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Temperature sensor

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ckj

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Hi all, this is my first post.
I want to make a circuit that measures water temperature, at 70c it will switch a heater on and at 80c it will switch off and i want to run it from a 6v or 12v battery.
Is this possible?????
 
Hi all, this is my first post.
I want to make a circuit that measures water temperature, at 70c it will switch a heater on and at 80c it will switch off and i want to run it from a 6v or 12v battery.
Is this possible?????

Yes, it is possible and very common. There are a few ways to go about it and those you need to decide. You could for example use a PIC (micro-controller) or build with discreet components.

Starting at rthe beginning you need to decide what you would like to use as a temperature sensor? Since you are looking at a range of about 70C to 80C you want a sensor that will work well in that range. Once you have a sensor the sensor will provide an output proportional to temperature. That signal will need signal conditioning to get it to a usable level and conversion.

You also mention water. How much water figures into things as to what heating element you will be using as well as you mentioned 12 VDC. So you need a heater element that will run off 12 VDC that also will deliver enough heat for the quantity of water you have.

There are quite a few threads in these forums that cover temperature measurment and control so you may want to search for a few.

Ron
 
Thanks Ron,
I will be using a 2 million candle power halogen bulb as a heating element and can be used on 6v
Cheers
 
I want to keep the battery as light as possible, i want the battery charge to last as long as possible and the bulb is small so will fit into the housing that i plan to use.
 
OK, if you want to use the bulb as a heating element then candle power is here nor there as there is no way to convert from candle power to watts. That is not necessarily a bad thing but eventually you will need to know how much current the bulb draws.

Back to a sensor. I would suggest a common type J thermocouple. You could also use a PRT so there is a decision to make. There was just a very, very good write up using a PIC in the forums that included a display. Do you have any experience at all in programming and using micro controllers?

Ron
 
Thankyou ron,
The bulb is 55watt. Can you put the J type thermocouple directly into water?
I Have no experience programming or using micro-controllers but will begin research.
I want to do this project as cheap as possible.
 
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I suggest one of the digital temperature sensors. No need to mess around with analogue.

If you go with analogue (for whatever reason, be it a good learning experience for instance), research about the PIC (if your using PIC) analogue to digital converter (ADC). In short an ADC will convert the analogue voltages to a digital value (usually 0 - 256 in binary 0 being 0% and 256 being 100% of the micro controllers supply voltage). With this value you can set your code to test the 'voltage' which will correspond to a temperature (check the thermocouple's datasheet) and so doing once the 'voltage' hits the voltage set in the code you can then make the code switch a relay or transistor on or off!

All the best,
James
 
Thankyou ron,
The bulb is 55watt. Can you put the J type thermocouple directly into water?
I Have no experience programming or using micro-controllers but will begin research.
I want to do this project as cheap as possible.

OK, keeping "cheap as possible" in mind and also keeping simple in mind maybe the best route here is to use a thermistor as a temperature sensor. Also your initial requirement shows a pretty wide temperature range to control. I believe a simple and cheap thermistor sensor would work in that range and provide the uncertainty you want. Later today I'll get a simple circuit posted.

Also in the interest of cheap you could use the thermistor to control the temperature (turn your heater on and off) and buy a cheap digital indicator to display the actual temperature of the water. Digital temperature indicators are cheap anymore. They just don't control.

Later today I should have a basic circuit for you, enough to give you an idea. Also, yes, the right one can be immersed in the water.

Ron
 
Attached is a very basic thermistor circuit that should do pretty much what you want it to do.

However, there are a few caveats to all of this. First and most important is you mention a 55 watt load for your heater (lamp) and you mention 6 Volts as a battery source of power. A 55 watt lamp running at 6 volts will draw 55 watts / 6 volts = 9.17 amps of current. Unless you have a very, very large 6 volt battery which would be very heavy your battery will have a very, very short life. Next, you never mentioned the volume of water or container type or exactly how this is to be configured. If I place a calculated heating element in a cup of water I can likely raise the water temperature. If I place that same element in a lake it won't have any effect. Using a lamp to heat water as you mention likely will be a very inefficient way of going about this.

The attached circuit is a basic on/off controller. The relay it drives would need to be slaved to a much larger relay capable of handling the heater load current. RY 1 needs to have a coil voltage of 5 volts that draws less than 150 mA so transistor Q1 can drive it. Again, the circuit is an overview of a basic on/off heater control.

Ron
 

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Have a look at these two links for a method using a Dallas 1-Wire temperature sensor and your choice of free Basic compilers. The main advantage of the DS18B20 is a calibrated output without the need for any additional signal conditioning. Additionally, the probe shown in these examples (available on ebay for little more than the cost of the sensor) is already waterproof with a 3' long cable.

Amicus Version

Swordfish Version
 
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