DIY DDT2

DDT2 is based on Atmega328PB and DS18B20+ integrated circuits. All functions are implemented in software, therefore not many passive elements are required. It needs only 5V DC to operate.

BOM

Atmega328PB — U1
DS18B20+ (as temperature probe)
HD44780 compatible 16x2 LCD — DS1
100n capacitor — C1, C2, C3, C4, C5, C6, C7 (mostly as decoupling capacitor)
4,7 µF capacitor — C11
47 µF capacitor — C16
10 µH inductor — L1, L2 (can be omitted)
BC857 PNP — Q1 (LED background switch)
4,7 kΩ resistor — R1, R2, R3, R4
10 kΩ resistor — R5, R6
470 Ω resistor — R7 (LCD's background LED current limiting)
100 kΩ resistor — R13
10 kΩ trim potentiometer — RV1 (LCD contrast)
tactile switch — SW1, SW2

Relay is connected over SCK (B5). Logical zero means relay ON, so another PNP transistor and pull-up resistor for SCK are needed. Do not forget flyback diode for relay coil.

First DS18B20+ is connected to VCC, its data pin to SDA (C4) and its ground pin to MOSI (B3). Second DS18B20+ is connected to VCC, its data pin to SCL (C5) and its ground pin to MISO (B4).

Schematics

differential thermostat microprocessor circuit

Software

FLASH image: diyddt2.hex
EEPROM image: diyddt2.epp

If you're using USBASP programmer, flash with:

avrdude -c usbasp -p m328pb -B 5 \
  -U flash:w:diyddt2.hex \
  -U eeprom:w:diyddt2.epp

No (additional) fuse bits needed.

Troubleshooting

Questions? Drop mail to diy@[this domain].