Nuts and Volts - June 2014

READER-TO-READER TECHFORUM

>>> QUESTIONS

FM Instead of Wi-Fi

I would like to use a device such as a Pi or Arduino to connect to an Ethernet. I want to send/receive data wirelessly via digital FM instead of Wi-Fi to communicate with devices where the Wi-Fi/internet connection is unavailable, but still relatively close (suitable for low power FM where FCC regulations would not be an issue).

High speed data transfer such as for streaming video is not a must, but 100-200 kps would be ideal. This would, of course, vary depending on the strength of my signal.

Can I buy a board with Ethernet and simply I/O data to an FM transceiver? Any suggestions to help me get started would be great.

#6141 Gary McPherson

Wyoming, MI

Delay Circuit Needed

I need a little help with an automatic above ground pool filler circuit I constructed. It’s nothing complicated — a water level switch with a power supply. When the water level drops below the set point of the float switch, it completes the circuit which applies 12 VDC to a solenoid which activates and allows the water to flow.

When the water level comes up and the float switch raises up to the preset point, the reverse happens and the water stops flowing.

My problem is I need some sort of delay before the power supply sends

12 VDC to the water relay. The float switch — due to motion by the wind or vibration — normally keeps the water level switch bouncing. This, in turn, keeps the water constantly turning on and off. If there was a 15 to 30 second delay until the float switch stabilized in one condition — either on or off — it would prevent the water switch from constantly going on and off. Anyone out there with a circuit that can do this? It would be even better if the same timer also supplied the voltage for the water relay. Thanks in advance. #6142 Gary Lichtenstein Las Vegas, NV

Battery Charging Indicator

My truck only has a voltmeter for battery condition indication. I don't have room for an ammeter. Is there a circuit I could build with a red and green LED to indicate if the battery is charging or discharging?

#6143 Ray via email

Call Blocker

Telemarketing has gotten both ultra sophisticated and out of control.

Do Not Call listings are a waste.

I'd like to stop all incoming calls short at the point of entry, unless the caller — on receiving a prompt — dials an additional pass code to access my stand-alone answering machine and simultaneously ring any phones on which the ringer is turned on.

Is a DIY circuit available? Is anything off-the-shelf available? I

checked with AT&T marketing and all they offer is blocking of all calls where no telephone number or caller name is available. That's not what I want. #6144 Robert Wheaton San Antonio, TX

Renewing Permanent Magnets

I have an old style hand crank telephone generator. It works, but output voltage is low due to the four horseshoe magnets it uses being weak. What is the best way to revitalize these magnets to bring the generator back to like-new operating condition?

Also, how can I tell when — as a result of the revitalization process — the magnets have achieved maximum strength and won't get any stronger? #6145 David LeFevre

Ontario, NY

[#3143 - March 2014]

Sweep Generator Range

I have a Heathkit sweep generator 1274 which I bought with the hope I could quickly determine the frequency response of amplifiers I play with from time to time. However, there is no way to determine the range of the pattern on my scope — at least none that I have figured out. I’m not very advanced in such matters. Is there any easy way to add blips to the pattern say at 12 kHz and or 15 kHz?

One way to do this is to connect a series L-C tuned circuit (high Q) across the input of the amp under test. Since capacitors are higher Q than inductors, a high value cap and low value inductor would be best. I chose a 470 uH inductor (Mouser 580-22R474C, $0.59) and 0.374 µF capacitor for a 12 kHz notch.

All questions AND answers are submitted by Nuts&Volts readers and are intended to promote the exchange of ideas and provide assistance for solving technical problems. All submissions are subject to editing and will be published on a space available basis if deemed suitable by the publisher. Answers are submitted by readers and NO GUARANTEES WHATSOEVER are made by the publisher. The implementation of any answer printed in this column may require varying degrees of technical experience and should only be attempted by qualified individuals. Always use common sense and good judgment!