Right Starting Point for my project?

Tim Hall

Hi all,

I have some basic hobby-electronics experience, but I'm new to microcontrollers. Could anyone tell me if the BASIC Stamp 1 or 2 is right for this project?

I have a very clear, specific program for this project, but I'm still trying to wrap my head around some of the basics of microcontroller applications. Here's my program...it's basically an all-in-one line-voltage climate controller:

A) Temperature and Humidity Sensing
Four Programmable Timer functions
C) Control Three 2Amp (minimum) Line Voltage (110VAC) Relays
D) Control Two Low-voltage DC Relays
E) Menu-based interface w/ LCD 2x16 Character Display + six Tactile Switches

The timers are count-down functions, and do not need to be based on "real" date/time reference.

I realize this is a pretty advanced project for a beginner, but I'm die hard at rolling my own. Again, I'm just not sure if the BASIC Stamp is the right starting point for this project. Any input would would be much appreciated.

Thanks,

Tim

Tim Hall

Haha, I see just a few threads before me is someone working on a beer-brewing application.

Just FYI to everyone, my application here is also fermentation, but much broader. This is going to be a controller for a fermentation cabinet that will hold all sorts of things...beer, wine, cheese, tempeh, miso, kraut...

Again, thanks for any input.

Mike Green

A BS1 will be too small for your project. I would recommend getting a Board of Education for learning about the Stamp and for the experimentation phase of your project. I would also recommend getting a BS2p rather than a BS2 for your Stamp in that the BS2p/pe/px has built-in statements that directly support I2C and 1-Wire devices. I also recommend you use an external real-time clock (RTC) rather than implementing a count-down clock on the Stamp. It will be more accurate and, most importantly, simpler to program the function. Some RTC chips use I2C and there are some good thermometers that use the 1-Wire protocol.

Consider using solid-state relays for the AC. They can be driven directly from the Stamp without an external driver transistor.

Depending on the DC circuits that have to be switched, you may want to directly switch them with a power transistor or use a relay. Some reed relays can be directly driven from the Stamp while other relays need an external switching transistor.

The BS2p/pe/px also has statements that directly support a Parallel LCD display. You can use that or a serial LCD display using the SEROUT statement.

For sensing switches, consider using either a 74HC165 shift register or a PCF8574 I2C I/O Expander. There are examples of all of the things mentioned scattered among various Parallax tutorials and manuals and among the Nuts and Volts Columns. Start at the Resources tab on Parallax's main webpage.

Tim Hall

Thank you, Mike. Yes, I've been going through the Nuts & Volts archives and already learned quite a bit. It appeared the BS2p24 would do what I wanted to. All very good input.

Now I guess I need to do some research on specifying relays, which I know nothing about.

Mike Green

You want contacts on one relay to be rated at 2A current carrying capacity and to switch at least 120VAC. On the other relay, you want contacts rated at the current you plan to switch and a DC voltage higher than what you need to switch. Both relays need to have a coil voltage equal to your power supply voltage, probably 6V, and a coil current that can be switched by a simple switching transistor, probably on the order of 100mA.

If you're going to use a 12V power supply, you might have better luck getting the relays at RadioShack since most of their small selection of relays have a 12VDC coil.

Even though the BOE is rated for a 6-9V power supply, it will run fine off a 12V power supply if you limit how much current is drawn through the BOE's regulator. If you can limit the current to 250mA or 300mA, you shouldn't have overheating problems. Normally the BOE's regulator can supply 1A (with a 6V-9V supply voltage). It tends to run hot near the upper end of the voltage and current range.