This week’s EYE on NPI focuses in on Infrared thermal sensing, a very ‘hot’ market right now (ha ha). Amphenol has just released a new low cost IR temperature sensor that is optimized for human temperature reading – the ZTP-148SRC1.
Since one of the symptoms of Covid is fever, chances are you have gone to an office or store and had someone take your temperature using one of these very handy ‘non contact’ temperature sensors. Unlike your classic glass or digital thermometers, these can give an instant reading, and no need to prod in someone’s mouth or armpit. They’re a little more complex, and cost more – but for rapid temperature taking, nothing compares!
These non-contact thermometers all consist of the same setup: an infrared temperature element, analog circuitry, microcontroller to read the analog signals and compute the temperature, LED or LCD display to show the measured temperature. For the last few months, it has been nearly impossible to locate infrared temperature elements – either single or matrix style – they were immediately purchased as the demand for non-contact thermometers and thermal cameras skyrocketed. We were consigned to the quoted 50 week lead times until we saw this NPI on Digi-Key for a very low cost analog IR sensor with over 30K pieces in stock!
The idea behind these sensors is pretty simple. Every object in this world emits infrared energy as it heats up, and the infrared wavelength and magnitude change depending on the heat. The sensors contain a thermopile, a silicon sensor that increases in milli-voltage as more infrared energy is absorbed. Point the sensor at the object and it will measure that IR light, and with a little math and calibration we can back-calculate the temperature of the object. We use the same technique to measure the temperature of distant stars, pretty cool!
These sensors come in a cross-compatible package and configuration, although the packaging size may vary. They’re in a metal tin with 4 leads. There are some more expensive sensors that contain a microcontroller with I2C control, and these pins would be power/ground/data/clock. However, these lower cost sensors are purely analog devices. Two pins are connected to a thermistor that is inside the can. The thermistor is used to compensate and correct for the internal temperature of the IR sensing element. The other two pins are for the thermopile, which is the actual infrared sensing element. The top of the cap is open and has a very thin layer of silicon glass which is transparent to Infrared but will block other light so you’re only measuring what you want.
Thermopiles are very sensitive and the voltage generated per degree is measured in tens of microvolts, so you definitely need a good op-amp for amplifying the voltage precisely to a level that is readable. Amphenol provides a schematic you can follow, and we also found tons of example projects and papers online that use this part that you can follow along. Don’t forget you’ll also want to do 2-point calibration by pointing your sensor at known-temperature items so you can correct for any offset introduced by the op-amp or ADC.
You can pick up as many as your heart desires over at Digi-Key, just search for ZTP-148SRC on digikey.com or visit https://www.digikey.com/