An Inexpensive Interface Cable for Baofeng Transceivers

Much less expensive than the individual components 

I know many of us have Baofeng and Wouxon transceivers. I recently came across this tip to cannibalize an inexpensive mic for the cable, which can then be easily interfaced to a TNC or other digital device.

The Baofeng UV-5R and similar radios are extremely inexpensive dual band (2m/70cm) HT's that are widely used for FM voice communication. But what about using them on packet radio? Yes, this is possible, but a number of people have had problems with either home made cables are some that were bought commercially. The major problem is that the radio keys, but does not unkey. It seems to be a grounding problem because when you bring your hand close to the radio, it then unkeys. More information on how to fix this momentarily. 

One way to overcome this is to make your own radio/TNC cable using a speaker/microphone that is designed for the Baofeng radio. You then cut off the microphone and just use the cable and connector. This may sound expensive, but as it turns out, speaker/mics are available for the Baofeng on line in the US$5 range. Do a search on Amazon for "Baofeng Speaker" and you will find them. You will also need a 5 pin DIN or 9 pin serial plug for the other end of the cable. 

You can buy these DIN Plugs on the TNC-X web site for $2 and serial plugs are universally available. Locally Lee’s Electronics is my choice for supplier.

Here is the Baofeng Speaker/Mic purchased from Amazon.The pin out for the plug is:

• TX Audio: Ring on big plug
• Ground: Sleeve on small plug 
• PTT: Sleeve on big plug 
• RX Audio: Tip on small plug

In the photo above, the 3 screws that hold the microphone rear plate in place have been removed and it is opened up. You can see that the wires are labelled on the printed circuit board, which makes it easy to figure out which wire is which. On this microphone the connections are as follows:

• Red = TXAudio
• White = Ground
• Black = PTT
• Green = RXAudio

NOTE 1: Some of these mics have the M- (ground) and SP+ (RXAudio) wires reversed. Since these wires are connected to the speaker, this doesn't matter for the operation of the speaker/mic, but it does matter for TNC connections. Typically the white wire is ground and the green wire is RXAudio. To be certain, clip the microphone off the cord and check the continuity between the white wire and the sleeve on the small plug. 

NOTE 2: A few Baofeng speaker/mics don't work.  It is suggested that before you cut the cable, make sure the speaker/mic works with your Baofeng HT.  If it does not work, the cable probably won't work either. 

The next step is to strip the wires and tin them with solder. Notice that the black wire is significantly shorter than the other wires. 

Solder a 2.2K resistor onto the pin where the PTT (black) wire is going to be attached. (This will solve the problem of the TNC getting stuck in transmit. Use as small a resistor as will fit, wattage is not important. Next solder the connector on to the remaining wires. 

Here's the completed cable, ready to go! As you can see it is not a difficult assembly process. However, if you would like to buy one already built, they are available for $20 plus shipping from https://www.tnc-x.com/ This company also sells TNC kits.

 ~ John VE7TI 

   19-02

Tech Topics: Review Of An SDR Dongle

The SDR Dongle 

SDR = software defined radio

Having already a conversation with VE7TI (John) about an older generation of SDR dongles I felt compelled to buy a new one, in 2018, a much smaller one, also from China. Most probably what I bought is a knockoff of a NooElec micro dongle. It was in sale at the time, for $7.87 CAD, shipping and taxes included, from aliexpress.com. It came with a remote control, an antenna and a CD with drivers. I discarded all those accessories, which are totally unusable if somebody wants to use the SDR dongle as a general receiver, and not as a DVB-T PC adapter, as intended.

I would like to start my review by underlining exactly that, the SDR dongle I am reviewing was not designed to be a general receiver, as I use it.

My first action was to install it on the computer, on a USB port, and to install drivers and software for it. I followed the instructions from www.rtl-sdr.com. It is tricky to have the drivers work in Windows 10, but if the instructions are followed exactly as in the given website, it works.

Some conclusions

• 
  The only software that completely works in Windows 10 is SDR sharp. It has various useful plugins, like a plugin for detecting the CTSS tones. Many plugins do not work with the last version of SDR sharp. It is free. A close competitor is HDSDR, which does not know how to decode stereo FM. All other programs I tried partially worked (they do not know all modulations types, have unclear settings, and so on).
  
