This circuit is intended for precision centigrade temperature measurement, with a transmitter section converting to frequency the sensors 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 diagram :

 
Receiver Circuit diagram :


Receiver Parts :

R1 = 100K 1/4W Resistor
R2 = 1K 1/4W Resistor
R3 = 12K 1/4W Resistors
R4 = 12K 1/4W Resistors
R5 = 47K 1/4W Resistor
R6 = 12K 1/4W Resistors
R8 = 12K 1/4W Resistors
R9-R15=470R 1/4W Resistors
R16 = 680R 1/4W Resistor
C1 = 47nF 400V Polyester Capacitors
C2 = 47nF 400V Polyester Capacitors
C3 = 1nF 63V Polyester Capacitors
C4 = 10nF 63V Polyester Capacitor
C7 = 1nF 63V Polyester Capacitors
C5 = 220nF 63V Polyester Capacitors
C6 = 220nF 63V Polyester Capacitors
C8 = 1000µF 25V Electrolytic Capacitor
C9 = 100pF 63V Ceramic Capacitor
C10 = 220nF 63V Polyester Capacitors
D1 = 1N4148 75V 150mA Diodes
D2 = 1N4148 75V 150mA Diodes
D3 = 1N4002 100V 1A Diodes
D4 = 1N4002 100V 1A Diodes
D5 = 1N4148 75V 150mA Diodes
D6 = Common-cathode 7-segment LED mini-displays
D7 = Common-cathode 7-segment LED mini-displays
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 = BC327 45V 800mA PNP Transistors
Q3 = BC327 45V 800mA PNP Transistors
Q4 = BC327 45V 800mA PNP Transistors
PL = Male Mains plug & cable
T1 = 220V Primary, 12+12V Secondary 3VA Mains transformer
L1 = Primary (Connected to C1 & C2): 10 turns
Secondary: 100 turns
Wire diameter: O.2mm. enameled
Plastic 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 receivers 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.
  

The complete circuit diagram is shown in Fig. 1. Heart of the system is lC2, National’s MM5402 clock chip. It is an MOS monolithic large scale integrated circuit.
Its pin configuration is shown in Fig. 2. The supply is derived from the mains through a low-voltage stepdown transformerXl . As shown in the diagram, 9-volt DC supply derived by diode D3 and capacitor Cl is fed to the counting circuit of the clock chip at pin 29 through a lk resistor. A bypass capacitor C5 is used at r input of clock chip for protection from high-frequency transients. The current requirement for the entire clock chip is around 4.5 mA. .  The clock input pin 35 is fed by 60Hz clocked pulses. These are derived by 3.579 MHz quartz crystal oscillator formed with ICI, Nationals MM5369 chip, which is also ia frequency divider. Its output at pin I is 60Hz square wave For trimming the crystal frequency, a trimmer (C4) is also provided which could be used for improving the accuracy of time. For display, Fairchild’s FNDSOO seven-segment light emitting diodes (LEDs), D7 to D10, are used. These are common-cathode LEDs.

Since the clock chip’s segment output is of the order of lO mA, no interfacing is required. The various LED segments are only to be connected to the respective clock outputs. In all, four LED displays are required, and these can be straightaway soldered on to a separate PCB or breadboard. Supply to the display LED is obtained from the 3V tap of transformer Xl.  The pin configuration of FND500 is shown in Fig. 3. The blinking colon is also available in the FND500 display.

It is provided at pin 5 of the display, and may be connected through a 220-ohm current limiting resistor to pin 39 of the clock chip. The a.m. and pm. displays are obtained by using two LED lamps, D5 and D6, and connected to pins I and 40 of clock chip through 220-ohm current limiting resistances. All the U’Eplay functions are obtained by simply connecting push-button switches at various control inputs, and grounding the respective input. In the prototype a self- assembled seven-reed assembly of switches was used. However,though costlier, the miniature calculator keyboard switches made by O/E/N India Ltd are the best for this purpose. Alarm output is available at pin 25 of the clock chip. Since this output is quite low, a current driver is used.

This driver is formed by the transistor Tl and a few supporting components, as shown. Fora gentle alarm sound, tone is generated by lC3, which is an Indian timer IC 555. The tone can be varied by changing the value of capacitor C7. An 8—ohm, 0.5- watt speaker of 6.3 cm dia should produce sufficient sound to be heard across the room. _ The sleep output at pin 27 of the clock chip is similarly amplified and used to drive a relay which can control any audio or electrical equipment. The function of sleep timer is to switch off a radio set, tape recorder, TV etc after a preset time, within the maximum duration of 59 minutes.

Since the clock chip as well as the crystal oscillator-cum- divider IC operate on 9V supply (actual operating range : 7 to ll volt), a battery back—up system can be used to keep the clock running during mains failure. There will be no display of time while the clock is running on this back-up system, but on the resumption of power through AC mains the clock will automatically display the correct time. lf due to any reason supply to the clock chip is cut off, all the digits will flash at lHz to indicate that time displayed is incorrect. 

Download the full project file here.