My interest in energy efficient PIC projects began when I decided to adapt an earlier wall-powered project into standalone, self-powered devices. While I haven't yet settled on the best way to power these devices, I have found some useful ways to cut milliamp usage to a minimum. This is not often a concern for students, but as we investigate new ways of scavenging power, a basic knowledge of some energy efficient circuitry might help projects run longeer from a single power source.

The Starting Point

When this class first met, I measured the energy consumption of a single word module, consisting of a PIC chip, an LED, and a LCD display. That figure -- 5 volts at 22 milliamps, or 110 milliwatts, at first seemed surprisingly low, but now seems exceedingly high. Over the course of this project I have been able to reduce the energy consumption by about 95 percent.

Here is the original circuit:

Gary Schober, guru of ITP's advanced tech, suggested I modify the circuit in a few ways:

• Cut voltage. Could this circuit run on 3 volts or less? Microchips offers the 18LF252, a low-powered version of the 18F252 chip I was using at 5v that could run on lower voltages.
• Change the MCLR resistor from 10k to 47k. Still not entirely sure whether this was suggested for low-power or other considerations.
• Remove the LEDs. Or at least dim them heavily with a big resistor. They guzzle power.
• Use a low drop-out, low amperage voltage regulator. I decided to try the LP2950CZ-5.0, available at Jameco.
• Use a slower clock. Lower frequency, lower power. He suggested a 3.85mhz resonator.

So, taking Gary's advice into consideration, I tested a reworked circuit using a variety of different components at two different voltages -- 5v and 3v. (The LCD I chose runs dimly at 3 volts, but is clearly visible from a vertical angle. Its power consumption characteristics are otherwise similar to the previous LCD.)

Each measurement given is the amperage of the entire circuit, with only the given component changed. The option in red is my recommendation.

Component	Type	@ 5v (ma)	@ 3v (ma)	Comments
MCLR resistor	10k	4.3	2.1	No difference here.
	47k	4.3	2.1
Clock	3.85 MHz resonator	4.3	2.0	Using a 20 MHz crystal effectively doubles the power consumption of this circuit. No reason to use it unless you need it.
	4.0 MHz crystal	4.5	2.1
	20.0 MHz crystal	9.0	4.5
LED power light	none	4.0	1.9	A 10k resistor gives a dim lamp, useful to avoid power confusion, and adds negligible current to the circuit. Worth it.
	w/ 10k resistor	4.3	2.0
	w/ 220 ohm resistor	16.7	6.5
LCD backlight	off	4.3	2.0	The LCD backlight, an array of about 8 LEDs, doesn't light at 3 volts. At 5 volts the brilliant green display comes at a steep cost, not really viable without a wall power supply.
	w/ 220 ohm resistor	10.1	off
	w/ 10 ohm resistor	62.5	off

Summary

It's clear that a lot of energy is wasted in student projects simply because, like me, many of us have never had the need to really count our milliamps. But for portable projects there's really no reason not to make every effort to prolong battery life and maximize the little energy provided by such renewable energy such as solar cells.

A bit of perspective: Con Edison charged me 16.0606 cents a kilowatt/hour last month. So each of my milliwatts saves me (or ITP) $0.0000160606 an hour. Still, batteries are more expensive. And if you can get your project to last twice as long on a single battery, why not?

That being said, here are my simple rules for making the most of your milliwatts:

• Lower your voltage.
  ITPers usually run things at 5v by default, but there's no magic in that number unless one of your components requires it. Limiting yourself to 3 volts widens the range of batteries you can use. For low-current circuits, check out some low drop-out voltage regulators; impress your friends with a oh-so-trendy TO-92 regulator on your breadboard.

• Get rid of useless LEDs.
  They're great for troubleshooting but absolute current hogs. If you must have them on your board, limit them with some hefty 10K resistors. They'll be dimmer but still visible.

• Use the slowest clock you can.
  Often circuits can run just as well with any speed clock. We all know how sexy higher clock speeds can be; I too remember the turbocharged 40MHz PC-XTs of my youth. But there's a big cost to higher clock speed, even with PICs.

By making these three changes, I was able to cut my circuit's power consumption from