Help With Designing Digital Clock with Temperature Sensor - Page 1
Forum section: electronics / beginners

author: Mr1234kaki
author_rank: Contributor
timestamp: July 21, 2012, 03:54:41 am
content: HI there everyone,I need some helps and advices to design a digital clock with temperature sensor,Im thinking of designing the temperature sensor with thermistor, but i have no idea how it works,can anyone here please explain this,Besides ive found there are several other components to sense temperature such as LM35 and BM945F,what is the differences and how they works?Thank you for your help, cheers!

author: Rerouter
author_rank: Super Contributor
timestamp: July 21, 2012, 04:08:34 am
content: a thermistor radically changes its resistance with temperature, you make it part of a voltage divider and it will give you a voltage corresponding to temperature out of the junction,the lm35 however seems to be internally compensated and removes the need for lookup tables with the trade off of a much much smaller signal, at 10mV per degree C, so at 20 degrees your only looking at a 200mV signal, so you might want to stick an op amp infront of it to boost it to something more reasonable,while the thermistor would be able to hook directly to the adc due to a nice higher voltage signalso here is how i see it, if you want low battery power use the lm35, with an op ampif you want less noise and less components use a themistor,

author: ejeffrey
author_rank: Super Contributor
timestamp: July 21, 2012, 09:17:27 am
content: NTC Thermistors are also not very accurate. They have good sensitivity (at least around room temperature), but each unit needs to be calibrated for its base resistance. They are really superb for temperature stabilization applications where you care more that the temperature is held constant than the exact value. Because they are super cheap and easy to use (high sensitivity means no need for fancy signal conditioning) they work great for crude applications like over-temperature sensors.Most IC sensors work off some variation of measuring the temperature dependence of a Vbe in a transistor. The signal here is smaller, but they have uniform sensitivity over a relatively wide range of temperature. For good resolution you generally need to pair them with a higher resolution ADC or some signal conditioning amplifiers/filters. You can also get digital IC temperature sensors that you can just read off with i2c or 1-wire. These have good resolution and are easy to use, but you pay for convenience.

author: firewalker
author_rank: Super Contributor
timestamp: July 21, 2012, 10:59:08 am
content: I have used NTC thermistor some time in thermostats/thermometers. I compared the results with a fairly accurate lab thermeter and it was out by 0.3~0.8 degrees Celsius (in a range of -10 to 70 degrees). I am using the Steinhart-Hart equation.Alexander.

author: David_AVD
author_rank: Super Contributor
timestamp: July 21, 2012, 11:33:03 am
content: The good ol' DS18S20 type digital temperature sensor is not too hard to get going. If you use an Arduino board there's probably code available to talk to that sensor.

author: FenderBender
author_rank: Super Contributor
timestamp: July 25, 2012, 02:29:58 am
content: lm35 would be very easy to interface with an MCU I'd assume. Never did it but I assume its very minimal.

author: free_electron
author_rank: Super Contributor
timestamp: July 25, 2012, 04:49:55 am
content: We did such an application for an oven range one time.The sensor was a simple 1n4148 diode.We used a simple audio transformer to apply a 600 hz simewave .The chip was directly mains powered. The transformer provided the galvanic isolation required.The thermoprobe was steel encased and could be plugged into a 5mm headphone kack inside the oven. You've probably seen those. Very popularThe forward drop of a diode changes -2mv per degree c ( actually -1.989 and something odd. )This gave a noticable enough change in transformer current to digitze it and show temperature.

author: amyk
author_rank: Super Contributor
timestamp: July 25, 2012, 10:31:19 am
content: Quote from: free_electron on July 25, 2012, 04:49:55 amI thought the diode wouldn't survive temperatures that high, but apparently the 1N4148 can have a maximum junction temperature of 175 - 200C, depending on whose datasheet you look at. I'm surprised it lasts for any length of time though.

author: johnboxall
author_rank: Unknown
timestamp: July 25, 2012, 01:09:58 pm
content: Quote from: David_AVD on July 21, 2012, 11:33:03 amE.g.http://www.freetronics.com/pages/temp-temperature-sensor-module-quickstart-guide

author: SeanB
author_rank: Super Contributor
timestamp: July 25, 2012, 03:03:21 pm
content: 5mm, not too common ( I want one or two myself as otherwise I have to change the sockets to a different one and space is limited) but still an available size.