Wireless temperature monitor has data-logging capabilities

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You can use a local temperature sensor and an ASK (amplitude-shift-keying) transmitter/receiver pair to design a simple wireless temperature-monitoring system with data-logging capabilities. A microcontroller processes and displays the temperature reading to the user. The microcontroller’s onboard UART (universal asynchronous receiver/transmitter) also allows for data-logging applications.

Local-temperature sensor IC₁ detects the ambient temperature at the device (Figure 1). The output of IC₁ is a square wave with a frequency proportional to temperature in kelvins. ASK transmitter IC₂ modulates the signal onto the carrier frequency of 315 MHz. You measure the output signal’s frequency with a frequency counter. The configured scalar multiplier is 1K/Hz when the TS1 pin connects to ground and the TS0 pin connects to V_DD. This scalar multiplier is configurable with pins TS1 and TS0. ASK receiver IC₃ demodulates the signal at the corresponding carrier frequency (Figure 2).

Comparator IC₄ connects to IC₃’s RSSI (received-signal-strength indicator) with an internal peak detector. The external RC follows the peak power of the received signal and compares it with a predetermined, resistor-voltage-divider-generated voltage level. Lab experiments show that a threshold of approximately 1.57V generates a valid output on the data-out pin without receiving false readings. Adjust this threshold to the proper level for optimal performance. The comparator’s output is low when the received signal is weak or invalid and high when the received signal is adequate.


The microcontroller monitors the signal power through P6.0, a general-purpose input pin. When the input is logic low, the LCD and UART output “no RF” to alert users of possible transmitter issues when the transmitter and receiver are too far apart from each other. The LCD connection follows the design in the IC’s evaluation kit. Using a look-up table in the data segment of the assembly code enables you to preserve the internal mapping of the display’s A through G segments. This preservation ensures that the display enables the correct segments. Using an RS-232 level converter, the UART output sends data to a data-logging device, such as a computer.