A simple Arduino code for Sky Quality Meter (SQM) based on TSL237 for use in dark conditions
Simple Sky Quality Meter (SQM)
DIY : Simple Arduino ESP32 code to compute TSL237 counts, frequency and magnitude
by yapo and jompo (07/2022), SQM for rather low frequencies (i.e. dark conditions)
This simple arduino program for ESP32 (download here)
displays the counts (in the given period), the frequency of
the TSL235/TLS237, and computed magnitude on a ST7789 IPS screen
(display in red color for amateur astronomers ;-)
The wiring scheme can be found here and see the picture here
The TLS 237 is behind a UV-IR cut filter and a LED collimator or lens
We have found information for this project on various forum discussion as
this one,
in project as this one or
that one and
SQM discussion as this web page
About the code :
-
- The magnitude is rather empirical :-). You can manipulate
the MAGNITUDECONST - see below - if you are not happy with this
empirical constant we have set for dark conditions).
-
- The counts vary with the period which is 20 seconds by default (see below for modify the period)
-
- the frequency (in Hz) do not vary with the period
The two following defines can be modified in the very beginning of the arduino code :
#define period 20000000UL // period of measure of TSL237 counts in micro-seconds,
longer is the period, more counts will be done
(i.e. in very dark condition, 20 seconds are ok).
For having more frequent measures, you can lower
to 10 seconds or 5 seconds (i.e. 5 000 000 micro-seconds)
#define MAGNITUDECONST 21 // magnitude constant in the following equation :
Magnitude = MAGNITUDECONST - 2.5*log10(frequency)
This code will work up to ~100KHz on an ESP32 chip,
above that frequency the interrupts saturate the processor.
The only external library needed is for the ST7789 IPS screen, i.e.
the Adafruit_ST7789 library
Digital Pin layout ESP32 and components
-
TLS237 PINS : IMPORTANT: a ceramic capacitor of 1uF must be wired between GND and VDD on the TSL237, the nearest possible of the TSL237 pins.
-
PIN 1 - GND
-
PIN 2 - VDD - 5V
-
PIN 3 - SIGNAL -> wired to pin 26 of the ESP32
-
screen ST7789 display
-
pin 23 -> SDA of ST7789 screen
-
pin 18 -> SCL of ST7789 screen
-
pin 22 -> DC of ST7789 screen (Data Command control pin)
-
pin 21 -> RST of ST7789 screen Reset pin (could connect to RST pin)
-
no CS pin on ST7789 screen
The parts needed for this project :
-
ESP32 chip approx. 4-5 euros
-
ST7789 IPS screen approx. 3-4 euros
-
Light to freq-sensor: TSL237 - approx. 3 euros
-
0.1 uF capacitor (few centimes)
-
20 degree lens - ebay 1 euro (or LED collimator, you can ask yapo if you want one, he needed two, but he has to got 10 :-)
-
UV/IR-cut filter between the lens and the sensor - ebay 1 euros
Powering of the ESP32 can be made by a Dual 18650 Lithium Battery Shield V8/V9 Mobile
Power Expansion Board Module 5V/3A 3V/1A Micro USB For Arduino ESP32 ESP8266 (see picture). This power board costs about 4-5 euros (without the 16850 batteries)
All these components can be found on ebay or aliexpress
The arduino code is here
