Implementing an Astro Sunrise/Sunset Function on Siemens LOGO! 8

Siemens LOGO! logic modules do not ship with a native Astro function block, yet lighting, irrigation, and shutter applications frequently require a digital sunrise/sunset switch that follows the annual solar cycle at a fixed installation site. This reference documents four field-proven approaches - almanac lookup, simplified astronomical formula, KNX/EIB timer integration, and photovoltaic sensor tracking - with worked LOGO! Soft Comfort FBD code, parameter tables, and verification steps sized for garden-lighting accuracy (≤ 20 minutes).

1. Problem Definition and Accuracy Targets

The user requirement is straightforward: a Boolean output on a LOGO! that goes HIGH at local sunrise and LOW at local sunset, automatically tracking the changing day length across a 12-month cycle at a single fixed geographic coordinate (latitude, longitude). No GPS is required; the site is static.

2. Hardware Selection

All four approaches below run on the LOGO! 8 system family (0BA8, 0BA9, 0BA10). The specific part numbers most commonly used for Astro switching applications are listed below.

Refer to the LOGO! 8 System Manual (Siemens Support, 109741041) for the full device specification, in particular the maximum number of function blocks (0BA8: 400 blocks; 0BA9: 800; 0BA10: 800 with multi-page program support).

3. Approach 1 - Almanac Lookup Table (Simplest, Recommended for 20-min Accuracy)

The most reliable method on LOGO! is to pre-compute the year's sunrise and sunset times in a spreadsheet, then load the values as constants. Because the LOGO! has no trig instructions but has shift registers and analog comparators, a 365-step lookup is trivially implementable.

3.1 Compute the Almanac Offline

Use the NOAA Solar Calculator algorithm. For latitude φ (positive north), longitude λ (negative west), in the northern hemisphere at solar declination δ, the hour angle H of sunrise/sunset is:

cos(H) = -tan(φ) · tan(δ)

with solar declination approximated as:

δ = 23.45° · sin(360° · (284 + n) / 365)

where n is the day-of-year. Solar noon in local mean time (LMT) is then:

LST = 12:00 + EoT - 4·λ + DST_offset

with the Equation of Time (minutes):

EoT = 9.87·sin(2B) - 7.53·cos(B) - 1.5·sin(B), B = 360°·(n - 81) / 365

Sunrise = LST - H, Sunset = LST + H. Worked example for Berlin (52.52° N, 13.40° E) yields:

3.2 LOGO! Soft Comfort Implementation

The implementation pattern uses the LOGO! 8 weekly timer and yearly timer blocks. Although a single yearly timer accepts a maximum of 16 date/time pairs, three yearly timers cover the 32 transitions of a typical year when combined with on/off-pulse variants. The minimal block diagram is:

1. Set "Yearly Timer 1" with the 8 day-of-year entries bracketing the spring-summer half (Feb 10 → Jun 5 sunrise). One output Q1 = "Sun is up".
2. Set "Yearly Timer 2" with the 8 entries for the summer-fall half (Jun 5 → Oct 28). This avoids midnight-crossing overlap with Timer 1.
3. Set "Yearly Timer 3" for the off-season entries (Oct 28 → Feb 10) where the day length is < 9 h.
4. OR-combine the three Q outputs into Q_SunUp.
5. Optional: add an "Off-delay" block (parameter = 20 min) so the lights remain ON for a buffer after sunset.

// LOGO! FBD pseudocode
[Yearly Timer 1: Feb10_on..Jun05_off] --Q1--\
[Yearly Timer 2: Jun05_on..Oct28_off] --Q2---+--[OR]--- Q_SunUp --[Off-delay 20 min] --Q1_output
[Yearly Timer 3: Oct28_on..Feb10_off] --Q3--/

3.3 Day-of-Year Resolution

If sub-10-minute accuracy is required, replace the three Yearly Timers with a free-running Shift Register holding 365 sunrise minutes and 365 sunset minutes. Generate the index from the RTC via an arithmetic block:

day_of_year = (month_code[1..12] lookup) + day - 1

Use the LOGO! Analog Multiplexer block to select today's pair, then compare against the RTC time-of-day with two Analog Threshold Triggers (one rising for sunrise, one falling for sunset). This consumes ~25 function blocks, well within 0BA8's 400-block budget.

4. Approach 2 - Simplified Astronomical Formula (On-Device, No Table)

LOGO! 0BA8 supports integer addition, subtraction, multiplication, and division, but not trigonometric functions. A direct on-device formula is therefore not feasible. The only on-device option without trig is a polynomial fit of sunrise/sunset over a 12-month period:

sunrise_minutes_from_midnight ≈ a0 + a1·cos(2π·n/365) + b1·sin(2π·n/365) + a2·cos(4π·n/365) + b2·sin(4π·n/365)

Coefficients a0, a1, b1, a2, b2 are pre-computed in a spreadsheet for the site latitude and hard-coded as LOGO! analog constants. The LOGO! can then implement the sum using five Math Instruction blocks chained together. Accuracy is typically ±25 min over the year - usable for garden lighting, marginal for shutter control.

