May 14, 2026
Description
Set the sundial to 12 o'clock & wait for the sun to come around everyday to alert you when it hits the “noon mark”, or set it to sound-off at any other time for that matter during sunny days. Essentially, this sundial works as a solar alarm clock!
An additional feature to this design is that the user no longer requires to follow a precise visual alignment of the nodus “sunspot” onto the analemma curve to tell the time. This can now be done without any visual confirmation by simply rotating the alidade until the sun is detected onto a sensor to sound an alert, therefore, notifying the user that the alignment is complete & the time reading is accurate.
Compatibility with Previous Heliochronometer Models
This adaptation is compatible with the following 3 heliochronometer designs:
Accurate Heliochronometer Sundial 2025 by 3DMason | Download free STL model | Printables.com
Sundial for the Visually Impaired by 3DMason | Download free STL model | Printables.com
This design modification leverages on the above heliochronometer models so you won't need to reprint all of the parts if you have already constructed one of these sundials. Only the analemma, alidade & nodus are affected + a few extra parts. Hour & minute dials, latitude crescents, mounts, etc. all remain the same.
In fairness, the first & third models listed above; i.e. clock hybrid, is more sturdy & therefore more capable of handling the extra weight imposed by this new design. I would highly recommend using this model over the second 2025 model, but either will work reliably.
Set an audible alert for any preset time;
Uses audio feedback when aligning the sun. No visual alignment of the nodus “sun-spot” onto the analemma plate is required anymore for reading actual time;
100% solar powered, no batteries required;
Alarm will sound off 3 minutes before the preset time, then remain on for 3 minutes past this time;
Time reading accuracy to less than ± 1 minute throughout the entire solar year from any location in the northern hemisphere. For maximum accuracy, the time is read at the midway points between where the alert first turns-on & where it turns-off. In practical terms, however, the accuracy will always be determined by the quality of it’s construction, assembly and precision of its final alignment;
As with previous designs, the alidade is coupled to a planetary gear system consisting of a sun gear & planetary gears traveling around an outer ring gear on the main dial plate;
The time reading is “tuned-in” by turning a knob located under the main hour dial. Rotating the knob moves the entire Alidade, Nodus & Analemma assembly via a series of gears which enables the precise positioning of the sun's shadow from the slits of the analemma vertical plate to the reception sensor situated at the nodus point. Once the audio is at it's maximum volume, the time is then read from the main hour & minute dials;
The main dial crescent has a coarse time reading resolution of 5 minutes;
The Alidade is coupled to a 0 to 60 secondary minute dial with a finer 1 minute reading resolution;
Adjustable Latitude Scale Dial with 1 degree reading resolution;
The main dial crescent, along with the secondary minute dial, displays Local Mean solar time directly by computing corrections to the true, or apparent solar time. It achieves this by utilizing a visual-mechanical computer, also known as an Analemma curve;
It converts Local Mean Time to Standard Local Time via meridian offset dial adjustments with 5 minute resolution;
Time compensation feature to the main dial to adjust for Daylight Savings Times;
4 base options are available; 3 with integrated magnetic compass and levels, and 1 without. For compass/level options, refer to these printable links:
Assembly measures 180x200x200mm (W x L x H).
The heliochronometer prototype was constructed out of ABS, but recommend using PETG or ASA for permanent outdoor installations. Use the included 3mf files to simplify the construction process or to make any print adjustments. Coating the parts with a transparent UV protective paint can also extend the life of your sundial.
In the late 18th and early 19th centuries, sundial cannons were installed in European parks. A small lens focused the sun’s rays onto a pan of gunpowder at solar noon, firing a miniature cannon to sound the hour—an early, albeit seasonal and maintenance-intensive, alarm Wikipedia.
In 2018, Caroline Hermans created an “Alarm Sundial” that mounted a photoresistor sensor on a rotating arm. Although not very accurate & fixed for a given latitude, it worked by having the sundial’s shadow cross the sensor, triggering an Arduino‐driven buzzer. The alarm time was set by moving the sensor around the dial studioforcreativeinquiry.org.
A parallel project at Carnegie Mellon’s Frank-Ratchye Studio likewise used an Arduino and light sensor: positioning the sensor at a chosen hour letting the shadow trip the alarm when that hour arrived.
