Nuts & Volts - May 2004

Tech Forum

3.2/.1 = 32 hours, but, in practice, you might get 60 hours out of it. Also, if you discharge at a high rate (like 10 amps), you might get much less usable power (watts) out of the battery. In any event, this battery is capable of supplying far more power than any "Supercap" solution you are likely to be able to afford. It's also much more compact.

The energy stored in a capacitor is equal to 1/2 CV ^ 2. A 1 farad capacitor at 5 V stores 1/2 x 1 x 5 ^ 2 joules = 12. 5 joules, which is 12. 5 watts for one second or .00347 watt hours. Since your battery supplies 47. 36 watt hours, the capacitor can supply .00347/47.36 = .0000733 of the power of the battery pack.

Robert Zusman

Scottsdale,AZ

on. The loud bass noises and vibrations penetrate our house, setting off resonances that rattle our walls and windows. The rumbling from other vehicles — buses, trucks, and trains — is also an aggravation.

Is there a technology available that could cancel out these noises? Or, perhaps, is there a metal screen that could be plastered into the walls and have a charge applied to it that would cancel out the low frequency sound waves — similar to the screen embedded in the glass of the microwave oven door?

We would appreciate help with this problem,

which is becoming more persistent.

"coherent" types of noise, such as the roar of an airplane engine or continuous traffic noise. Such technology has been applied in noise cancelling headsets worn by pilots and in the headrests of some automobiles. Microphones in each earpiece pick up the sound waves at about the same time as they hit your eardrum. The noise cancelling circuitry "inverts the signal" produced by the microphones and produces a sound wave at the speakers that is 180 degress out of phase from the original. When the two waves hit your eardrum, they cancel and the sound is heard at a reduced level or not at

[2042 — February 2004]

We are searching for a solution to the problem of the ultrabass sound from stereos in cars that drive down the busy street we live

#1 There is technology available that can cancel "predictable" or

F o r

E l e c t r o n i c s

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[1045 — January 2004]

I'd like to build a simple interval timer to run a 12 volt CPU cooling fan. It should turn the fan on for five minutes and then off for about three. If it is adjustable, that would be even better.

A very simple circuit using a CMOS 555 timer and a P-channel HEXFET can be used to operate a motor in an off/on sequence, as shown in this schematic diagram.

The 555 chip is operated in a slightly modified 50% duty cycle circuit where the on time of the motor is determined by the values of RA and CT. This time can be calculated from the expression T = 0.7 (RA) (CT) where RA is in megohms and CT is in microfarads. The motor is turned on when the output of the chip is low. RA can be made variable to adjust the on time of the motor.

Values of 1.3 MΩ and 330 µF will result in an on time of 300 seconds. A low-leakage electrolytic is recommended.

Without D1 and RB in the circuit, the off time of the motor would be the same as the on time. When D1 and RB are added as shown, CT is charged up faster during the time when pin 3 of the chip is high, resulting in a shorter off time for the motor. RB may be made variable to control the off time of the motor. A good starting value for RB would be about 1 MΩ.