explain signal analysis

3 files changed, 25 insertions, 2 deletions

diff --git a/additions/HW/thermostat/waveforms-large.png b/additions/HW/thermostat/waveforms-large.png
new file mode 100644
index 0000000..636dd13
--- /dev/null
+++ b/additions/HW/thermostat/waveforms-large.png
diff --git a/additions/HW/thermostat/waveforms-small.png b/additions/HW/thermostat/waveforms-small.png
new file mode 100644
index 0000000..d8b9239
--- /dev/null
+++ b/additions/HW/thermostat/waveforms-small.png
diff --git a/src/HW/thermostat/document.en.rest.txt b/src/HW/thermostat/document.en.rest.txt
index d3c282d..9ea1bcf 100644
--- a/src/HW/thermostat/document.en.rest.txt
+++ b/src/HW/thermostat/document.en.rest.txt
@@ -26,6 +26,8 @@ The current candidate is a ESP32_ running off (probably alkaline)
 batteries, using a DHT11_ sensor.
 
 .. figure:: thermometer-diagram.png
+   :alt: diagram showing a ESP32 microcontroller wired up to a DHT11
+         sensor
    :align: center
 
    Connections:
@@ -58,6 +60,8 @@ Harper`_ and probably others, I connected a 433MHz receiver to my
 soundcard:
 
 .. figure:: rf-receiver-diagram.png
+   :alt: diagram showing a 433MHz receiver wired up to a stereo audio
+         cable via resistors
    :align: center
 
    Connections:
@@ -73,12 +77,31 @@ soundcard:
 
 By turning the knob the thermostat that was installed with the boiler,
 I generated the on and off signals. I recorded them with Audacity_,
-and spent a few hours puzzling them out (I'll write more details about
-this part at some future time).
+and spent a few hours puzzling them out. This is what the recorded
+waveforms look like:
+
+.. figure:: waveforms-small.png
+   :alt: two similar but not identical saw-toothed waveforms, one
+         above the other
+   :target: waveforms-large.png
+   :align: center
+
+   The two signal trains (one for "on" and one for "off")
+
+It took a while to realise that those saw-toothed waves were actually
+square waves, mangled by all the filters that my soundcard has, and
+that the heights were different only because of those same filters.
+
+Measuring the number of samples between rising edge and falling edge
+gave me the timing. Then I had to do the whole thing again when I
+noticed that the "low" widths where not always equal to the preceding
+"high" width: it's not a 50/50 duty cycle square wave.
 
 Then I built a transmitter:
 
 .. figure:: rf-trasmitter-diagram.png
+   :alt: diagram showing an Arduino UNO wired up to a 433MHz
+         transmitter
    :align: center
 
    Connections: