How to measure a Thermistor wirelessly/contactless? - Page 1
Forum section: electronics / beginners

author: AQUAMAN
author_rank: Regular Contributor
timestamp: October 16, 2019, 04:22:37 pm
content: I need to make a circuit where a Thermistor measures temperature, and this information has to be transferred to the measurement IC without using any wires or any form of physical connection. i.e. it has to be completely isolatedWhat would be the best way to do this?I am thinking about using the Thermistor as the current limiting resistor for an LED, and measuring the LEDs brightnessBut LEDs usually need a certain level of forward current, so the thermistor would have to be selected carefully to avoid excessive self heating (1C or 2C heating is OK though)Would an IR LED be a better choice?I remember when I was about 12 and I made a morse code transmitter using an IR Emitter, so it seems like it would be feasible

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: October 16, 2019, 05:47:21 pm
content: If you have a power supply on the measuring side to power an LED, surely you have a power supply on the measuring side to feed a variety of small active circuits, which control that LED based on a much smaller current through the thermistor. Starting with the comparator-based schematic inhttps://www.eevblog.com/forum/projects/pwm-signal-from-555-timer/msg17855/#msg17855you would replace R5 (or R7) with the thermistor and R7 (or R5) with a fixed resistor of 1-3x the thermistor's nominal value, balancing the range of the output against self-heating. R6 can be substituted with a short circuit for the first iteration, and you can replace it with two fixed resistors to reduce the change in duty cycle per °C in case you need to increase measurement range.On the receiving side, first figure the minimum and maximum voltages of the pseudo-triangle wave at the - input of U1B. You can treat the oscillator as two separate voltage dividers, one with Rtop=(1k+R3)||R1 and Rbottom=R2, the other with Rtop=R3 and Rbottom=(R1||R2). Next, measure the duty cycle of the received signal by your favorite method, then map 0% duty to the minimum voltage and 100% duty to the maximum voltage. Now you know the voltage on the + input as seen by U1B, and you can now solve the R5-R6-R7 voltage divider for Rtherm, from which you can look up or calculate Ttherm by the usual equations/tables.Between the two circuits, you can place optocouplers, discrete LEDs/phototransistors, ultrasonic transmitter/receivers, or carrier pigeons, as available.(Tip: all the important voltages in Zero999's comparator circuit are proportional to Vcc, so Vcc drops out of all the equations and its exact value is irrelevant. For the purpose of processing the received signal, you need only concern yourself with the ratio of any given voltage to Vcc. However, Vcc must not change quickly during operation, as any variations over short-time will be coupled into the transmitter output.)

author: soldar
author_rank: Super Contributor
timestamp: October 16, 2019, 06:05:30 pm
content: You can control the frequency of an oscillator which sends IR or visible pulses or radio pulses or whatever.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2019, 06:12:17 pm
content: IoT temp sensor...What range? Temp range, accuracy? Distance? Can it be battery powered? Can it be plugged in for power? Does it have to be passively powered [by the connection]? Does it need to be analog or digital? Sample rate/bandwidth?Tim

author: AQUAMAN
author_rank: Regular Contributor
timestamp: October 17, 2019, 10:15:54 am
content: Quote from: T3sl4co1l on October 16, 2019, 06:12:17 pmQuote from: soldar on October 16, 2019, 06:05:30 pmQuote from: jhpadjustable on October 16, 2019, 05:47:21 pmSounds good. Thanks for the replies.I have to use the power supply on the measuring side, but the supply is intermittent. I'll be sampling only when the the supply is available. I only need to travel max 10cm with the signal, and the temperature range is 25 - 150CBut I have little space. I need to use 0.6x0.3mm LED and Thermistor. It also needs to be very fast if possible. The 0.6x0.3x0.15mm NTCs also have a fast time response and I need that. The application has very fast temperature swings and an average each second isn't good enough.Since the supply is intermittent I don't think I can use the duty cycle method. As the switching supply will create a duty cycle itself. I think I will have to trigger the sampling of the LED specifically when the supply is ON.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 17, 2019, 12:46:17 pm
content: If it needs to be fast, might it be better to eyeball it? That is, use temp sensitive paint and a spectrophotometer? (Which might be realized with a white light, lens, diffraction grating and line array CCD/camera.)Tim

author: AQUAMAN
author_rank: Regular Contributor
timestamp: October 17, 2019, 01:09:51 pm
content: Quote from: T3sl4co1l on October 17, 2019, 12:46:17 pmI do use uniform emmissivity black paint + an IR camera sometimes. But this requires direct line of sight to the surface, and it's also really expensive.