The thermometer measures the temperature between -55°C and +125°C. It was built exclusively with THT components and a ready-made waterproof temperature sensor.
• Temperature range -55°C…+125°C.
• LED display update every 2 seconds.
• Waterproof sensor based on DS18B20.
• Does not require adjustment or calibration.
• Power supply 7-15V DC/0.3A.
• PCB dimensions: display: 59×21mm; motherboard: 38×37mm.
• Electronic Kit – soldering required.
Schematic of the thermometer with LED display is shown in Figure 1. The device should be supplied with DC voltage within range from 7 to 15V connected to the POWER connector. It can be any DC power supply unit with 0.2A load current or more. Diode D1 protects device from faulty input polarity. Input voltage is applied to voltage stabilizer U1. Microcontroller U2 (ATtiny2313) is responsible for all functionality of the thermometer.
DS1820, DS18B20 or DS18S20 is used as temperature sensor. Individual sensors differ in their resolution and software driver.
The microcontroller software automatically detects the type of connected sensor after switching power on. The result of temperature measurement is displayed on the 4 digit LED display. Bipolar transistors T1-T4 are supplying anodes of the LED digits while cathodes are directly driven from microcontrollers outputs via limit resistors R6-R13. The first position of the LED display shows a minus sign, if the measured temperature is less than 0. The maximum resolution of the temperature measurement is 0.1°C. The measurement result is updated every 2 seconds.
Assembly and test
The assembly is typical and should not cause the problems. It runs in standard way starting from the smallest components and ending with the largest ones. After assembling both PCBs should be connected together with the angled goldpins.
The next step is to attach the temperature sensor. For this purpose use a screw connector mounted on the PCB: the white sensor cable is connected to the 1W pin the blue and red cables are shorted and connected to the pin marked GND.
R1–R5, R14: 4,7kΩ
C1, C2: 100nF
C3, C4: 100µF
U2: ATtiny2313 with 20-pin IC socket
Goldpin connector 14pin
VCC: 2-pin terminal block connector
1W: 2-pin terminal block connector