ESP32 S3 UNO - GUVA-S12SD UV Sensor

Hardware Preparation

Or you can buy the following kits:

Overview of GUVA-S12SD UV Sensor

The DIYables GUVA-S12SD module comes two to a pack, with the header pins already soldered. Each one is a small purple PCB of about 11mm x 27mm, with a mounting hole on each end so you can screw it into a weatherproof box.

Look for the tiny white square on the board. That is the GUVA-S12SD photodiode, the part that reacts to ultraviolet light. When UV-A or UV-B light lands on it, it lets through a very small current, far too small for any ADC to read. Beside it, a VLM358 op-amp chip makes that current bigger and hands it to the SIG pin as a voltage. The ESP32 S3 Uno only has to measure that voltage on an analog pin.

The header has three pins. Read them from the print on the board: SIG, GND, VCC.

• SIG: the analog output. It rests near 0V when there is no UV and climbs to about 1V in strong summer sun. Wire it to A0 (GPIO2).
• GND: ground. Wire it to any GND pin of the ESP32 S3 Uno.
• VCC: power in. The module runs on 2.5V to 5V DC. On this board, wire it to the 3.3V pin.

From SIG voltage to UV index

Most makers use one simple rule with this module: UV index ≈ SIG voltage ÷ 0.1. Said another way, each 0.1V on SIG is roughly one UV index point, so 0.5V means about UV index 5. The rule is good enough for a hobby project or a "put on sunscreen" reminder. It is not a calibrated meter, so do not use it for medical or safety decisions.

These are the five UV index levels used by the World Health Organization:

ESP32 S3 Uno Pinout

The picture below shows the pinout of the ESP32 S3 Uno form board. You need the 3.3V pin, a GND pin and the A0 header. A0 is GPIO2 on the chip, one of the ADC1 inputs, so it keeps working even when WiFi is on.

Wiring Diagram

The module plugs straight onto three jumper wires. There is no resistor to add and no breadboard to set up.

This image is created using Fritzing. Click to enlarge image

Why A0? On the ESP32-S3 chip, the ADC2 pins stop working while WiFi is busy. GPIO2 belongs to ADC1, so you can add a web page or cloud upload to this project later without moving the wire.

How To Program For GUVA-S12SD UV Sensor

The SIG pin gives a plain analog voltage, so no extra library is needed. Three things matter on this board.

1. Write the GPIO number, not the Uno label. A0 on the ESP32 S3 Uno is GPIO2:

1. Open the full input range in setup(). The 11 dB setting lets the pin measure up to about 3.3V:

1. Ask for millivolts instead of doing the math yourself. analogReadMilliVolts() uses the calibration data stored in the chip at the factory, so the result is closer to the true voltage than raw * 3.3 / 4095:

ESP32 S3 Uno Code - Raw ADC Count and SIG Voltage

Start with a check of the hardware. This sketch reads A0 twice in each round: once with analogRead() for the raw 12-bit count (0 to 4095), and once with analogReadMilliVolts() for the calibrated voltage. Both go to the Serial Monitor on one line, a little under once per second.

Detailed Instructions

1. Never used this board before? Install the ESP32 core first with ESP32 S3 Uno - Getting Started.
2. Connect three wires: SIG to A0, GND to GND, VCC to 3.3V.
3. Plug the board into your computer with a USB Type-C cable.
4. Select the board ESP32S3 Dev Module and its COM port in Arduino IDE.
5. Paste the code above into a new sketch.
6. Press Upload and wait for "Done uploading".

1. Open the Serial Monitor and set it to 9600 baud.
2. Read the numbers at your desk first, then take the board (still on the USB cable of a laptop) out into direct sun:

1. Did the indoor lines stay near zero? Good, that is normal. The jump only shows up under real sky. If the value stays near zero outside too, check the SIG wire and make sure the white square faces the sun.

Notice that the raw count and the millivolts are not in a fixed 3.3V / 4095 ratio. That gap is the reason to use analogReadMilliVolts(): each ESP32-S3 chip is a little different, and the stored calibration fixes it.