5. Approach 3 - External KNX/EIB Astro Timer

When a KNX installation already exists, the cleanest approach is to delegate the Astro calculation to a KNX actuator with a built-in Astro function. The Siemens LOGO! 8 CM EIB/KNX (6ED1057-4EA00-0BA1) integrates the LOGO! into the KNX bus. The Theben TR 648 top2 RC or Jung TRDA-2 KNX Astro timers publish a 1-bit sunrise/sunset object daily over KNX.

5.1 Topology

5.2 LOGO! Side Configuration

In LOGO! Soft Comfort, open the EIB/KNX editor and bind:

• Network input NI 1 ← KNX group 1/0/15 (1-bit, sun below horizon)
• Network input NI 2 ← KNX group 1/0/16 (1-bit, sun above horizon)

The FBD reduces to a single OR block feeding the lighting relay. This is the most compact approach (~5 function blocks) and the most accurate (±1 min, since the KNX timer does the trig).

6. Approach 4 - Photovoltaic Sensor Tracking (Closed-Loop, No Almanac)

For installations that need a truly self-calibrating solution and where pointing accuracy matters more than civil-twilight accuracy (e.g. solar panel positioning, greenhouse louvre control), two photovoltaic cells can be wired in a differential configuration. The LOGO! reads the differential and drives a pair of motor outputs. The advantage: no almanac, no latitude data, automatically follows weather and actual cloud cover. The disadvantage: it tracks the brightest source, not necessarily the sun.

6.1 Sensor Circuit

6.2 LOGO! Soft Comfort FBD

The corresponding LOGO! blocks:

1. Two Analog Amplifier blocks scale AI0 and AI1 to 0-1000 (raw mV / 10).
2. One Math Instruction block computes Δ = AI0 - AI1.
3. Two Analog Threshold Triggers detect Δ > +0.2 V and Δ < -0.2 V.
4. An RS-flip-flop holds the drive direction; an off-delay of 60 s prevents hunting.
5. Q1 → motor contactor east, Q2 → motor contactor west.

7. Cross-Platform Comparison

8. Commissioning Procedure (Approach 1, Almanac)

8.1 Prerequisites

• LOGO! Soft Comfort V8.2 or newer (V9.0 for 0BA9/0BA10)
• Ethernet or micro-SD connection to the LOGO! 8 BM
• Site coordinates (decimal degrees, WGS84)
• Reference year (2025) NOAA sunrise/sunset CSV export for the site

8.2 Step-by-Step

1. Set the LOGO! clock. On the LOGO! HMI, navigate to Settings → Date/Time, set the timezone, and enable DST auto-adjustment. This is the single most common commissioning mistake - an incorrectly set timezone will shift sunrise/sunset by exactly 1 hour.
2. Open LOGO! Soft Comfort and create a new program. From the toolbar, drag three Yearly Timer blocks (Y1, Y2, Y3) and one Off-Delay block (T1) onto the worksheet.
3. Configure Y1 with the day-of-year on/off pairs for the spring-summer half. Use the table editor in LOGO! Soft Comfort: click the block, select Yearly Timer, and add rows. Format: MM/DD hh:mm for the ON, MM/DD hh:mm for the OFF.
4. Configure Y2 and Y3 for the remaining months. Verify that the intervals are disjoint - if a single minute is covered by two timers, the OR will still produce the correct output, but the project becomes harder to audit.
5. Wire Y1, Y2, Y3 outputs to a 3-input OR gate, then to the Off-Delay T1 with parameter Th = 00:20. T1 output drives Q1.
6. Compile with F5 and check the resource count. The Info dialog should show < 20 blocks used.
7. Download the program over Ethernet. The LOGO! 8 web server can also accept .lsc uploads - log in as admin, go to Toolbox → Upload.
8. Force the output via Online → Force Q1 = 1 for 5 s to confirm the wiring reaches the load.

8.3 Verification Tests

9. Memory and Cycle Time Considerations

LOGO! 0BA8 executes a fixed scan cycle; for the almanac approach the cycle time is unaffected because the Yearly Timer is event-driven. For the shift-register approach, an additional Analog Multiplexer adds ~2 ms per scan. On a 0BA8 with 25 function blocks, the typical cycle is 8-15 ms, well within the 50 ms budget for lighting control.

Memory footprint:

10. Troubleshooting Matrix

11. Best-Practice Checklist

• Always pre-compute the almanac with a known-good spreadsheet (e.g. NOAA Solar Calculator, suncalc.js) and paste the result into LOGO! Soft Comfort - never trust an on-the-fly trig approximation on the LOGO!.
• Define a single "Astro_SunUp" flag and use it as an enable for downstream logic, not as a direct relay drive. This makes overrides (holiday, maintenance) easy to implement.
• Add a 20-min off-delay buffer to absorb sub-minute cloud cover at sunrise/sunset - this matches the original requirement of ±20 min accuracy for garden lighting.
• Configure DST in the LOGO! menu, not in the program. The RTC hardware handles the transition; user programs should treat clock time as opaque.
• Document the latitude, longitude, and time-zone on the inside of the cabinet door. A future commissioning engineer will need them to update the almanac next year.
• For a multi-year deployment, consider switching to a 0BA9 with a CR2032 battery - the super-cap on 0BA8 is rated for ~480 hours, and a long winter power outage will lose the clock.

12. Frequently Asked Questions