Handheld infrared forehead temperature gun solution

Handheld infrared forehead temperature gun can achieve non-contact close-range measurement of human body temperature. In special cases, in order to avoid direct contact with human body and prevent cross infection, handheld infrared forehead temperature gun is needed in many places. The provided handheld infrared forehead temperature gun solution is fast and accurate, and the measurement range is wider.

1. The principle of infrared forehead temperature gun

Infrared forehead temperature gun, also known as infrared thermometer, can accurately measure the temperature of the human body within 1 second. When in use, just point the probe to the forehead and press the button. The working principle is actually very simple: when the temperature of the object is higher than "absolute zero", that is -273 ℃, the object will radiate infrared rays outward. Using infrared probes to obtain the infrared light intensity of different objects, the temperature data can be displayed on the liquid crystal display through the amplification circuit and the analog-to-digital conversion circuit.

Second, the infrared block diagram

In order to expand the temperature measurement range, this circuit is powered by a 9V battery. The voltage of the battery is converted to 3V through a high-voltage LDO RS3002 for the entire system, including power supply for microcontrollers, backlights, op amps, etc. The consumption of LDO RS3236 is reduced from 3V to 1.5V. The purpose of this is to provide a reference voltage for the thermopile, so that the entire measurement temperature range can be expanded. In order to calculate correctly at different ambient temperatures, the microcontroller must also read the current ambient temperature, which is calculated by reading the voltage point of the NTC resistance inside the thermopile and the fixed 100kΩ partial pressure.

Because the output resistance of the thermopile is high, the topology of the op amp part should use a non-inverting amplifier. The amplification factor needs to be more than 100 times. A low-pass filter needs to be reserved to reduce the output noise. To maximize the temperature resolution, the op amp is required. The 1/f noise is less than 5uVpp, and the DC offset voltage is less than 10uV. In this circuit, the operational amplifier RS8551 has an ultra-low offset voltage, suitable for DC or low-frequency signal amplification.

3. Detailed chip parameters

The main features of RS8551 are:

◆Wide working voltage range: 2.5V~5.5V;

◆Ultra-low offset voltage: typical value 1uV, maximum value 5uV;

◆Ultra-low temperature drift: typical value 5nV/°C, maximum value 50nV/°C;

◆Ultra low input bias current: 50pA;

◆Unity gain bandwidth: 4.5MHz;

◆Static current: 640uA per channel;

◆0.01Hz~10Hz noise: 0.75uVpp.

High-pressure LDO and low-pressure LDO: RS3002 series and RS3236 series.

The main features of RS3002 are:

◆Wide input voltage range: 2.5V~36V, suitable for common 9V battery input;

◆Continuous output current: 150mA;

◆Ultra-low quiescent current: 2.5uA;

◆Low voltage difference: 10mV/mA;

◆Super high PSRR: 54dB@1KHz;

◆Optional output voltage accuracy: 1% or 2.5%;

◆A variety of fixed output voltages are available, available in SOT-23, SOT-89-3, SOT-23-5 packages.

The main features of RS3236 are:

◆Wide input voltage range: 1.7V~7.5V, support 4 batteries input;

◆Internal maximum output current limit: >500mA;

◆Ultra-low quiescent current: 20uA;

◆Ultra low dropout: 1mV/mA;

◆High PSRR: 70dB@1KHz;

◆Optional output voltage accuracy: 1% or 2.5%;

◆A variety of fixed output voltages are available, available in SOT-23, SOT-89-3, SOT-23-5, UTDFN-1*1-4L packages