Monday, July 23, 2012

Say hello to my tiny friend!... Or an update on the ATtiny 45/85 development board.

I've been working on an ATtiny 45/85 dev board for a while now. It went from it's infancy of no external power input to being a toddler and using an FTDI cable to supply the 5v needed. Now it's a teenager and has on-board power with 5v regulation:

It's the third revision of the board I've done, and it's also the largest of them. Still, it's not too expensive from OSH Park or BatchPCB. It now has a power jack and 5v power regulation (should be able to plug that Arduino power adaptor in and get her to fly) circuitry, ISP header, reset switch and two sets of broken out headers. The two sets of headers do serve a purpose. The first and inner set are for 'shields', while the second, outer, set is for both breadboard use and to give access to any unused pins that a 'shield' may not be using. It doesn't have an external crystal etc... as it shouldn't need one (as far as I understand), but the next revision will likely include a ceramic resonator, or at least be able to plug one in if wanted.

The board came from the want and need for an easy to use board for the ATtiny 45 and 85 (mainly the 85) that included all I would need to use it to make a Toaster Reflow Oven, but also be viable for use with other projects.

To further this idea of ease of use I designed an AD595 K-type thermocouple amplifier 'shield' for the dev board:

This is the second revision of the breakout board. It started life as something very basic, with the bare minimum for what I needed... I also cocked up my first prototypes. The thermocouple solder pads (which aid in getting better readings) were not accessible to solder to. So in this version I rectified this, in a very ugly way. But it works.

This revision also has an alarm LED, which will alert you if a thermocouple isn't connected or if the chip is damaged in some way. I also now use a 3-pin screw terminal to connect to the relay board I will be using (

The AD595 is rather useful as you only need one pin on the microcontroller side of things and it's dead easy to work out the temperature:

Signal Values

The nice thing about the AD595 is that the signal it generates is linear with temperature. This means that it is really easy to convert from an analog reading to a real temperature. Secondly, the AD595 outputs 10mV/C which is simple to work with. at 1V, the temperature is 100C, at 2V, the temperature is 200C, etc. With a reference voltage of 5V, the sensor can make readings up to 500C.
The way to read the temperature is easy. First you convert an analog reading to voltage, then you multiply by 100 to get the temperature in celsius:
Voltage = analogRead(X) * 5.0 / 1024.0; //Note that 5.0 is the A/D reference voltage. Celsius = Voltage * 100;
Or, to simplify:
Celsius = ( 5.0 * analogRead(X) * 100.0) / 1024.0;
It does mean that the breakout board can only output up to 500°C since it only has 5v to work with, but we don't need to go higher than that for a reflow oven (I imagine it wouldn't be hard to bring the 9v from the power adaptor up to the AD595 to give it a 900°C working range). I'm going to be using the firmware from the Pleasant Hardware reflow controller ( as the basis for my code. He managed to get full PID controlled temperature management, 3 'programs' and a minimalistic UI into the limited memory of the ATtiny 45!

Anyway, this is just an update of what's been happening with the dev board. More to come about the reflow oven!

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