water heater with temperature controller

[jemilsan]

My instructor ask me to build a water heater for our school project. The circuit should trigger up to 5 heaters depending on the preset temperature. The idea goes like this....

The first heater will turn on.... if the temperature doesn't reach the required temperature, the second heater will turn on also so that there will be 2 heaters turned on. But if the required temperature was reached by the first heater, the remaining heaters will not turn on.

I've already constructed the AC part of the circuit which will turn the heater on.... my problem is the circuit that will trigger it.

I was told to use a counter, a shift register, a 555 timer, and a thermistor.

Can anyone help me please????

 

[ThermalRunaway]

I would probably use a microcontroller for that kind of thing because it would be easy to modify the software for the purpose of adding features or improving the design etc. However, you could easily complete that project without a microcontroller and it sounds like this would be your best bet.

You'll need some form of temperature sensor for the front end of the project, and a sensor which converts temperature to a voltage would be ideal (there's loads of these on the market). You wouldn't need a linear sensor provided you're willing to add in some form of calibration so that the preset can be setup, although there are linear sensors out there which will convert a temperature, in degrees C or F to a voltage in a linear fashion. You'll want a comparator for the temperature sensor output, which will compare the temperature of the tank with your preset (a bias voltage on the other pin) and then make a decision at the output as to whether the tank is hot enough or not. Convert the output to a logic level so that, for example, not hot enough= logic0 and hot enough=logic1.

A 555 timer could monitor a time period and, via some simple logic control, a circuit could be designed which would check the temperature of the tanks compared to the preset at certain time periods. If, after one time period (the length of time of which would be set by the 555 timer), the tank was not hot enough, a second heater could be turned on. If, after 2 time periods the tank wasn't hot enough then a third heater could be turned on etc.

The tank heater control could simply be a decade counter which is incremented if the tank isn't hot enough, or even a simple binary counter with some decoding logic on the output.

I'm not going to design your project for you, because clearly that's your job - but I hope this has given you a few ideas to work with. I haven't given you a solution, but there are a few ideas there which should allow you to come to a solution yourself.

Let us know how you get on and, once you're a bit further into the design process maybe you can ask some more specific questions about how to tackle certain problems you're having.

Brian

 

[mramos1]

A microcontroller would be best if you are allowed to use them. You have output pins to fire the heaters/blink LEDs/etc, input pin to read the temperature (thermistor), interrupts to time your checks. All in one chip. And as ThermalRunnaway said, you can change the program and add features

If you use a thermistor, look at Nigel's joystick tutorial. The terminstor and a cap and you can do a lot on a microcontroller.

The code will be the fun part.

 

[jemilsan]

I'm not allowed to use a uC, just the parts I mentioned.

I already have the temperature to voltage conversion circuit which uses a thermistor in a voltage divider configuration and will trigger an NPN transistor. The rest of the circuit, as ThermalRunaway says, will be my problem. But I hope that you can guide an electronics neo until the project is finished.

Thanks guys!!!

 

[akg]

ok..a logic .. u haven't specified only about the warming up phase.

ok , along the componnets bu specified u'll need a comparator(using an opamp or build it around 555) to check the preset temp with the thermistor.

A 555 is wired as a timer . During start, one heater element will be on and the counter counts the pulses from 555 osc. when the counter overflows it compares the signal from the comparator(should be low if the preset is not crossed), and shiftes one in the shift register which switches on the next heater element and so on.. Once the preset temp is crossed , the comp: o/p goes high and it inhibits the counter.

 

[ThermalRunaway]

when the counter overflows it compares the signal from the comparator(should be low if the preset is not crossed), and shiftes one in the shift register which switches on the next heater element and so on..

I don't think he could shift a Logic 1 through a shift register in order to control the heaters, because then only one heater would come on at a time. My understanding was that he wanted more and more of the heaters to come on until the preset temperature was reached.

I suppose he could initially fill the Shift Register with all 0's and then clock Logic 1's into it everytime the following statement was true: "If Counter has overflowed AND Temperature is below preset".

