Frankenstein's Thermometer Mk2

Well it's been just over a year since this blog started and what better way to celebrate than revising an old project? I'd had a look at Frankenstein's thermometer before, using a neat trick with a mono jack plug and a stereo socket to switch the power. However I wasn't really pleased with this arrangement, because it separated the thermistor from the main box when it wasn't in use.

The idea was to upgrade the firmware to put the ATtiny into deep sleep after a short time. The complete code is now:

	/*
	* Set IDE to "ATtiny84@1MHz, BOD disabled"
	* fuses: H:FF, E:DF, L:62
	* (Can program with Arduino as ISP using one of these:
	* http://programmablehardware.blogspot.ie/2011/05/in-system-programming.html )
	*/
	#include <math.h>
	#include <Interrupted.h>
	// thermistor parameters
	const double r0 = 10000;
	const double beta = 3977.0;
	// bias resistor
	const double rb = 9990;
	const double zeroC = 273.15;
	const double tzero = 25 + zeroC;
	#define IDLE_MS 10000L
	#define DIGIT_MS 500L
	#define DEBOUNCE_MS 500L
	#define THERMISTOR A7
	#define BUTTON 9
	#define DIVIDER_GND 8
	#define A_PIN 6
	#define TIMER 0
	#define IDLE 1
	Analog thermistor(THERMISTOR);
	Port pins;
	Pin button(BUTTON, pins);
	Timer timer(TIMER, DIGIT_MS);
	Delay idle(IDLE, IDLE_MS);
	Devices devices;
	const uint8_t A = _BV(0);
	const uint8_t B = _BV(1);
	const uint8_t C = _BV(2);
	const uint8_t D = _BV(3);
	const uint8_t E = _BV(4);
	const uint8_t F = _BV(5);
	const uint8_t G = _BV(6);
	static const uint8_t digits[] = {
	A | B | C | D | E | F,
	B | C,
	A | B | G | E | D,
	A | B | G | C | D,
	F | G | B | C,
	A | F | G | C | D,
	A | F | E | G | C | D,
	A | B | C,
	A | B | C | D | E | F | G,
	A | B | C | F | G,
	};
	static const uint8_t minus = G;
	#define CENTIGRADE 0
	#define FAHRENHEIT 1
	#define ABSOLUTE 2
	static const uint8_t units[] = {
	A | F | E | D,
	A | F | E | G,
	A | B | C | E | F | G,
	};
	void display(uint8_t bits)
	{
	uint8_t b = 1;
	for (int i = 0; i < 7; i++, b <<= 1)
	digitalWrite(A_PIN - i, bits & b);
	}
	double read_temp()
	{
	double rt = rb * (1023.0 / thermistor.read() - 1);
	double rtk = 1.0 / tzero + log(rt / r0) / beta;
	return 1.0 / rtk;
	}
	uint8_t *reformat(int u, double t, uint8_t *buf)
	{
	uint8_t *p = buf;
	*p++ = units[u];
	if (u != ABSOLUTE) {
	t -= zeroC;
	if (u == FAHRENHEIT)
	t = t * 9.0 / 5.0 + 32;
	}
	if (t < 0) {
	t = -t;
	*p++ = minus;
	}
	int temp = (int)(t + 0.5);
	bool first = true;
	for (int pow = 100; pow > 0; pow /= 10) {
	int dig = temp / pow;
	if (!first || dig != 0 || pow == 1) {
	temp -= dig * pow;
	*p++ = digits[dig];
	first = false;
	}
	}
	*p++ = 0;
	display(buf[0]);
	return buf+1;
	}
	void setup(void)
	{
	devices.add(button);
	devices.add(thermistor);
	devices.add(timer);
	devices.add(idle);
	devices.begin();
	for (int i = 0; i < 7; i++)
	pinMode(A_PIN - i, OUTPUT);
	pinMode(DIVIDER_GND, OUTPUT);
	digitalWrite(DIVIDER_GND, LOW);
	thermistor.enable();
	idle.enable();
	}
	void loop(void)
	{
	static int u = CENTIGRADE;
	static bool sleeping = false;
	static uint8_t buf[8], *p = buf;
	static double curr_temp = 0.0;
	static uint32_t last;
	uint32_t now = millis();
	switch (devices.select()) {
	case BUTTON:
	if (sleeping) {
	sleeping = false;
	last = now;
	pinMode(DIVIDER_GND, OUTPUT);
	digitalWrite(DIVIDER_GND, LOW);
	thermistor.wake();
	thermistor.enable();
	idle.enable();
	} else if (now - last > DEBOUNCE_MS) {
	last = now;
	if (++u > ABSOLUTE)
	u = CENTIGRADE;
	p = reformat(u, curr_temp, buf);
	}
	break;
	case TIMER:
	if (*p) {
	display(*p++);
	timer.enable();
	} else
	thermistor.enable();
	break;
	case THERMISTOR:
	curr_temp = read_temp();
	p = reformat(u, curr_temp, buf);
	timer.enable();
	break;
	case IDLE:
	idle.disable();
	timer.disable();
	thermistor.disable();
	thermistor.sleep();
	pinMode(DIVIDER_GND, INPUT);
	display(0);
	sleeping = true;
	break;
	}
	}

view raw frankenmeter.ino hosted with ❤ by GitHub

The main additions are:

• the sleep() routine which causes the ATtiny to enter a power-down mode,
• the PCINT1_vect which wakes it up again when the push-button is pressed.

The port PB1, which the push-button connects to ground, generates PCINT9. On the ATtiny84, there are two pin-change interrupts, PCINT0 and PCINT1. The former handles pin change signals on Port A, while the latter handles those from Port B. Our handler simply checks whether the interrupt has woken us up, or was to change the units of temperature.

The sleep routine switches off all of the LEDs and the ADC before putting the ATtiny to sleep. When it wakes up again, the ADC must be re-enabled.

With the new software installed,  the device consumes about 11mA when on but 0.16mA when off. With a nominal capacity of 1000mAh, this would drain the batteries in about 6000 hours (or 250 days). This was a bit surprising, as was the discovery that this current was still drawn when the AVR was removed altogether. This indicated that the source of the drain was the voltage divider comprising the thermistor and a 10k resistor. (See the circuit diagram in the original posting.) At room temperature, the current drawn would be 3v/20k or 0.15mA.

Rather than complicate the circuit with a MOSFET switch, we decided simply to increase the value of the bias resistor from 10k to 47k. This reduced the current by a factor of 4 increasing the battery life to over 2 years. (The calculation of the Thermistor equation has been cleaned up a bit too: the original made the assumption that the 10k resistor would cancel the thermistor's resistance at 25C.)