Cheap Control, Microcontrollers under $3

[OperaHouse]

I've bought more than a dozen of these.  The little blue board for $2.33 shipped is the same as a UNO, only stripped down to
its basic essentials, crystal, regulator, reset button and two LED. I've bought just as many of the $3 versions which are
better constructed with larger vias to solder the pins to for those projects I really care about.  The entire assembly on a
breadboard costs less than just getting the DIP microprocessor chip alone.  To program it I purchased the red USB FTDI Basic
board for $3.99 shipped made by geeetech.com  There are some older versions of these that may not match op pin for pin so check
pictures in the listings.  It would be nice if everything was plug and play.  There was no mention of it but the CDM v2.12.00
software driver needs to be downloaded for the FTDI board to work.  There are a number of online tutorials for FTDI programming.
They all seem to be poor and leave out a detail or two. In the TOOLS section of the Arduino compiler the BOARD will have to be
selected as MINI AT328 instead of UNO.  Otherwise program will not download.
Pin connectors are included. Pin outs should match up as follows:

USB FTDI  BLUE MINI   RED MINI

DTR GRN   DTR         GRN
RX1          TX0         TX0
TX0          RX1         RX1
3V3          VCC         VCC  NOTE: FTDI board is set to 5V and has 3V3 option
CTS          GND         GND
GND         GND         BLK

The UNO is recommended for first time users since it eliminates possible communication errors in setup.  The MINI looks much smaller
but it has exactly the same pin out spacing.  The full UNO board is often just too big to use in many applications.  I don't think I
will ever buy a full UNO board again, the MINI has everything I need (it even has two extra A/D converter inputs).  The serial board
uses a mini USB connector and  is not included. Many people have these around from old cameras.  I was just at a Radio Shack and they
wanted $35 for the MINI 328 board in one of their maker kits.

I will be posting a low voltage disconnect project for one of these with voltage averaging, upper and lower set points, and a time
delay to prevent unwanted disconnects due to initial current surge as a starter project.

First picture shows FTDI ban MINI board connected.  Second picture shows $3 better board, not physically larger processor chip and use of GOLD.

[stofanel]

I am not sure if UNO's are such a great deal. My microcontroller of choice is the PIC made by Microchip. Most low end chips are less than $2. The beauty of the PIC's is that all they need to function is power and ground. No external crystals are required, since they have their own internal clocks. Programming can be done in assembly, C, basic, etc. A Pick'it 3 programmer, which will program 99% of the chips is less than $35. 

[DamonHD]

The ATMegas in (say) the UNO can also run entirely from internal oscillators; for the design we use in OpenTRV I use a 32768Hz crystal to maintain an accurate realtime clock, and the CPU runs on the internal RC clock while it's awake.

Our first prototype was on PICAXE (just got taken out of service in Denmark a couple of days ago) running BASIC, but I had to add an external RTC.  To keep reasonably accurate time (wristwatch accurate) I think an external crystal is needed whatever the CPU/MCU family.

http://lists.opentrv.org.uk/pipermail/opentrv-dev/2015-March/001551.html

Rgds

Damon

[OperaHouse]

Where I used to work we used Microchip.   I actually got them to switch to the small PIC from Motorola for small projects (old ways die hard).  A majority of these projects were self oscillating using the internal RC.  This is not scientific at all.  We had  this old cattle prod that we would zap boards with to try and make them fail.  It always seemed that the internal oscillators were more immune and those with a crystal would tend to reset or lock up more often.

[OperaHouse]

I've ordered a single 100W 12V panel to do a little research at home.  I have several posts covering my power
point hot water system at the camp.  That used a 36V array (this year it will be 48V) operating at about 50V
power point and the heating element is fed a PWM voltage.  At home I use a heat pump water heater and my typical
cost per day is about 37 cents. During a 12 hour daylight period the heat loss with no water used is 800WH. Figuring
a COP of 2 for the heat pump that becomes 1.6KWH.  From camp experience I would need 300-400W of panels to reduce
daylight running of the heat pump to about zero.  A 100W panel will not make a big dent in this but will provide
data necessary to scale up the project.

Instead of PWM high voltage from a series of panels, this version will use 150W boost converters available on ebay
for $5 to boost the voltage up (about double) so that a standard 240V heating element can use all the power of the
panel. A MINI328 processor will keep the panel voltage at the power point and a PWM output pin with some filtering
will control the output of the boost converters operating in parallel.  One boost converter will be used for about
every 70W of panels.  With a slight modification of the software this could be used as a dump load on a battery.
I'll be trying several methods to see which is easier.  After my HPWH conversion the tank has a heating element which
is not being used and dropping a wire down from the roof is easy going down the flue pipe from the roof since that
is not used anymore. 

[joestue]

I think this is relevant to the thread, looks like the same arduino clone...

http://www.instructables.com/id/ARDUINO-SOLAR-CHARGE-CONTROLLER-Version-30/?ALLSTEPS

looks like the parts list might be under 50$.
this is rather open ended.. the high side current sensor used could be moved to the low side.. and the input voltage as high as 600 volts given he uses a standard high and low side fet driver....

[Yyrkoon]

The TI MSP430G2553 used to be available for ~$1.25 a couple years ago. Back around the time the MSP430 Launchpad was $3.99 ( complete dev board and micro USB cable included ). Since then though the board is now $10.99, and I think the MCU's are now selling for $3.00 ea in lots under 1ku.

I'd still consider it a good micro for the fact that if I recall correctly it only needs a single cap and resistor to get running. Has an onchip watchdog, ADC, SPI x 2, I2C( if you forgo one SPI ), and a couple timers / PWMs. Oh, and right a UART too. All this in a package that can literally run for years off a button cell battery. It's nothing special with only 16 pin outs( in DIP package ), and up to 16Mhz. But if you need a small power efficient micro for a limited task it can come in real handy.

Lately though I've gone the other way. I'm using a beaglebone black + CAN cape to read from our Schneider inverters  . . .

EDIT: Ah and I forgot to mention the MSP can be programmed using TI's code composer studio, but one of the major attactions for me on the software side. Is that you can use gcc, and other opensource software to write code, and upload to it. There is even an MSP430 port of gdb . . .