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Project design & component selection help

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NewGeek

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I am designing a project with an LCD screen that displays inputs from two sensors: temperature and fluid level.
The fluid level sender is a standard automotive 33-240 Ohm variable resistor.
Im trying to decide what to use for the temp sensor. Analog or digital? The only real restriction is that it must be small TO-92 size or smaller, for a remote mount. Are there any inexpesive digital sensors this small?
Should I use a PIC with A2D or just do the A2D in my program? Keep in mind that I will have to learn how to program A2D anyway because of the fluid level sender. Should I just use a thermistor and figure out how to write code? It doesnt need to be super accurate, 1-2 degrees is fine.

Currently Im learning to program PIC16F84, is there a better PIC choice that is just as easy to learn?

I imagine that the only way to do analog-to-digital in software is with a lookup table, correct?

Can anyone help point me in the right direction for selecting components?
Primary criteria are: easy to program, low cost.

Im also looking for help with code, Ive got the LCD part pretty well figured out, now I just need help with the A2D.
Will I need some type of comparator for the analog sensors (16F628?)?

BTW, I already have some PIC16F84 and F628. Which is probably what I'll end up using unless there are major disadvantages.
Thanks to all.
 
NewGeek said:
Im trying to decide what to use for the temp sensor. Analog or digital? The only real restriction is that it must be small TO-92 size or smaller, for a remote mount. Are there any inexpesive digital sensors this small?
Should I use a PIC with A2D or just do the A2D in my program? Keep in mind that I will have to learn how to program A2D anyway because of the fluid level sender. Should I just use a thermistor and figure out how to write code? It doesnt need to be super accurate, 1-2 degrees is fine.

The DS18S20 is a really cool digital temp sensor in a TO-92 package, $3.88 from **broken link removed** . The mfg will send you free samples if you request it from their website. Its 1-wire protocol is a bit tricky to write for the first time.

I hate to break it to you, but 1-2 deg F IS "super accurate". Digital displays which do 1/10ths of a degree are often unable to give that with absolute accuracy.

Thermistors work well. However, the PIC's ADC will be have slight offset errors because the thermistor circuit's output impedance is probably going to be greater than the 2.5k max the PIC needs. So you may want to buffer it, which can add its own offset error. Also, check the error in the thermistor's resistance too.
 
NewGeek said:
Currently Im learning to program PIC16F84, is there a better PIC choice that is just as easy to learn?

The 16F628 is a better choice, or the 16F819 which has 10 bit A2D in the same size chip. The 16F84 is essentially obselete, replaced by the 628.

I imagine that the only way to do analog-to-digital in software is with a lookup table, correct?

No, I don't see how that would do A2D at all?. You could use a lookup table to correct any non-linearity in the sensor though.

If you want to read a thermistor you can use a simple capacitor charging technique, checkout my joystick tutorial for an example of doing this.
 
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