Elektor_Mag_-_January-February_2021

46 January & February 2021 http://www.elektormagazine.com

rest, I now had to do automations inside ESPHome. This programming

and configuring is done in the YAML file of the thermostat’s ESPHome

project (see “Home Automation Made Easy” [3]).

Measure Room Temperature

First, declare the temperature sensor that measures the room tempera-

ture. As the sensor used is a DS18B20 (originally from Dallas, now Maxim

or even Analog Devices) connected to GPIO pin 5, and since ESPHome

has a special Dallas component, this translates to the following entries:

dallas:

  - pin: GPIO5

sensor:

  - platform: dallas

    address: 0x6D00000C24013928

    name: “Measured temperature”

    id: t_room

    filters:

      - offset: 0.0

The first line tells ESPHome to include its Dallas 1-wire communica-

tion module and connect GPIO5 to it. The sensor then is of the dallas

platform. The address is optional. However, if you specify it, it must

be correct; you can get it from the ESPHome log (don’t use mine, it is

unique). Specifying an id (t_room, pun not intended) is required here

because we will need to refer to the sensor from somewhere else inside

I added a potentiometer with a voltage limiting resistor because the Wi-Fi
module cannot handle voltages higher than 1.1 V. I kept the two pushbut-
tons and the three LEDs as they might come in handy at some point.

It was a lucky coincidence that the AC/DC module was just small
enough to fit under the plastic knob support of the potentiometer. The
potentiometer, power switch and mains terminal block were recovered
from the old thermostat (Figure 3). I had to replace the relay by a 5 V
type as the old thermostat’s relay was a 48-V type. I did not manage
to fit the desktop thermostat’s relay, but, lucky me once more, the old
thermostat’s relay was a type from an industry-standard family that
is still available and that exists in 5 V.

Putting it all on a PCB that fitted in the original enclosure required a
lot of measuring, but I succeeded in the end. All the SMT components
including the Wi-Fi module went on the bottom side of the board
(Figure 4), while all the through-hole parts found a place on the top
side. A little bit extra board surface was gained by obstructing a couple
of unused mounting holes of the enclosure. The fitting also implied
cutting away some plastic obstacles inside the enclosure. To route all
the tracks, a flexible approach of recommended isolation standards
was unfortunately unavoidable.

Software
The ESPHome firmware that I had compiled for my first setup also
required rethinking. Instead of simply exposing all the sensors and
actuators of the thermostat and letting Home Assistant take care of the

ESP-12E

GPIO16

GPIO14

GPIO12

GPIO13 GPIO15

GPIO10

MOD1

CH_ PD

GPIO2

GPIO0

GPIO4

GPIO5

GPIO9

REST

CS0MISO MOSISCLK

ADC

VCC GND

RX

TX^2221

16

17

18

19

20

10 11 12 13 14

15

9

1 2 3 4 5 6 7 8

K2

1

2

3

4

5

K1

1 2 3 4 5 6

R4

10k

+3V3

R11

120k

P1

47k

C3

100n

R3

10k

S2 S1 R2

220

LED3

YELLOW

LED1

RED

LED2

GREEN

R10

10k

R9

220

R8

10k

R7

10k

R1

220

R6

1k

DS18B 20

IC2

1

2

R5 3

10k C1

100n

T1

2N7002 S

D

G

RY211006Re1 D1

1N4148

N

L

NC

COM

NO

S3

LM1117-3.3

IC1

C4

1

+3V3

IRM-02-5

AC/DC1

L

N

+5V

200519-001

TXD

RXD

RST

GND

P0

C2

100n

C5

(^100) 6V3
Figure 2: Basically, the new discreetly
connected thermostat is the Elektor
Desktop Thermostat from 2018
with a new power supply. Also, a
potentiometer was added for setting
the target temperature.