Electronic-heat-sniffer

Description: The sensing element Q1 is a 2N3904 general-purpose NPN transistor, although any general-purpose NPN unit in a TO-92 style case will suffice. IC1, an LM334, provides Q1 with a constant current that remains stable regardless of temperature variations. An LM324 quad op-amp, designated as IC2, is configured as a high input-impedance differential amplifier (IC2a, IC2b, and IC2c) with a gain of approximately 99. IC2d functions as a voltage comparator. When Q1 detects a change in temperature, the base-to-emitter voltage decreases. This reduction in voltage causes the input to IC2a at pin 3 to deviate from the reference voltage supplied to IC2b at pin 5, which is adjusted by potentiometer R5. The difference between the input and the reference voltage is amplified by IC2c. The amplified output is then sent to IC2d, where it is compared to a control voltage set by potentiometer R13. The setting of R13 establishes the threshold, calibrated to match the ambient temperature. The output of IC2d at pin 14 drives the base of transistor Q2. When the output of IC2d is high, LED1 illuminates, and Q2 activates. With Q2 in the on state, a ground path is provided through the transistor for buzzer PB1. The circuit can be constructed on perforated construction board using point-to-point wiring. All components, except Q1, are mounted on the board, while transistor Q1 is positioned at the tip of the heat-sensing probe.

The circuit described is a temperature sensing and alert system utilizing a combination of transistors and operational amplifiers to achieve precise temperature monitoring. The 2N3904 transistor serves as the primary sensing element, detecting temperature changes through variations in its base-to-emitter voltage. The LM334 integrated circuit ensures that Q1 receives a consistent current, eliminating temperature-induced fluctuations in performance.

The LM324 quad op-amp plays a crucial role in signal amplification and comparison. IC2a, IC2b, and IC2c work together to amplify the small voltage changes detected by Q1, with IC2a acting as the initial amplifier, IC2b providing a stable reference, and IC2c amplifying the difference to a measurable level. The output from IC2c is then processed by IC2d, which acts as a comparator. This configuration allows for a highly sensitive response to temperature changes, essential for applications requiring precise temperature monitoring.

Potentiometers R5 and R13 allow for user calibration of the circuit. R5 adjusts the reference voltage for the differential amplifier, while R13 sets the threshold for temperature detection, enabling the system to be tailored to specific ambient conditions. The output from IC2d directly controls the state of transistor Q2, which in turn activates LED1 and buzzer PB1 to provide visual and audible alerts when the temperature exceeds the predetermined threshold.

The construction of the circuit on a perforated board simplifies the assembly process, allowing for easy modifications and troubleshooting. The strategic placement of Q1 at the tip of the heat-sensing probe ensures accurate temperature readings by placing the sensing element directly in the environment being monitored. Overall, this circuit design is effective for applications requiring reliable temperature sensing and alerting mechanisms.Sensing element Ql is a 2N3904 general-purpose npn transistor, although any general-purpose npn unit in a T0-92 style case will do. IC1, an LM334, supplies Ql with a constant current that is independent of temperature. An LM324 quad op amp, IC2, forms a high input-impedance differential amplifier (IC2a, IC2b, and IC2c) with a gain of about 99.

IC2d is used as a voltage comparator. When Ql senses a rise or fall in temperature, the base-to-emitter voltage decreases. That decrease in voltage causes the input to IC2a at pin 3 to deviate from the reference voltage that"s fed to IC2b at pin 5, which is set by potentiometers R5. The difference between the input and the reference is amplified by IC2c. That amplified voltage is fed to IC2d where it is compared to a control voltage set by potentiometer R13.

The setting of Rl3 determines the threshold and is set at a point that"s equal to the ambient temperature. The output of IC2d at pin 14 is fed to the base of transistor Q2. When the output of IC2d is high, LEDllights and Q2 turns on. With Q2 turned on, a ground path through the transistor is provided for buzzer PBl. The circuit can be built on perforated construction board using point-to-point wiring. All components, except Ql, are mounted on the board. Transistor Ql is mounted at the tip of the heat-sensing probe.