Nuts and Volts - September 2014

READER-TO-READER TECHFORUM

>>> QUESTIONS

Antenna Preamp

Is there a CMOS op-amp with low noise, good common-mode isolation, and unbalanced output available? I would like to use it in a loop antenna preamp in a location that has an extremely high power-line noise level.

E-mode whips with a preamp are definitely unusable. This antenna would be untuned and intended for use below 30 MHz.

#9141 John R. Seeley KI4LTB Palm Bay, FL

UV LED VS UV Bulb

I'm been working a lot with high power LEDs now that they're so affordable. I'm especially fond of the UV LEDs for visual effects at night, but I've heard that some UV LEDs can cause blindness, and that I should use a UV bulb to create the effects. Is this true? Can someone explain the difference?

#9142 NickRobbins Ft Wayne, IN

Noise Cancelling

Is there such a thing as a noise cancelling technology that would allow me to play my guitar without disturbing the neighbors?

I get complaints about noise from my apartment when I play. I just can't use headphones and I've tried putting blankets on the wall for soundproofing, but I still get complaints.

How about an electronic solution?

#9143 Brian Tate Madison, WI

[#7143 - July 2014]

Lithium Charging

I'm thinking of building a solar charger for my iPhone, but don't know how to handle the internal lithium battery in terms of taper current, etc. — especially when I have the phone on all day. What I've found online is information on charging disconnected lithium batteries, not ones under load.

Any hints?

#1 Charging your iPhone should not require any special accommodations. The OEM charger from Apple supplies 5V at 1A via a USB connection with proper cable. Either the 30-pin connector or the new five-pin "lightning" connector cable both deliver the same power.

The phone (as all phones now) has built-in current limiting, temperature sensing, and voltage sensing to protect the battery.

You just need to deliver 5V regulated to the cable with, of course, polarity protection for the solar charger. A simple LM7805 regulator IC is what most chargers use for the 5V.

It really is NOT a good practice to leave the phone on the charger continually, as most rechargable batteries 'like' to be exercised. Charge it up, let it run off the battery until it shows recharge is needed, then reconnect the charger. You should get several years of service from it.

Rod Hogg REVCOM Electronics Scott City, KS

#2 First, under load shouldn't be that much different from being off as long as the circuit can handle the current needed to do both jobs. That you will need to measure and then find a pre-built circuit to do the trick.

I suggest pre-built since they are so cheap. I did the same kind of thing for a phone that refused to recognize its own charger as a valid charger. A good 'ol Motorola phone; I'll never buy another phone that has some type of recognition routine in it to check the charger.

In any event, the first thing I noticed was that my camera charger was for the same type of battery, same voltage, same Ahr rating, same Li Ion battery. I hooked up wires from the charger into the phone and it worked just fine.

Next, I wanted a more universal Li-Ion battery charger and found more than one online. The one I settled on was from dx.com:

1A Lithium battery charging module - blue $1.70/e

http://dx.com/p/1a-lithium-battery- charging-module-blue-205188

There are others as well; a red module at 3A for under $9

http://dx.com/p/1a-lithium-battery- charging-module-red-318740

These are circuit cards usually with a USB power port to give it 5V/ 1A to work with, so you will need to do some soldering for connections to the battery from the board.

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