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# How to build Digital Remote Thermometer

2015-01-18 17:34
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## Description

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.

## Transmitter parts:

R1 = 100K 1/4W ResistorsR2 = 47R 1/4W ResistorR3 = 100K 1/4W ResistorsR4 = 5K 1/2W Trimmer CermetR5 = 12K 1/4W ResistorR6 = 10K 1/4W ResistorR7 = 6K8 1/4W ResistorR8 = 1K 1/4W ResistorsR9 = 1K 1/4W ResistorsC1 = 220nF 63V Polyester CapacitorC2 = 10nF 63V Polyester CapacitorC3 = 1μF 63V Polyester CapacitorC4 = 1nF 63V Polyester CapacitorsC5 = 2n2 63V Polyester CapacitorC6 = 1nF 63V Polyester CapacitorsC7 = 47nF 400V Polyester CapacitorsC8 = 47nF 400V Polyester CapacitorsC9 = 1000μF 25V Electrolytic CapacitorD1 = 1N4148 75V 150mA DiodeD2 = 1N4002 100V 1A DiodesD3 = 1N4002 100V 1A DiodesD4 = 5mm. Red LEDIC1 = LM35 Linear temperature sensor ICIC2 = LM331 Voltage-frequency converter ICIC3 = 78L06 6V 100mA Voltage regulator ICQ1 = BC238 25V 100mA NPN TransistorQ2 = BD139 80V 1.5A NPN TransistorT1 = 220V Primary, 12 12V Secondary 3VA Mains transformerPL = Male Mains plug & cableL1 = Primary (Connected to Q2 Collector): 100 turnsSecondary: 10 turnsWire diameter: O.2mm. enameledPlastic former with ferrite core. Outer diameter: 4mm.

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.

## Circuit diagram:

R1 = 100K 1/4W ResistorR2 = 1K 1/4W ResistorR3 = 12K 1/4W ResistorsR4 = 12K 1/4W ResistorsR5 = 47K 1/4W ResistorR6 = 12K 1/4W ResistorsR8 = 12K 1/4W ResistorsR9-R15=470R 1/4W ResistorsR16 = 680R 1/4W ResistorC1 = 47nF 400V Polyester CapacitorsC2 = 47nF 400V Polyester CapacitorsC3 = 1nF 63V Polyester CapacitorsC4 = 10nF 63V Polyester CapacitorC7 = 1nF 63V Polyester CapacitorsC5 = 220nF 63V Polyester CapacitorsC6 = 220nF 63V Polyester CapacitorsC8 = 1000μF 25V Electrolytic CapacitorC9 = 100pF 63V Ceramic CapacitorC10 = 220nF 63V Polyester CapacitorsD1 = 1N4148 75V 150mA DiodesD2 = 1N4148 75V 150mA DiodesD3 = 1N4002 100V 1A DiodesD4 = 1N4002 100V 1A DiodesD5 = 1N4148 75V 150mA DiodesD6 = Common-cathode 7-segment LED mini-displaysD7 = Common-cathode 7-segment LED mini-displaysD8 = Common-cathode 7-segment LED mini-displaysIC1 = 4093 Quad 2 input Schmitt NAND Gate ICIC2 = 4518 Dual BCD Up-Counter ICIC3 = 78L12 12V 100mA Voltage regulator ICIC4 = 4017 Decade Counter with 10 decoded outputs ICIC5 = 4553 Three-digit BCD Counter ICIC6 = 4511 BCD-to-7-Segment Latch/Decoder/Driver ICQ1 = BC239C 25V 100mA NPN TransistorQ2 = BC327 45V 800mA PNP TransistorsQ3 = BC327 45V 800mA PNP TransistorsQ4 = BC327 45V 800mA PNP TransistorsPL = Male Mains plug & cableT1 = 220V Primary, 12 12V Secondary 3VA Mains transformerL1 = Primary (Connected to C1 & C2): 10 turnsSecondary: 100 turnsWire diameter: O.2mm. enameledPlastic former with ferrite core. Outer diameter: 4mm.

## Notes:

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.