01:00. Repeat for 12:00, 15:00, and 18:00.
3. Verify the sounder audibly rings for exactly 60 seconds. Use a stopwatch and listen for the tone to drop cleanly at the one-minute mark.
4. Disconnect mains power for 10 minutes, reconnect, and confirm the LOGO! retained the RTC and the next scheduled bell rings at the correct time.
5. Disconnect mains power for 90 hours or longer (more than the rated supercap retention), reconnect, and confirm the LOGO! did not lose its program but did lose the clock. Verify the operator notification (via the LOGO! display message text or a flag in the Web Server) that the clock needs to be reset.
6. Force a power loss mid-bell (e.g. trip the breaker 30 seconds into the 12:00 bell). On power restore, the bell should not retrigger because the Weekly Timer cam is edge-of-time based and not retriggerable.
7. Verify the manual pushbutton on I1 rings the bell for exactly 60 seconds regardless of schedule, and that pressing the silence input immediately drops Q1.

12. Troubleshooting Matrix

13. Related Application Examples from Siemens

Siemens publishes an application note titled 'Bell systems for schools' in the LOGO! Application Examples collection, accessible via the Siemens Industry Online Support portal. The reference implementation uses two Weekly Timer blocks for weekday bells and two more for Saturday and Sunday bells, which maps directly to the workshop schedule in this article. Other useful examples in the same collection cover staircase lighting timers, parking-lot gate controllers, and irrigation controllers - all built around the same Weekly Timer and Pulse Generator primitives.

14. Standards and Compliance Notes

LOGO! 8 base modules carry CE, UL, and cULus listings and are rated for industrial environments per IEC 61131-2. Install per IEC 60204-1 (Safety of machinery - Electrical equipment of machines) and the local electrical code. The sounder circuit should be protected by an appropriately sized overcurrent device - typically a 6 A Type B MCB on a 230 V AC ring, or a 2 A DC fuse on a 24 V DC sounder circuit. De-energise the LOGO! supply and lock out the upstream disconnect before performing any wiring changes; the relay contacts can carry lethal voltage even when the LOGO! is in stop mode.

15. Output Sizing Worked Example

Consider a typical 110 dB industrial horn rated 24 V DC at 0.5 A continuous, with an inrush of 1.2 A for 20 ms. Two design cases apply:

For Case B the apparent power through the contactor coil is P = V x I = 24 x 0.05 = 1.2 W (a typical contactor pickup) and the LOGO! relay contact sees only the contactor coil inrush (around 0.1 A), well within the 10 A rating. For Case C the LOGO! transistor output can switch the buzzer directly provided the total 24 V sensor supply load (sounder plus sensor loads) stays under 0.3 A for the 24CE or 0.2 A for the 230RCE.