  
• It has to be connected on the USB computer port with an extender, otherwise the electric noise generated by the computer makes it unusable, completely deaf for useful radio signals. I used my own accessories, in order to adapt the MCX antenna connector from the dongle to my antennas: 
  
  
• Caging the SDR dongle does not help much; if it is not case to case to the electric noise generator, but several centimeters apart, it is fine. I tried to cage it in metal and it did not make any difference in various test situations. I suspect it is already somehow shielded or partly shielded inside.
  
  
• In the commercial FM band it is a cheap stereo and more important, a RDS (radio data system) receiver. It knows how to display the name of the station, the songs that are played at that moment and whatever digital info the station sends in addition to the analog signal. The sensitivity in FM is way worse than 2 microvolts. Any dedicated commercial receiver-amplifier, including my roommate’s Yamaha 2 microV, every single FM radio in the apartment we have, including clock radios, and MP3 portables (the radio part) are more sensitive than the SDR dongle. I am using a proper horizontal dipole antenna on the balcony measuring 71 cm each leg, connected with coax cable to the SDR dongle, while all other 7 receivers have just a small piece of wire as antenna. I estimate the sensitivity in the 88 – 108 MHz band somewhere at 30 microvolts . It is expected the SDR dongle would be less sensitive in the FM band, due to the wide frequency bandwidth. I limited the bandwidth from 250 KHz to 180 KHz and there was a slight improvement.
  
  
  
• The sound in the FM band is not great. Even at 250 KHz, wide band FM (maximum in SDR sharp program), has audio quality that is just bearable. This is not exactly acceptable. I will not replace any of the radios with this SDR dongle, even though it displays data.
  
  
• The characteristics differ very much on the Rx bands and require adjustment at the RTL dongle settings. That means RF Gain; RTL AGC; Tuner AGC. 
  
  
• It is stable. I did not feel the need for a more stable oscillator. It did require adjustment in the software, 218 ppm as in the above picture for my dongle. This is considered a huge adjustment. I verified this with encapsulated quartz oscillators (32 MHz, 125 MHz, 150 MHz, the 28.197 CW beacon), and indeed it needs that huge adjustment.
  
  
• It seems it does not like the 50 MHz band, and the sensitivity is not great in this band. I confirmed the poor reports as everybody writing about this issue on the Internet experienced the same result, although I hear some local ham radios almost every evening. They never say their callsigns, so I just presume they are ham radios since they are in a ham band.
  
  
• On the 144 MHz band, with a good dipole, it receives everything the Kenwood 7950 and the Chinese walkie-talkie receives. It likes this band and it has a good sensitivity. All repeaters from Victoria, Port Angeles, Nanaimo, and Cowichan are 59. Probably the path is more important than the sensitivity in this case, too. I am at 103 meters above sea level. There are some images for strong local repeaters.
• It also likes the marine band, air traffic band and the weather band. They are all around 150 MHz and once the settings are done for one station, they can be kept for the weather, marine, 2 meter bands.
  
  
• The CB band and the beacon on 28.197 MHz (VE7MTY, Pitt Meadows, continuous, CW) are in a band where the RTL dongle is not so sensitive. The beacon (nearby me) booms in on my SONY ICF7600G portable radio, with its telescopic antenna. The SDR dongle with a CB whip on the balcony receives it almost OK, but only because I was hunting for the beacon and I knew where it was. The beacon’s signal barely produces a trace in the display spectrum, and I am nearby it (exactly 13.89 km).
  
  
• There are images everywhere. The FM band (88 – 108 MHz) can also be received on 30-50 MHz. The worse thing to do is to use an upconverter, as I saw so many do on the Internet, with a NE612 integrated circuit, and wide non-tuned input. I tried, and the images kill any useful signal. In the end I did 2 converters, in order to cover 3.5 MHz to 30 MHz, one for the lower part and one for the upper part. I used an NE612, attached to an amplifier with a BF998 in front. I have a tuning circuit just at the antenna, and a 2.2K resistor + coil in the output of the BF998’s drain. The source terminal is connected directly at the ground and the BF998 is supplied with 9 Volts (12 V is max in the datasheet, and it does burn after 12 V). The oscillator is an encapsulated 3.3 V powered oscillator, in a socket, to easily change it. The best the dongle worked for me is in the 150 MHz band, to stay away from the FM commercial band and to upconvert the shortwave into a sensitive band that the SDR dongle likes. I can adjust the signal from the oscillator to the value from the NE612 datasheet, but it actually does not make any difference, even if it is attached with 2 Volts (NE612 has a buffer in it before the mixer).

