Thermostat project

Projects General Guidance
1

Good evening,

I am interested in building a home thermostat to control my conventional HVAC system. I wrote up some very brief plans below. My question is electronic, can you use transistors to bridge the 24VAC to appropriate lines, or will relays need to be used? I have many transistors and few relays on hand. My will to build this stems from an autistic child that will mess with a conventional panel and wish to have more control - even remote control (via a simple WWW server possibly).

components
 RTC – Real Time Clock Module
 WiFi card - ESP 8266
 16x2 LCD Display
 Capacitive touch board
 Temp / Humidity Sensor
 Four LEDs: Red, Orange, Yellow, Blue
 HVAC Interface
 24VAC Operation! Is it possible to avoid use of relays?
 Red – 24VAC
 Yellow – Cooling
 White – Heat
 Green - Fan
 Display
 LEDs
 Red: System Fault / Unlocked (slow flashing)
 Orange: Heating / Heating Standby (slow flashing).
 Yellow: Fan on. / Fan with H/C (slow flashing). / Fan Only (fast flashing).
 Blue: Cooling / Cooling Standby (slow flashing).
 Default Display
 TIME Set / Amb.
 22:30 70 / 72
 Menu Display
 * Schedule >
 Lock Settings >
 Function
 Unlocking
 Simultaneously touch both unlock areas.
 This is to prevent misuse by kids.
 The red LED flashes
 screen says “unlocked”.
 Menu
 Schedule
• Set time blocks
 Lock Settings
• Unlocked period
• Lock active / inactive
 Time Settings
• Set time
• Set timeout for fan mode changes
• Set timeout for temp setting changes
 Hysteresis
• Cool at x degrees above
• Heat at y degrees below
 Fan Modes
 On all time
 On with H/C (default)
 On with no H/C
 Buttons
 Capacitive Touch
 Unlock / Fan Mode
• In menu moves back
 Unlock / On/Off
• In menu moves forward
 Menu
 Up
 Down

2

Start simple and get familiar with all those components first, before you start to attach to and manipulate your HVAC.

I cannot answer your electronic questions due to the lack of a circuit diagram and/or data sheets. Dunno how 24V AC is related to your project.

3

The short answer is that normal transistors cannot switch AC. There is a type of transistor called a triac which can do this, but you don't often see them used in Arduino projects. Relays may be the simpler answer for you.

4

SSRs are triacs with opto couplers for electric insulation. Unlike relays they only can switch AC, not DC.

5

You may not need the RTC. With the ESP chip, you can easily get the time from a Network Time Server, at startup and then once per day. The ESP's internal clock can keep time accurately enough over a day.

6

PaulRB:
You may not need the RTC. With the ESP chip, you can easily get the time from a Network Time Server, at startup and then once per day. The ESP's internal clock can keep time accurately enough over a day.

Thanks, that saves a few bucks.