LCD-LED Display

MicroMinimal Thermometer Schematic Circuit Diagram

Micro Thermometer with ATtiny13A: Miniature Temperature Sensing

In this innovative design, the thermometer is labeled “micro” due to its utilization of the ATtiny13 microcontroller and its potential to be constructed as a miniature device when incorporating surface-mount device (SMD) components.

Temperature Measurement with Precision

The temperature measurement process employs the DS18S20 high precision 1-Wire® digital thermometer from Maxim. Within the ATtiny13A microcontroller, a programmed sequence triggers a single temperature conversion. The microcontroller then patiently waits until the conversion is complete before reading and displaying the obtained result. The temperature data is ingeniously conveyed using a two-color LED, which emits red and green blinks corresponding to the measured temperature. For instance, a temperature of 23°C might be represented by 2 red blinks followed by 3 green blinks. The simplicity of this system is enhanced by the readability of the blinks. Each blink lasts around 135 ms, clearly distinguishable due to the subsequent 400 ms pause between blinks.

MicroMinimal Thermometer Schematic Circuit Diagram

The same LED pair is used to display other events, too:

  1. When the temperature is negative (centigrade value), an R-G-R-G sequence with no intermediate pauses stands for the “–” sign (red and green blinks are clearly visible);
  2. 0°C is displayed as a 1 second long sequence of short red and green blinks (red and green light blend together);
  3. A communication error is displayed as a 1- second long red light. As indicated in the circuit diagram, two different two-color (red + green) LED types may be used: 3-terminal (with common cathode) or 2-terminal (with red and green LEDs in antiparallel connection). The ATtiny program is the same for both versions. Since LEDs consume most of the power, choose an appropriate value for R2 to suit your own needs. A 100 Ω resistor results in an 8 mA current flow through the LED that’s switched on.

Efficient LED Display with Low Power Consumption

In this configuration, the LEDs are illuminated with a 25% duty cycle during the display phase. To conserve power, there are 1-second intervals between two display sequences, reducing the average LED consumption to approximately 1.5 mA. Using separate low-current red and green LEDs can significantly lower consumption. Even with standard 20-mA LEDs, the circuit can be powered by a small 3 V Lithium cell for an extended period.

Practical Temperature Range

While the device theoretically measures temperatures from –55°C to +125°C, it is advisable to operate it within the –15°C to +50°C range for practical purposes.

Versatile Sensor Placement

The DS1820 sensor can be detached from the main circuit up to the limit allowed by the 1-wire protocol; a 3-meter (10 ft.) connecting cable has been successfully tested. Proper insulation enables temperature measurement of water or other non-corrosive liquids. However, the primary use of this design is to create a compact, energy-efficient thermometer that is readily available and functional whenever needed.

Modular Display Modes

By closing JP1, the readout operates in ‘modulo 5’ mode: each red LED blink represents 5, while green blinks remain at unity. For instance, a temperature of 23°C is indicated by 4 red and 3 green blinks.

Power Optimization with Sleep Mode

Closing JP2 activates power-down mode after the initial temperature measurement and display, consuming minimal power. To initiate a new measurement, power off the thermometer, wait for 1 to 2 seconds, and power it on again.

Program Customization and Availability

The program, titled ‘EE_micro_T.bas,’ was crafted in BascomAVR for compilation and object code generation. A partial excerpt is presented here, and the complete program is available for free download [1]. For individuals without access to an ATtiny13A programmer or BascomAVR, pre-programmed ICs can be purchased through the designated webpage.

Fahrenheit Temperature Conversion

Readers desiring Fahrenheit temperature readings can adapt the BascomAVR program accordingly to align with their preferred temperature scale.

[1] www.elektor.com/090634

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