• The noise of the first element in the SDR dongle must be better than that in the  BFR91A. I tried a wide range untuned amplifier with one BFR91A, and it did not bring anything new, just noise. 
  
  
• The situation changed when I put a SAW 88 – 108 MHz (3 pin filter) in front of the BFR91A, and it helped.

• It does not run hot. Whatever other users noticed with old SDR dongles is no longer an issue with my 2018 SDR small dongle.

Final conclusions

• The SDR dongle is the cheapest 2 meter receiver a Ham can buy, and works as receiver on par with dedicated equipment, which is generally limited by line of sight, not by sensitivity. A beginner can listen to the weekly nets for some $8–11 CAD, shipping and taxes included.
  
  
• The SDR dongle is the cheapest FM commercial RDS receiver one can have, capable of displaying the digital data continuously and transmitted by almost all stations in Vancouver. 
  
  
• The SDR dongle was not meant as a general coverage receiver. It was designed as a DVB-T television European standard receiver, and it is probably better for that purpose.

A Postscript…

In the NI Multisim schematic you see a LED with a big resistor in series. It is not a mistake.

All LEDs I use are from China. The 3 mm ones I bought extremely cheap (I think they were 200 or 500 in the bag). The white ones are the most sensitive, and light at several microamps. I need to use resistors between 150 K (for BLUE) and 300 K (for WHITE) in series with the LEDs for 12 Volts power supply.

Now I understand what kind of LEDs they use in the portable lit antennas for walkie-talkies they sell for Baofeng. They light OK. Smaller resistances means burned LEDs. I tried the old values from various published schematics, and NO, they are not OK for the bags of LEDs I have.

~ Daniel VE7LCG

18/12

Not All Triplexers Are Equal: A Review of the Dunestar HF Triplexer

 

...Buyer Beware!

I had an opportunity to troubleshoot and repair SARC’s Dunestar triplexers. A couple of surface mount capacitors failed and had to be replaced. The triplexers are now in working order again. But...

These triplexers were very cheaply made. They contain only resonant circuits instead of filters. Better diplexers have low pass filters, band pass filters and high pass filters. So why are filters better than resonant circuit? Let’s see:

Here is a schematic diagram of our triplexer . There are 3 resonant circuits, one for 160, one for 80m and one for 40m. 

The frequency response curve of resonant circuits is narrow and may not cover a whole amateur radio band. In the case of our triplexer, the 80m bandpass is so narrow that does not cover both the CW and SSB portion of the bands. There is a jumper JP that adds more capacitance (C3) for the CW band by lowering the resonant frequency. Without the jumper the circuit is tuned to the SSB band. However, the jumper is located inside of the enclosure and it is very inconvenient to change.

Below are the 80m band graphs showing insertion loss (G) and VSWR of our Dunestar triplexer, with the jumper in the SSB position. As you can see it is useable only from 3.685 to 3.882 MHz if the practical VSWR limit is 2.  It would be slightly wider if the allowable SWR is 3, but it still covers only the centre of the band.

Triplexers which use filters (low, high, band pass) are much better. Below is a schematic diagram of a more sophisticated triplexer using filters rather than resonant circuits.

Let’s examine the characteristics of this 80 MHz bandpass filter. The graphs [above] show the scans taken the same way as for the Dunestar units. You can see that the band pass filter is flat and covers the entire 80m band (from 3.42 to 4.10 MHz) CW and SSB without need of a jumper. The low pass and high pass filters have similar characteristics; they are flat and cover the entire 160m and 40m bands.

So what have we learned here?

First, you get what you pay for.

Second, you have to be careful using this triplexer because outside the narrow pass bands you will run into high VSWR. It is worthwhile to check the specifications before you purchase any triplexer. 

