Precision Temp Frequency Meter Schematic

Description

This circuit is designed for precise centigrade temperature measurement, converting a temperature sensor’s output voltage into a frequency, which is then transmitted via mains supply cables. The receiver section counts these frequency bursts to display the temperature on three 7-segment LED displays. The least significant digit shows tenths of a degree Celsius, while the entire range extends from 0.0 to 99.9 °C. The transmitter and receiver distance can reach up to one hundred meters if both units are connected to the mains supply and operate under the control of the same light-meter.

IC1 is a linear temperature sensor IC, the LM35, providing a linear output of 10mV per degree centigrade. This output drives IC2, a voltage-frequency converter, generating 100Hz frequency pulses. For instance, a temperature of 20°C results in an output of 200mV, which IC2 then converts to a 2kHz frequency pulse. Q1 functions as a driver for the power output transistor Q2, coupled to the mains supply through L1 and C7, C8.

Transmitter circuit operation:

The LM35 linear temperature sensor IC (IC1) produces an output voltage proportional to the temperature, with a reading of 10mV per degree Celsius. This voltage is fed into IC2, a voltage-frequency converter that transforms it into 100Hz frequency pulses. Specifically, a temperature of 20°C is converted to 200mV by IC1 and subsequently converted into a 2kHz frequency pulse by IC2. The BC238 transistor (Q1) acts as a driver for the power output transistor Q2, facilitating its connection to the mains supply via L1 and C7, C8.

Circuit diagram:

Transmitter parts:

Receiver circuit operation:

The frequency pulses originating from the mains supply, safely isolated by C1, C2 and L1, are amplified by Q1, which incorporates diodes D1 and D2 to limit the peak voltages at its input. These pulses are then filtered by C5, squared by IC1B, divided by 10 in IC2B and finally transmitted to the clock input of IC5. IC4 functions as a time-base generator, providing reset pulses for IC1B and IC5 and enabling the latching and gate timing of IC5 at a frequency of 1Hz. This time base is driven by a 5Hz square wave, obtained from the 50Hz mains frequency, and then squared by IC1C and divided by 10 in IC2A. IC5 drives the cathodes of the 3 displays (D6, D7 and D8) via Q2, Q3, and Q4, utilizing a multiplexing frequency fixed by C7, and also controls the BCD-to-7-Segment decoder IC6, which facilitates the paralleled activation of the 3 displays. The resulting input pulses, at a frequency of 2KHz, are divided by 10 and displayed as 20.0°C.

Circuit diagram:

Receiver Parts: