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Precision Analog Thermal Regulator Circuit #26

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PROJECT PROPOSAL: Precision Analog Thermal Regulator Circuit

PCB Design Idea & Basic Functionality:

1. PCB Design Idea:
The core idea is an open-source, standalone Analog Thermal Regulator PCB designed for closed-loop liquid temperature control (such as pasteurization, fermentation, or laboratory heating baths).

Instead of relying on expensive microcontrollers, digital displays, or code that can freeze, this design uses a purely analog feedback loop built around an LM324 operational amplifier. It is designed as a compact, double-sided (2-layer) through-hole board so that it can be easily manufactured, hand-soldered, and deployed in low-cost or rural settings.

2. Basic Functionality:

Live Temperature Sensing: An external probe (NTC thermistor or RTD) connects to the board, converting liquid temperature into a proportional voltage signal fed to the LM324's inverting input (-).

Adjustable Setpoint Control: An onboard potentiometer forms a simple voltage divider connected to the LM324's non-inverting input (+). Turning the knob changes the reference voltage (V ref), allowing the user to set any desired temperature threshold (e.g., 72∘ C for milk pasteurization or 45 ∘C for yogurt making).

Comparator Action with Hysteresis: The LM324 constantly compares the sensor voltage against the setpoint voltage. A built-in resistor hysteresis loop creates a narrow deadband buffer, preventing the relay from rapidly turning on and off ("chattering") when the temperature fluctuates right at the target boundary.

Transistor Power Switching: When the liquid drops below the setpoint, the op-amp output goes HIGH, triggering a BC547 NPN transistor switch to drive a high-power heating relay.

Inductive Spike Protection: A 1N4007 flyback diode placed across the relay coil suppresses voltage spikes when switching off, protecting the on-board components.

Visual Status: An onboard LED illuminates whenever the heating element is actively powered.