As with any triplexer, you need to use extra band pass filters (one per band) because the isolation between the inputs is not sufficient (for example some RF power from the 80m transmitter could get into 40m receiver and damage the receiver’s front end).

~ Les Tocko VA7OM

18/12

The January/February 2020 Communicator

Here is the Latest SARC Communicator

Projects, News, Views and Reviews... 

Happy New Year from all of us here at SARC!

Here is the January/February 2020 SARC Communicator newsletter: 

http://bit.ly/SARC20JanFeb

This edition has 75 pages of projects, news, views, and reviews from the SW corner of Canada. 

• VY0ERC: What is life like at the farthest north Amateur Radio Club in Canada?
• Building a moonbounce (EME) station on VHF
• Make a 6m receiver with Arduino and a handful of parts
• A 3-pin radio IC
• A soldering primer
• Remote rotator control
• AA-600 Antenna analyzer review
• The BC QSO Party 2020
• Tech tips
• No-ham recipe
• and much more!

Past articles and issues are available on our blog at https://ve7sar.blogspot.ca

We always welcome contributions of news, stories and your Amateur Radio experiences. The deadline for the next issue is February 15th.

73,

John VE7TI
Communicator Editor

Back To Basics: Voltage Measurement

How is a voltmeter usually connected to a circuit under test?

Question B-005-013-001 - From The Canadian Basic Question Bank

A handy thing to know, particularly as basic digital multimeters (DMMs) are now very inexpensive, usually less than $10., and can be useful for many things around the home like checking the condition of batteries.

The two types of voltmeter you may encounter are digital and analog.  Analog meters are recognizable by their printed scale and a moving needle. 

Voltage is always measured in parallel with a device, current in series. If you recall Ohm’s Law in your (Canadian) Basic Qualification, you will remember series, parallel and series-parallel circuits. If not, check this link:  http://www.physicsclassroom.com/class/circuits/Lesson-4/Two-Types-of-Connections

On the meter, first set the knob to a voltage range greater than the expected voltage. If you don’t know what to expect, set it to the highest range.  DC Voltage range has a V- with a straight line next to it, AC generally a V~ with a wavy line. For example, 2V measures voltages up to 2 volts, and 20V measures voltages up to 20 volts. In our circuit the meter is hooked up in parallel to measure the voltage of the component under test.

The correct answer therefore: In parallel with the circuit.

~ John VE7TI

Make Your Own Single-layer Air-core Coil

Information and examples to make practical single-layer air-core coils.

Whenever you are working on an RF filter, choke, antenna or an oscillator design, most likely you will need a coil. Under some circumstances, the coil might not be available commercially or may be expensive. If this is the case, you have to make your own. This article will provide the information and examples to make practical single-layer air-core coils.

The well known Wheeler’s formula is used to calculate the approximate inductance of a single-layer air-core coil.  In 1925, Harold A. Wheeler published his formula as shown below. This is not a theoretical formula but an empirical one and is accurate to 3~4%.

Where:

     L: inductance (μH)

     d: diameter of coil (inch)     
     n: turns of winding                    
     l: length of coil (inch)

For best result by using this formula, the length of the coil (l) should be equal to or greater than 40% (0.4) of the coil diameter (d). If you study the formula carefully, you will find that the inductance is proportional to the square of the turns. That is, if you want to double the coil’s inductance, you don’t have to double the turns, you just add 40% more turns to the coil. For example, if you have a coil of 47μH and the winding has 100 turns, and you want to double the coil’s inductance to 94μH, you simply add another 40 turns to it, for a total of 140 turns.

Example 1: What is the inductance of a coil if the coil has 86 turns wound on a 1.25 inch diameter round form, and the coil’s length is 1.5 inches? In this case, d = 1.25,  l = 1.5 and n = 86.

Since the input data are only good to (at most) 3 significant figures, the result is only good to 3 figures so you would round the answer to 140µH.  To calculate the number of turns of a single-layer air-core coil for a given value of inductance, re-arrange the formula and it becomes:

Example 2: To build an AM radio, an inductance of 260μH is required. The form on which the coil is to be wound has a diameter of two inches and one inch is chosen to be the length of that coil. Then d = 2 inches, l = 1 inch and L = 260.