ESP32 S3 Uno Code - UV Index and Level Name

Now turn the voltage into something useful. This sketch takes 16 quick millivolt readings, averages them so one noisy sample does not jump the result, and divides the average by 0.1V to get the UV index. A small helper function then rounds the index and picks the WHO level name.

Detailed Instructions

1. Keep the same three wires. Nothing changes in the circuit.
2. Upload this sketch the same way as the first one.
3. Open the Serial Monitor at 9600 baud.
4. Walk outside and point the sensor at the open sky. Try the shade of a tree, then full sun. A sunny day near noon gave these lines:

1. Compare the result with the UV index in your weather app for the same hour. If yours is always a bit higher or lower, change the 0.1 in the code a little until the two agree. That is a quick, simple calibration for your own module.

Why does 5.6 say "High"? The level comes from the rounded index, the same way weather reports do it. 5.6 rounds to 6, and 6 is in the High group.

Video Tutorial

Watch the video below to see this ESP32 S3 Uno project step by step.

Additional Knowledge

Glass, clouds and angle

• Window glass stops most UV-B. A sensor behind a window reads much lower than one outside, so it is not a fair test.
• Thin clouds let a lot of UV through. A grey day can still reach Moderate, which is why people get sunburned on cloudy days.
• Angle changes the reading. The sensor sees a wide 130 degree cone, but it reads highest when it faces the sun. For a fixed station, point it straight up at the sky.
• Heat is not UV. A hot, bright lamp in your room can leave the reading at zero. That is correct.

Project ideas

1. Sunscreen alarm: beep a buzzer when the index reaches 3 (Moderate) or more.
2. Balcony UV station: show the index on an LCD and log the highest value of each day.
3. UV lamp checker: see if an old UV curing lamp or UV LED still gives out real UV light.
4. Smart blind: drive a servo motor to close a blind when the index goes over 6.
5. Web UV meter: add the ESP32 S3 Uno - Web Server code and read the UV index on your phone.

Function References

• analogRead()
• Serial.print()
• delay()

FAQ

Why does the sensor read 0 indoors?

It only reacts to light between 240nm and 370nm. Most indoor lamps give almost no light in that band, and window glass blocks a lot of the rest. A reading near 0 inside is normal. Test it outside in the sun, or with a UV flashlight.

Why use analogReadMilliVolts() and not analogRead()?

analogRead() gives a count from 0 to 4095, and turning that into volts with a simple formula can be off by tens of millivolts on the ESP32-S3. On this sensor, 0.1V is a whole UV index point, so that error matters. analogReadMilliVolts() uses the calibration stored in the chip and returns the voltage directly.

Do I still need analogSetAttenuation(ADC_11db)?

Keep it. In strong sun SIG can go above 1V, and the 11 dB setting gives the pin a range up to about 3.3V. It costs nothing and keeps the reading from hitting the top of the scale.

Can I power the module from 5V?

The module accepts 5V, and SIG would still stay around 1V in normal use. But on the ESP32 S3 Uno the safe choice is 3.3V, because the GPIO pins are not 5V tolerant. The sensor works the same at 3.3V.

How accurate is the UV index from this module?

It is a rough guide. The 0.1V-per-point rule is an approximation, and each sensor is a little different. It is fine for reminders, logging and fun projects. Do not use it as a medical or safety device.

Can I use another analog pin?

Yes. A1 (GPIO1), A2 (GPIO7), A3 (GPIO6), A4 (GPIO5) and A5 (GPIO4) are all ADC1 pins too, so any of them works, even with WiFi on. Change UV_SIG_PIN to the matching GPIO number. Avoid the ADC2 pins (for example GPIO15 and GPIO16 on the extra rows) if you plan to use WiFi.

Troubleshooting

Learn More

• ESP32 S3 UNO - Light Sensor
• ESP32 S3 UNO - LDR Module
• ESP32 S3 UNO - Light Sensor Controls LED
• ESP32 S3 UNO - Light Sensor Controls Relay
• ESP32 S3 UNO - Light Sensor Controls Servo Motor