Brian

 

[Paul Obrien]

Make the 555 with a looong time interxal triggered by the thermistor, this is fed to the counter ie if burner 1 isn't doing the job burner two starts. The shift register could be used to provide the long time interval from the 555

 

[Paul Obrien]

OK so it goes thermistor - 555 - shift register - counter - burner

 

[akg]

I don't think he could shift a Logic 1 through a shift register in order to control the heaters, because then only one heater would come on at a time. My understanding was that he wanted more and more of the heaters to come on until the preset temperature was reached.

Brian

the data i/p of shift reg is always one(high) (a serial in parallel out) . and if clocked , the u'll get a string of one

 

[Paul Obrien]

so the shift register would be better for the burner selection and use the counter to provide for the delay.
thermistor - 555 - counter - shift register - burner

 

[ThermalRunaway]

the data i/p of shift reg is always one(high) (a serial in parallel out) . and if clocked , the u'll get a string of one

The data input of a shift register is not always logic 1, it can be either 1 or 0. If you clock one Logic 1 into a shift register and then continue to clock, you'll end up with that logic level "shifting" through the register to the right. Unless of course you tie the input to +5V so that it's always at logic 1 - is this what you meant?

Brian

 

[akg]

The data input of a shift register is not always logic 1, it can be either 1 or 0. If you clock one Logic 1 into a shift register and then continue to clock, you'll end up with that logic level "shifting" through the register to the right. Unless of course you tie the input to +5V so that it's always at logic 1 - is this what you meant?

Brian

yes i meant that it to be tied to logic 1

 

[jemilsan]

I can't find a 5-bit counter that will be the input of the shift register. And also does the clock input of the shift register should come from the output of the 555 timer just like what I did?


I've made all the circuit except for the counter and I want to post it for your comments but I can't figure out how.....

 

[Nigel Goodwin]

**broken link removed**

That's so silly! - you've input a graphic link to your own hard drive directory, how is anyone else supposed to see it?.

 

[jemilsan]

That's so silly! - you've input a graphic link to your own hard drive directory, how is anyone else supposed to see it?.

sorry....I've already edited my post.... can you tell me how to post an image?

 

[Nigel Goodwin]

Click on 'Manage attachments' and upload the file to the server.

 

[ThermalRunaway]

I can't find a 5-bit counter that will be the input of the shift register. And also does the clock input of the shift register should come from the output of the 555 timer just like what I did?


I've made all the circuit except for the counter and I want to post it for your comments but I can't figure out how.....

Ok so I assume the rest of your project is working as you'd like, but you haven't decided on how to build a 5-bit counter, right?

Firstly, I don't quite see where the counter fits into your design, and I don't see how the counter would be an input to the shift register either, but to answer the question you actually asked, there are a couple of ways to build a 5-bit counter.
The way I'd do it is to buy an 8-bit counter IC and set it up to reset once it had counted to the number I wanted. In your case, a counter which resets at 31(decimal) 11111(binary) would give you a 5-bit counter.
Another way to do it is to actually make the counter yourself out of D-type flip-flops, then you can build any type of counter you like!

Regarding the Shift register you can clock it off anything you like, provided you're using TTL logic levels and you don't try to clock it faster than the device can handle. In your case, you only want to clock the shift register when the output from your comparator circuit is saying that the water isn't hot enough, AND the 555-timer time_duration has expired. So you're clocking the shift register via the output of an AND gate which has the 555 timer and your comparator circuit as inputs to it.

Brian

 

[ThermalRunaway]

I can recommend the book "Electronic Systems" which I reviewed here.

It has quite a lot about Shift Registers, Flip Flops, Counters, Synchronous and Asynchronous stuff - you'd be in your oils

Brian

 

[mramos1]

That's so silly! - you've input a graphic link to your own hard drive directory, how is anyone else supposed to see it?.

Well if it is Windows. If we get his IP we might have a shot at it

In the post (and you scroll down), there is a "Manage Attachments", in there you can browse to the file, once you find it, click OK, then click upload (that will send it to the forum).

(SORRY, I clicked to a post from early today)

 

[jemilsan]

You mean my design is wrong.....can you pls comment on the attached image..