Since the coil is 1 inch long, the number of turns per inch is 70 / 1 = 70. Consulting the chart at the end of this article, we find that 28 AWG enameled wire can be used.

To make it easy for you to build your single-layer air-core coil, the author has written a script with PHP to do all the calculations for you. All you have to do is just plug in the desired inductance, the diameter of the coil form, the wire gauge and, if wanted, the operating frequency (for the Q or quality factor of the coil). Then you will be given the number of turns of the coil, the length of the coil and the length of wire needed. Since you know the length of the coil, you just wind the coil tightly to that length, which saves you from having to count the turns.  I hate to do the counting because it is tedious and frustrating when you lose count, believe me. You can fool around with the diameter of the coil form and/or the wire gauge to optimize your coil.

I’ve placed the script on our website.  You can try it out from the link below.

http://www.ve7sar.net/CoilCalculator/CoilCalculator-en.php

The above picture shows the final product of the example 2 in this article.

The coil is wound with 28 AWG enameled wire on a 2 inch diameter pill bottle. 

254μH is measured, which is 2.3% less than the target value 260μH. 

This result is more than adequate for most applications.

Have fun on winding and I hope this has been useful.

~ Hiu VE7YXG

The Power Gate: Keeping The Voltage On

A Communicator Reprise...

 November 2015

The commercial alternatives are good, but pricey. Here is an option for less than $10

In the September and October 2015 issues of the SARC Communicator [and on this blog], we featured circuits that will provide you with a reliable, robust power source. In September 2015 it was Hiu Yee VE7YXG’s simple Gel-Cell Battery Charger, and in October 2015 John Brodie VA7XB’s low cost Battery Monitor Project. This time we’ll round out this series with a device that will automatically switch your station to battery power if the AC fails, and switch it back when the power comes on. It is both inexpensive and simple, yet reliable, as there is only one part.

First, lets look at the commercial alternatives. There are a number of solutions on the market including one, quite expensive, at US$140, known as the West Mountain PwrGate. This device uses solid state devices to charge and automatically insert a backup battery if there is a power outage, and to switch back to the power supply if it is restored.

You will note that the PwrGate above is housed in a large heat sink. This device used Schottky diodes which can generate significant heat. Those fins are there to dissipate that heat. Heat is wasted energy, so we look at an alternative device that is more efficient.

The low-loss PWRGate is billed as being simple, safe, and reliable, and easily able to add backup battery power to your home station or go-kit. The Low Loss PWRgate uses MOSFET power transistors to switch the load between power sources with less than a 20 miliVolt drop, much smaller than systems that use Schottky diodes.  This keeps the power losses to a minimum and delivers full battery power to the load. The device is rated at 25 Amp total, with 3 power outlet ports, ARES standard Anderson Pole Connectors, 3 ozs, and US$49.95 plus shipping by USPS Priority mail. Note that there is no heat sink here, and it does not charge the battery. The distributor, Flint Hills Radio Inc. will also sell you a solar battery charge controller for US$ 39.95 plus shipping and a Smart Lead-Acid Battery Monitor and alarm for another US$ 29.95 plus shipping.

Makes our projects seem pretty reasonable doesn’t it?

So back to the low cost alternative. This device transfers up to 40 amperes at up to 14 volts DC continuously.  It is a safe way to connect both a 12 volt battery and a 13.8 volt power supply to a load, while electrically isolating both from each other.  Whenever your power supply is on, the supply feeds the load, and if you add Hiu’s charger, will also charge the battery, keeping it healthy and ready for use when the power supply is off, or loses AC power, all at a cost of about CA$ 10.00

I did some time in the seventies as a service technician while in my early twenties. One of the products I had some exposure to was alarm systems. In those days before PWRgate, a simple single pole double throw (SPDT) relay was used for the same purpose. The relay is the same as used in many automotive systems. In this application, if the magnetic relay coil is activated, when normal power is on, the contacts switch in the power supply. If the power supply loses voltage, as in a power failure, the magnetic coil is no longer activated and releases the contacts, which then switch in the battery backup. The coil, now deactivated does not rob the battery of any current. A very simple solution with no loss through excessive circuitry or heat. The coil uses a bit of current from the power supply to remain activated, but this is minimal.

