Make Electronics

Getting Somewhat More Serious 123

Experiment 14: A Pulsing Glow

Because the LED will flash at about the speed of a human heart while the per-
son is resting, it may look as if it’s measuring your pulse, especially if you mount
it on the center of your chest or in a strap around your wrist. If you enjoy hoax-
ing people, you can suggest that you’re in such amazingly good shape, your
pulse rate remains constant even when you’re taking strenuous exercises.

To make a good-looking enclosure for the circuit, I can think of options rang-
ing from embedding the whole thing in clear epoxy to finding a Victorian-style
locket. I’ll leave you to consider alternatives, because this is a book about elec-
tronics rather than handicrafts.

However, I will address one final issue: how long will this gadget continue
flashing?

If you check the following section “Essentials: Battery life,” you’ll find that a reg-
ular alkaline 9-volt battery should keep the LED flashing for about 50 hours.

essentIAls

Battery life

Any time you finish a circuit that you intend to run from a battery, you’ll want

to calculate the likely battery life. This is easily done, because manufacturers

rate their batteries according to the “ampere hours” they can deliver. Keep the

following in mind:

• The abbreviation for amp-hours is Ah, sometimes printed as AH. Milliampere-
  hours are abbreviated mAh.


• The rating of a battery in amp-hours is equal to the current, in amps, multi-
  plied by the number of hours that the battery can deliver it.
  Thus, in theory 1 amp-hour can mean 1 amp for 1 hour, or 0.1 amp for 10 hours,
  or 0.01 amp for 100 hours—and so on. In reality, it’s not as simple as this, be-
  cause the chemicals inside a battery become depleted more quickly when you
  draw a heavy current, especially if the battery gets hot. You have to stay within
  limits that are appropriate to the size of the battery.
  For instance, if a small battery is rated for 0.5 amp-hours, you can’t expect to
  draw 30 amperes from it for 1 minute. But you should be able to get 0.005 amps
  (i.e., 5 milliamps) for 100 hours without any trouble. Remember, though, that
  the voltage delivered by a battery will be greater than its rated voltage when
  the battery is fresh, and will diminish below its rated voltage while the battery is
  delivering power.
  According to some test data that I trust (I think they are a little more realistic
  than the estimates supplied by battery manufacturers), here are some numbers
  for typical batteries:


• Typical 9 volt alkaline battery: 0.3 amp-hours, while delivering 100 mA.


• Typical AA size, 1.5-volt alkaline battery: 2.2 amp-hours, while delivering
  100 mA.


• Rechargeable nickel-metal hydride battery: about twice the endurance of a
  comparably sized alkaline battery.


• Lithium battery: maybe three times the endurance of an alkaline battery.