Digital Remote Thermometer

Posted in Misc power, Temperature sensor, Tools and measuring, on 2015-08-22

Transmitter circuit diagram:

Digital Remote Thermometer

Transmitter parts:

  • R1,R3________100K 1/4W Resistors
  • R2___________47R 1/4W Resistor
  • R4____________5K 1/2W Trimmer Cermet
  • R5___________12K 1/4W Resistor
  • R6___________10K 1/4W Resistor
  • R7____________6K8 1/4W Resistor
  • R8,R9_________1K 1/4W Resistors
  • C1___________220nF 63V Polyester Capacitor
  • C2____________10nF 63V Polyester Capacitor
  • C3_____________1µF 63V Polyester Capacitor
  • C4,C6__________1nF 63V Polyester Capacitors
  • C5_____________2n2 63V Polyester Capacitor
  • C7,C8_________47nF 400V Polyester Capacitors
  • C9__________1000µF 25V Electrolytic Capacitor
  • D1__________1N4148 75V 150mA Diode
  • D2,D3_______1N4002 100V 1A Diodes
  • D4____________5mm. Red LED
  • IC1___________LM35 Linear temperature sensor IC
  • IC2__________LM331 Voltage-frequency converter IC
  • IC3__________78L06 6V 100mA Voltage regulator IC
  • Q1___________BC238 25V 100mA NPN Transistor
  • Q2___________BD139 80V 1.5A NPN Transistor
  • L1___________Primary (Connected to Q2 Collector): 100 turns Secondary: 10 turns Wire diameter: O.2mm. enameled Plastic former with ferrite core. Outer diameter: 4mm.
  • T1___________220V Primary, 12+12V Secondary 3VA Mains transformer
  • PL1__________Male Mains plug & cable

Receiver circuit diagram:

Digital Remote Thermometer

Receiver Parts:

  • R1__________100K 1/4W Resistor
  • R2____________1K 1/4W Resistor
  • R3,R4,R6-R8__12K 1/4W Resistors
  • R5___________47K 1/4W Resistor
  • R9-R15______470R 1/4W Resistors
  • R16_________680R 1/4W Resistor
  • C1,C2_________47nF 400V Polyester Capacitors
  • C3,C7__________1nF 63V Polyester Capacitors
  • C4____________10nF 63V Polyester Capacitor
  • C5,C6,C10____220nF 63V Polyester Capacitors
  • C8__________1000µF 25V Electrolytic Capacitor
  • C9___________100pF 63V Ceramic Capacitor
  • D1,D2,D5____1N4148 75V 150mA Diodes
  • D4,D4_______1N4002 100V 1A Diodes
  • D6-D8_______Common-cathode 7-segment LED mini-displays
  • IC1__________4093 Quad 2 input Schmitt NAND Gate IC
  • IC2__________4518 Dual BCD Up-Counter IC
  • IC3__________78L12 12V 100mA Voltage regulator IC
  • IC4__________4017 Decade Counter with 10 decoded outputs IC
  • IC5__________4553 Three-digit BCD Counter IC
  • IC6__________4511 BCD-to-7-Segment Latch/Decoder/Driver IC
  • Q1___________BC239C 25V 100mA NPN Transistor
  • Q2-Q4________BC327 45V 800mA PNP Transistors
  • L1___________Primary (Connected to C1 & C2): 10 turns Secondary: 100 turns Wire diameter: O.2mm. enameled Plastic former with ferrite core. Outer diameter: 4mm.
  • T1___________220V Primary, 12+12V Secondary 3VA Mains transformer
  • PL1__________Male Mains plug & cable

Device purpose:

This circuit is intended for precision centigrade temperature measurement, with a transmitter section converting to frequency the sensor's output voltage, which is proportional to the measured temperature. The output frequency bursts are conveyed into the mains supply cables.
The receiver section counts the bursts coming from mains supply and shows the counting on three 7-segment LED displays. The least significant digit displays tenths of degree and then a 00.0 to 99.9 °C range is obtained.
Transmitter-receiver distance can reach hundred meters, provided both units are connected to the mains supply within the control of the same light-meter.

Transmitter circuit operation:

IC1 is a precision centigrade temperature sensor with a linear output of 10mV/°C driving IC2, a voltage-frequency converter. At its output pin (3), an input of 10mV is converted to 100Hz frequency pulses. Thus, for example, a temperature of 20°C is converted by IC1 to 200mV and then by IC2 to 2KHz. Q1 is the driver of the power output transistor Q2, coupled to the mains supply by L1 and C7, C8.

Receiver circuit operation:

The frequency pulses coming from mains supply and safely insulated by C1, C2 & L1 are amplified by Q1; diodes D1 and D2 limiting peaks at its input. Pulses are filtered by C5, squared by IC1B, divided by 10 in IC2B and sent for the final count to the clock input of IC5.
IC4 is the time-base generator: it provides reset pulses for IC1B and IC5 and enables latches and gate-time of IC5 at 1Hz frequency. It is driven by a 5Hz square wave obtained from 50Hz mains frequency picked-up from T1 secondary, squared by IC1C and divided by 10 in IC2A.
IC5 drives the displays' cathodes via Q2, Q3 & Q4 at a multiplexing rate frequency fixed by C7. It drives also the 3 displays' paralleled anodes via the BCD-to-7 segment decoder IC6.

Summing up, input pulses from mains supply at, say, 2KHz frequency, are divided by 10 and displayed as 20.0°C.


  • D6 is the Most Significant Digit and D8 is the Least Significant Digit.
  • R16 is connected to the Dot anode of D7 to illuminate permanently the decimal point.
  • Set the ferrite cores of both inductors for maximum output (best measured with an oscilloscope, but not critical).
  • Set trimmer R4 in the transmitter to obtain a frequency of 5KHz at pin 3 of IC2 with an input of 0.5Vcc at pin 7 (a digital frequency meter is required).
  • More simple setup: place a thermometer close to IC1 sensor, then set R4 to obtain the same reading of the thermometer in the receiver's display.
  • Keep the sensor (IC1) well away from heating sources (e.g. Mains Transformer T1).
  • Linearity is very good.
  • Warning! Both circuits are connected to 230Vac mains, then some parts in the circuit boards are subjected to lethal potential! Avoid touching the circuits when plugged and enclose them in plastic boxes.

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