These relays are commonly available at auto supply dealers but I ordered mine through eBay and received two, with sockets and mounting brackets, for US$ 3 shipped. They are rated for 12-14 Volts DC and 40 Amps, more than enough to handle the current that most transceivers would draw. Wiring is fairly straight forward and I used three sets of Anderson PowerPole connectors. One for the battery, one for the power supply and one set for the load, being my transceiver. A numbered connection diagram was stamped on the top of the relay I received. The relay coil is wired in parallel with the power supply. If the power supply is on, the relay keeps it feeding the supply circuit. If the power supply goes off current is diverted from the battery.

Once I figured out the contact layout, the actual construction took me only about half an hour, definitely something that can be tackled even by a beginner. Pair this with Hiu’s charger and John’s low voltage alarm and you’re good to go uninterrupted if the power goes out.

~  John VE7TI

Two Battery Monitor Projects

A Communicator Reprise...

October 2015

John demonstrating his meter to the group

Last spring, SARC initiated a competition to see who could construct the most suitable and innovative 12 volt battery monitor for use at Field Day.  Here is what I came up with.

  

The following design criteria were used for my version of the monitor: a) it should provide an analog reading of voltage, accurate to 0.1 volt; b) it should have an alarm that would warn of critical low voltage at an adjustable level; c) the alarm should be prominent but not disruptive to other operators; and d) it should be cheap and easy to build.  Anderson power poles would be the connector of choice.
  
I prefer an analog display as it is easier to discern conditions at a glance without having to read a series of digits on a digital display which may be fluctuating rapidly.

The monitor was constructed around a low voltage FK915 alarm kit purchased on line for US$ 5.95 from www.Qkits.com.  

For the voltage indicator, I found an old analog meter in my junk box, but I needed to change it from a 1 mA full-scale ammeter to a 9-16 volts voltmeter.   

I purchased locally a large red LED to substitute for the buzzer and a cast aluminum box to put it in.  Anderson power poles plus mounting blocks were obtained from QuickSilver Radio Products.   As will be described later, a few other small components were also required.

In order to change the 1 mA meter scale to read 9-16 volts, I calculated that a 16k resistor was needed in series with the meter (R=E/I = 16 volts/.001 amp = 16,000 ohms).  The resistance of the meter itself is not significant in this case.  To provide this resistance and allow calibration of the meter, a 10k ohm potentiometer was put in series with an 10k ohm fixed resistor.  I also added a 9 volt Zener diode so the scale would read 9-16 volts rather than 0-16 volts.
  
The meter, series resistor, potentiometer and Zener diode (all in series) were connected across the input of TR5 transistor and PZ buzzer (or LED in my case).  A potentiometer on the circuit board allows setting of the desired trigger voltage for the alarm.

A free scale drawing program called “Meter Basic” by Jim Tonne W4ENE (figure right) is available on the Internet.  A more sophisticated program simply called “Meter” is also available at a modest cost.  I found the former was adequate for my needs, and allowed me to change the appearance of the meter scale as shown in the figure.

It’s simple but it works.

~ John Brodie VA7XB


-----------------------------------------------------------------

And another monitor...

Keenan VE7XEN also showed off his design for the voltage monitor at the September meeting and promptly walked away with first prize. It was a very impressive professionally produced board with surface mount components.

It reports both under and over voltage and provides both a visual and audible alarm when voltage deviates from the set parameters. Other features:

•   Multiple Alarm Methods
•   Voltage to better than ±0.1V; range 5-20V
•   Parts cost $30 per unit - single supplier
•   Safe, “field serviceable” input connection
•   Small size
•   Programmable Thresholds & Alarms
•   “Mute” button
•   Temperature Readout

See a video demo at URL: https://youtu.be/2tfH_2MmHvI and Keenan’s slides at https://goo.gl/is40MR

Nice work Keenan!

Gel Cel Battery Charger 12 Volt—7.3 Amp

A Communicator Reprise...

