Cricket (thermostat kit concept) 16F917 & LCD

[David James]

Edit: A 3xAA (4.5V Alkaline) battery power supply as the battery level drops you can measure it by switching RA3 to analog (2.5v ref) as the A/D value will increase as VDD drops.

I think I understand this, can I ask a question to see if I am understanding correctly? Are you saying that you would measure the 2.5V reference voltage (on RA3) with reference to VDD?

Also, what is J1 for in the schematic, I'm not clear how this fits with the thought above as it looks to disconnect the reference voltage from RA3 - is the 'EDIT' (above) a later thought than the schematic?

 

[blueroomelectronics]

Actually it's on the + reference. So the 10bit (1024 step) A/D converter will measure between VSS (GND) and Vref+
Therefore each step would be 2.5/1024 (2.44140625mV)

So at 23C the LM35 puts out 0.23V and the A/D RES will be 94
0.23/(2.5/1024)

The technique works great with the LM35 (C), I would like to use the LM34 (F) and do the Celcius conversion in software (larger practical temperature range with zero additional components).

The basic conversion from A/D RES back to decimal (by my calculations) is
ADRES * 125 / 512
So 94*125/512 = 22.949 (I'll round it up in software) Same formula works with the LM34.
The integer routine avoids a floating point library and /512 is extremly easy

As for J1; when removed R5 would pull that pin to VDD. The idea was on reset to measure RA3 compared to VDD to determine run/setup.

 

[simrantogether]

Hi Bill...

Hi..

I congrats you Bill.. and a good thing i've learnt from you...

Your devices are Simpler and powerful.. and also they are beautiful...

one good law about embedded devices can be termed as : " simple and beautiful... "

Regards,

Simran

 

[blueroomelectronics]

The right PIC for the job can make many projects seem trivial. The 16F917 is perfect for a thermostat with display. Its packed with many great features; LCD controller built in, 10bit A/D, USART, Nanowatt tech, low power oscillator, low vdd detect.
I considered seven segment LED displays in the original design but on a small PCB the heat (even the 4ma HE displays) was picked up by the temperature sensor (about 2c)
Even the mighty 16F917 has a couple of quibbles though such as no MSSP & USART at the same time (they share I/O pins). But I'm not using the MSSP (almost did though)
Another really nice featue I've yet to play with is the low vdd detect mode. Should be handy as a on battery indicator (sleep & save power but keep the clock updating)

 

[Triode]

Nice post,
I got a 16F917 and some various LCD displays that I'm planning on figureing out how to use together this weekend. So this helps.

 

[blueroomelectronics]

It's a project back on the secondary burners. I can only work on it when I can find free time.
The main burners are a paired project. The control head (Ladybug) for the Cricket.
I was inspired by this Honeywell touchscreen thermostat and though, now there's something I could do with my iPhone / iTouch and a Ladybug*
Gizmodo - Honeywell Touchscreen Thermostat and Portable Remote Review - Honeywell thermostat review

*Ladybug is a PIC18F67J60 based Ethernet, RS484 (Cricktet), SDCard controller with 8 relays, 8 opto inputs and some spare I/O (including a couple of analog inputs)