Car Ice ararm circuit

The circuit utilizes a thermistor and three sections of a LM3900 quad operational amplifier (op amp) integrated circuit. When the temperature decreases to 36°F, an LED indicator flashes approximately once per second. As the temperature continues to drop to 32°F, the flashing rate increases, and the LED remains continuously lit. Amplifier I compares the resistance of the thermistor to that of a standard resistor network connected to its non-inverting input. Its output, which is fed to the non-inverting input of op amp III, varies with temperature changes. Additionally, op amp II functions as a free-running multivibrator, generating a pulse signal of about 1 Hz that is directed to the inverting input of op amp III. This configuration allows op amp III to compare the outputs of op amps I and II, activating the LED when the multivibrator's output level falls below that of op amp I. Calibration of the monitor is achieved by immersing the thermistor in a mixture of crushed ice and water while adjusting a 20 kΩ potentiometer to ensure that the LED remains lit.

The circuit operates based on the principle of temperature sensing through the thermistor, which exhibits a change in resistance with temperature variations. The LM3900 quad op amp IC is utilized for its versatility, allowing for multiple configurations within a single package. The first op amp (op amp I) serves as a comparator, measuring the thermistor's resistance against a known reference resistance. This comparator output is crucial for determining the temperature threshold at which the LED indicator will respond.

The second op amp (op amp II) is configured as a free-running multivibrator, generating a square wave signal at a frequency of approximately 1 Hz. This signal is essential for providing a time-based reference to op amp III, which compares the output from op amp I with the periodic signal from op amp II. When the temperature drops sufficiently, causing the output of op amp I to fall below the level of the multivibrator's output, op amp III activates the LED, signaling a critical temperature condition.

The calibration process is vital for ensuring accuracy in temperature detection. By placing the thermistor in a controlled environment of crushed ice and water, the resistance can be accurately measured and adjusted using the 20 kΩ potentiometer. This adjustment ensures that the LED indicator functions correctly at the desired temperature thresholds, providing a reliable visual indication of temperature changes.

Overall, this circuit design exemplifies an effective application of operational amplifiers for temperature monitoring, combining analog signal processing with straightforward calibration techniques to deliver a functional and user-friendly temperature indicator.The circuit uses a thermistor and three sections of a LM3900 quad op amp IC. When the temperature drops to 36°F the LED indicator flashes about once each second. The flashing rate increases as temperature drops to 32°F when the LED remains on. Amplifier I compares the thermistor's resistance to the resistance of the standard network connected to its noninverting input. Its output—fed to the noninverting input of op amp III—varies with temperature. Op amp is a free-running multivibrator feeding a pulse signal of about 1 Hz to the inverting input of op amp III. This amplifier compares the outputs of op amps I and II and turns on the LED when the multivibrator's output level drops below op amp I.

The monitor is calibrated by placing the thermistor in a mixture of crushed ice and water and adjusting the 20 kfi pot so the LED stays on. 🔗 External reference