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Renesas Electronics Corporation

Low-Cost Thermostat

Overview

Description

Growing demand for affordable and easy‑to‑install indoor climate control is driving the adoption of low‑cost thermostats in residential and light commercial environments. Users expect familiar operation, clear visual feedback, and reliable HVAC control without added complexity or cost.

This thermostat design comprises a low‑power MCU with an integrated segment LCD interface and supports temperature, humidity, and indoor air quality sensing, along with direct relay control. The MCU processes sensor data locally and drives a relay output to directly control HVAC equipment or electrical loads. A cost‑optimized user interface based on push buttons and a segmented LCD displays essential information. Flexible power and sensing options allow developers to prioritize either ultra‑low power consumption or minimum system cost, making the design well-suited for battery‑powered or mains‑powered installations.

System Benefits:

  • Delivers simple and intuitive user operation using push buttons and a clear segmented LCD
  • Supports temperature, humidity, and indoor air quality sensing for effective indoor climate monitoring
  • Enables direct relay control of HVAC systems or electrical loads without additional control circuitry
  • Processes sensor data locally for responsive and reliable thermostat operation
  • Offers flexible design options to optimize for ultra‑low power consumption or minimum system cost

Comparison

Applications

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WebConnector Arrow.142Connector Arrow.128Connector dotted arrowSheet.1Sheet.2Sheet.3Sheet.4Connector ArrowConnector Arrow.9Bridge RectifierSheet.11Sheet.12Sheet.13Sheet.14Sheet.15Sheet.16Sheet.17Sheet.18Sheet.19Sheet.20Sheet.21Sheet.22Sheet.23Sheet.24Sheet.25Sheet.26Connector Arrow.27Connector Arrow.28Connector Line24V24V24V3.3V3.3V3.3VConnector Arrow.333.3V3.3V3.3VBattery 1-CellSheet.38Sheet.39Line 1pt.1552Line 1pt.1583Connector Line.42Connector Line.43Notes1 Can be replaced with two diodes for lower cost 2 Can be rep...1 Can be replaced with two diodes for lower cost2 Can be replaced with bypass for lower cost3 Can be replaced with two load switches and a bistatic relay for lower power or one load switch and a legacy relay for lower costBoost ConverterBoost ConverterBoost ConverterConnected signals arrowOptional Boost, depending on Relay voltageOptional boost, depending on relay voltageOptional boost, depending on relay voltage3.3V3.3V3.3VPush ButtonsPush ButtonsPush ButtonsConnector Arrow.59LCD/LED Segment DisplaySheet.89Sheet.90Sheet.91Sheet.92Sheet.93Sheet.94Sheet.95Sheet.96Sheet.97Sheet.98Sheet.99Sheet.100Sheet.101Sheet.102Sheet.103Sheet.104Sheet.105Sheet.106Sheet.107Sheet.108Sheet.109Sheet.110Sheet.111Sheet.112Sheet.113Sheet.114Sheet.115Sheet.116Sheet.117ONONONOFFOFFOFFSheet.120Sheet.121Sheet.122Sheet.123Sheet.124Sheet.125Sheet.126Sheet.127Connector Arrow.129VSYSVSYSVSYSSegment LCD DisplaySegment LCD DisplaySegment LCD DisplayTemperature & HumidityTemperature & HumidityTemperature & Humidity Bi-directionalConnector Arrow.135 3.3V3.3V3.3VIAQIAQIAQConnected signals arrow.138Connector Arrow.139 3.3V3.3V3.3VVertical Wire Bundle222Sheet.146(Bistatic) Relay(Bistatic)3 Relay(Bistatic)3RelayConnector Arrow.149Horizontal Wire BundleCommunication Lines B.795222Connector Arrow.153VrelayVrelayVrelayHVAC ControlHVAC ControlHVAC ControlWS149WS149WS149Buck RegulatorBuck RegulatorBuck RegulatorBuck-BoostBuck-Boost2Buck-Boost2Load SwitchLoad Switch1Load Switch11-2x Load Switch1-2x3 Load Switch1-2x3Load SwitchSheet.169MCUMCUMCUGPIOGPIOGPIOGPIOGPIOGPIOSLCDCSLCDCSLCDCI2CI2CI2C2x GPIO2x GPIO2x GPIO
Exiting Interactive Block Diagram

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