September 2015

Two of my hobbies are hiking and camping.  Usually I take my QRP gear with me, including a low capacity 12V gel cell battery, which must be properly maintained or its life is short.  Some time ago, I bought two gel cell batteries at the same time. I used one of them frequently, and put the other one aside as a backup. When the time came that I wanted to use the backup battery, I found it was no longer useful as the battery voltage had dropped to a few volts and couldn't be charged.   A brand new battery went dead simply from disuse.

There is a simple and effective way to maintain the gel cell battery properly, and that is by charging the battery under a constant voltage. If a fresh battery is not being charged from time to time, it will eventually die due to the internal leakage. I use the constant voltage charger that is shown in Fig.1 to maintain my gel cell batteries.

The operating principle of this charger is quite simple. The heart of the charger is a 3-terminal LM317 adjustable regulator. The output voltage is set at 13.5V for a gel cell, but other batteries may require a slightly different voltage.

Diodes D1, D2, D3 and D4 form a full-wave bridge rectifier. The rectified current is smoothed by capacitor C1 and applied to the input of LM317. The output voltage of the LM317 is set by the resistor R1 and potentiometer Pot1. There is an internal voltage 1.25V between the output and adjust terminals of the regulator. As shown in the circuit, R1 is placed in between these terminals. Therefore, the current flow through R1 is 1.25 volts/270 ohms = 0.00463 amp or 4.63 mA. This current also flows through Pot1 and establishes a voltage drop across it. What voltage drop is needed?  As mentioned above, the output voltage of the charger is set at 13.5 V, and we get this output voltage after diode D5.   Since D5 has a voltage drop of 0.6 V in the forward direction, the output voltage of the regulator must be 13.5V + 0.6V = 14.1V. Hence the voltage drop across Pot1 should be 14.1V – 1.25V = 12.85V. Because there is 4.63mA on Pot1, the resistance of the pot should be 12.85V/4.63mA = 2775 ohms. Since there is no standard value resistor of 2775 ohms, instead a 5K potentiometer is used.

Transistor Q1, resistors R2 and R3 form a current limiter. To complete the circuit, the charging current must go through  R2 and R3. When this happen, a voltage is developed between the emitter and base of Q1. If this voltage is higher than 0.6V, Q1 turns on and it's collector draws down the voltage across Pot1.  Hence the output voltage of the charger is reduced and so is the charging current. What should the charging current be to charge a low capacity 12V gel battery? As the rule of thumb, the charging current is one twentieth of the capacity of  that battery. For example, if you have a 12V, 7.3A gel cell battery, 1/20 of 7.3A is 365mA. When 365mA goes through R2 and R3, it establishes 0.73V to the base of Q1. This voltage turns on Q1, therefore the charging current is always limited to 365mA.

Measurement of the voltage between the test points T1 and T2 will tell you the charging current. If the reading is 0.27 V, the charging current is 270 mA; if 76 mV, the current is 76 mA, and so on; 1mV corresponds 1mA.

In the circuit, D5 serves two purposes: first, it prevents damage to the charger and the battery if the output polarity is incorrect; second, it prevents the battery discharging itself through the charger while there is low or no power to the charger.

Under normal operation, there is a 3V voltage drop between the input and output terminals of the LM317, as we have calculated, the output voltage of the LM317 is 14.1V, therefore, the input voltage to the LM317 should be  14.1V + 3V =  17.1V or higher.

If you use DC only to power the charger, you can omit D1, D2, D3, D4 and C1 to simplify the circuit. You can simplify the circuit even more by removing Q1, R2, and R3 if you don't need the current limiter.  Fig.2 shows the much simplified circuit.

When a solar panel is used to power the charger, apply the output of the panel directly to the input of the LM317 rather than through the bridge rectifier, because the bridge rectifier will cause a 1.2V drop.  Even a loss of 1.2V represents a significant power loss under a cloudy sky.

Since the bridge rectifier takes away 1.2V from the power source, then why use it? Well, the reason is that with it, either an AC or DC power source can be used. When only DC power is used, it helps to prevent incorrect polarity hook up. With the rectifier being used, the input voltage of the charger should be 17.1V + 1.2V = 18.3V or higher. 18V is acceptable.