Unfortunately, both old & modern implementations only work as alarm sundials; i.e. when the sun hits a precise position on the sundial which is preset by the user. They do not facilitate or confirm the readings of the current solar time at anytime during the day without user & visual intervention.
My design goal was to keep the solution as simple as possible & in keeping to a sundial's traditional function, without compromising setting accuracy or ease of use. I therefore opted not to use an Arduino, or any complicated electronics. The design only consists of a simple light sensor which activates a power gate to drive an active piezo element beeper. The circuit is also battery-free, only utilizing a small 52x52mm solar panel as a power source.
As with previous implementations, this design also utilizes an audible alert when the sun is aligned, not just for noon, but for any time, with an preset accuracy of approximately 1 minute. Besides gaining the alarm feature, the alignment of the “sunspot” onto the analemma to determine the actual time is now done without any visual confirmation, relying entirely on audio feedback. This is a feature that I haven't seen in any other sundial designs. As a result, I also plan to adapt this design for use by the visually impaired or the blind in a future release. UPDATE (08-11-25): Refer to: Sundial for the Visually Impaired by 3DMason | Download free STL model | Printables.com
A photo-transistor light sensor is located at the nodus reception point, with the analemma plate positioned at the opposite end of the alidade. Note that this is not a typical configuration as far as most heliochronometers go since the analemma is directly facing the sun instead of the nodus. The alidade, nodus & analemma are completely shielded from any stray light, except for narrow slits which forms the shape of the analemma curve. When properly aligned, sunlight will pass through the slits & reach the nodus sensor, which in turn activates a P-channel MOSFET to deliver power from the solar panel to the beeper. The circuit schematics & parts list are provided below.
Figure 1: Schematic for the the Alarm Sundial Circuit
Ref | Qty | Value / Type | Example Part # / Notes |
|---|---|---|---|
Q1 | 1 | P-channel MOSFET | Microchip LP0701, or similar |
Q2 | 1 | IR-Blocking Silicon Photo-transistor | Excelitas VTT9812FH, VTT9814FH, or similar |
R1 | 1 | 10 kΩ, ¼ W resistor | Gate pull-up & photosensitivity |
BP1 | 1 | Active piezo beeper | PUI AI-2429-TWT-12V-3-R, or similar |
SC1 | 1 | Solar Cell, 5V, 0.3W | P122 Adafruit ETFE Solar Cell, (52 x 52 x 3.1 mm), or similar |
Table 1: Electronics Parts List for the Alarm Sundial Circuit
The VTT9812FH photo-transistor has an IR-blocking feature incorporated in the plastic epoxy package. As such, it gives an excellent response in the visible spectral range of 450 to 700 nm, with a peak spectral response at 585 nm. It is typically used for dusk/dawn switching at low light levels around 0.2 to 1 fc; i.e. 2 to 10 lux. The light sensitivity can also be adjusted by choosing an appropriate value resistor at R1; i.e. 100K very sensitive, 47K medium sensitivity, 10K low sensitivity. For this application, 10K was found to be the optimum value.
The P122 solar cell is a small & durable ETFE solar panel designed for industrial IoT applications. It is IPX7 waterproof rated with a 10+ years UV exposure testing with an efficiency of 21.5%. It is therefore a good choice for this application.
The PUI AI-2429-TWT-12V-3-R beeper has an active driver built-in & operates from 3 to 12VDC. It has a resonant frequency of 2.9kHz with a 95dBA sound pressure level.
Comment: I find this beeper a bit annoying due to it's high frequency & loudness. I only picked it because it was what I had on hand. I would strongly suggest choosing something different with a frequency around 1kHz & a lower sound output, but still able to operated down to 3Vdc.
Note that most of these components can be substituted but may require some modifications to the design.
The power requirement for this circuit is <2 mA @ 5Vdc when beeper is active and 1 µA in standby.
Refer to the file section.
Refer to the instructions provided with the original heliochronometer designs.
Please leave a like or comment—it would mean a lot to me! And if you have any feedback, I’d love to hear it!
License:
Creative Commons — Attribution — Noncommercial — Share Alike
3DMason