After the charger is built, a simple adjustment is needed. Adjust Pot1 to the mid-range before power is applied to the charger, then use a voltmeter to monitor the output voltage. Adjust Pot1 until the voltmeter reads 13.5V. This is it, all done. Since this charger provides a low charging current, a heat-sink is not needed for the regulator LM317, but no harm in using one.  Fig.3 and Fig.4 show the construction and physical size of this charger.

Because this is a constant voltage charger, there is no over-voltage to the battery to harm it. Also, because the charging current is limited to 365 mA, not much heat will be generated inside the battery. The charger can be connected to the battery for a long period of time and will keep the battery fully charged all the time even there is internal leakage of the battery itself. This charger may be suitable if you want to have emergency power handy to operate your QRP rigs. All the parts used to build this charger are from my junk box. Happy home brewing!

~ Hiu VE7YXG

A Simple Touch 'Code' Keyer

An Inexpensive Circuit With An IC And A Few Parts

Imagine tapping the table to generate Morse Code! This simple code practice oscillator is for those who want to practice Morse Code in a different way, without the Morse key. It can be also used as a touch operated door bell.

The popular timer IC555 is wired as astable multi-vibrator. The frequency (tone) can be changed by varying the 100 K variable resistor between pin 7 and 6 of timer IC555. The volume can be changed by varying the 10 K variable resistor and the sensitivity of touch plate can be controlled by adjusting the 1 K Ohms preset at pin 4 of IC555.

The touch plate is connected to the base of transistor BC147B. In this circuit the length of wire between the base of the transistor and the touch plate is not critical. Typical is a 9 cm wire and a 3 x 6 cm 3mm thick aluminum plate. The addition of a relay or additional circuitry could key your transceiver.

A LED Replacement Light For Your Soldering Iron

A Communicator Reprise: January 2012

One of handiest tools for the Ham Shack workbench is a Weller soldering gun.   It’s a 100 and 140 watt gun in a good old Bakelite case. This year it’s celebrating its 40th year on the work bench.  The thing  has been dropped so many times, it’s a miracle the gun still works, but thanks to several tubes of 5-minute epoxy and some crazy glue, it’s still in fine working condition.

But recently the little incandescent pilot lamp/tip illuminator burnt out and I just couldn’t find the right replacement bulb… a 2.0 volt bulb with a focusing lens at the end, something you used to be able to find in a common flashlight… a common flashlight 40 years ago. So while hunting in my parts bin, I came across a white light LED and decided to update the old gun with a modern light source.

So I removed the bulb from it’s screw base, found the right current-limiting resistor for 20ma at 2 volts and proceeded to assemble my new LED spot light.
There are a few things to ponder… the gun supplies 2v AC, your LED is a diode, so it’s going to work on one half the cycle, which translates into less light output…
Also the junction breakdown voltage is rather low on a LED, so if one were to use  higher AC voltages, it would most likely “expire” rather quickly.  But it’s only 2 volts, so I wasn’t worried.  You could put a 1N4007 in series on the other lead to help the LED deal with reverse voltages, but only if you were working with higher than 6-9 VAC.

So to hold things in place, I potted the resistor in epoxy putty, which set in 3 minutes and then soldered the LED to it.  I only had a ½ watt resistor, larger than I needed but it fit nicely in the screw base.  So if I wanted a 20mA current draw, that would be R = E / I = 2/.02 = 100 ohms and I just happened to have a 100 ohm resistor in my parts cabinet.  P (in watts) = E x I = 2 x .02 = .04 watts or 40 mW of heat dissipation so a ½ watt resistor wasn’t necessary, but it was the only size I have in stock and size wasn’t an issue.

A few minutes later I had a modern light source in an antique tool… but would it work?  But of course… for ½ the cycles per second -- so the light from it wasn’t as bright as I was hoping, but good enough to shed a bit of illumination on what was being soldered and certainly adequate for a pilot light to verify the gun was on.   OK for younger eyes, but this old buzzard needs a few more candle power!  Why didn’t I put a tiny bridge rectifier on the power leads to feed the LED with better DC?  Cause it was only a 2 volt tap off the coil inside the gun.. and for every diode you insert, you lose 0.7 of a volt.   Why did I need this in the first place you ask?  Well the tips of my fingers and tongue hadn’t recovered from my earlier attempts to see if the soldering tip was getting hot!