K9HZ Full Rig Power Control Unit

Bill Schmidt, K9HZ has  designed a fool-proof control circuit for the uBITx for power control.  This circuit prevents bad things from happening by shutting down the radio before any damage is done. 

It faults on:

  1. reverse voltage
  2. over power
  3. High SWR
  4. High PA Current 
  5. High voltage. 

It provides a visual indication of WHAT fault occurred, and the individual fault LEDs begin to flicker BEFORE the trip so you can fix the problem before you hit a hard trip. 

The fault conditions listed above can be expanded to any number by adding more SCR Trip components (they are set to trip at 1.8V whatever the fault is).

 When initially turned on, the transistorised RS Flipflop circuit comes up in the “Operate” mode.  If a trip occurs, it flips into “FAULT” and shuts down the PA. 

The circuit is reset with the “RESET” button, but ONLY if the fault has been resolved.  Turning the power off and on resets the circuit too.

Bill bread-boarded the circuit last week and has been using it on his radio for a while and found that it works flawlessly (yes transmitting and yanking the coax off the back of the radio shuts down the PA nicely!).  The circuit and a build list can be found in the BITX20 list’s files section.

Parts List for the uBITx Power Control Circuit

Capacitors Value Voltage
C1 10uf 16VDC
C2 0.01uf 50V
C3 0.01uf 50V
C4 0.1uf 50V
C5 0.1uf 50V
C6 0.01uf 50V
C7 0.01uf 50V
C8 0.01uf 50V
C9,C10, C11 0.01uf 50V
D1 Green LED
D2 1N4148/ 1N4001
D3 1N4148/ 1N4001
D4 Red LED
D5 1N4148/ 1N4001
D6 12V 0.5W zener 1N759, or 1N5242, or 1N6002
D10 BT149G SCR
D11 1N4148/ 1N4001
D12 1N4148/ 1N4002
D13 1N4148/ 1N4003
D14 1N4148/ 1N4004
D15 Red LED
D16 Red LED
D17 Red LED
D18 Red LED
D19 SB530
Q1 2N3904
Q2 2N3904
Q3 2N3904
Q4 2N3904
Q5 2N2222
Resistors Value Watts
R1 1K 0.125
R2 1K 0.125
R3 10K 0.125
R4 10K 0.125
R5 10K 0.125
R6 10K 0.125
R7 10K 0.125
R8 100K 0.125
R9 2.2K 0.125
R10 47K 0.125
R11 1K 0.125
R12 2.2K 0.125
R13 1K 0.125
R14 1K 10-turn POT
R15 10K 10-turn POT
R16 88K 0.125 Can just use a 100K POT set appropriately
R17 12K 0.125 Can just use a 100K POT set appropriately
R18 10K 0.125
R19 100K 0.125
R20 1K 0.125
R21 1K 0.125
R22 1K 0.125
R23 1K 0.125
R24 10K 0.125
R25 10K 0.125
R26 10K 0.125
R27 10K 0.125
R28 62 OHM 2
R29 1K 0.125
R30 1K 10-turn POT 0.125
R31 10K 0.125
R32 100K 0.125
SPST Momentary contact
Integrated Circuits
U1 LM339 (Make sure to connect Vdd and ground!!!!).
U2 BTS660P
Fuses Value Voltage
F1 1A Poly Fuse 50V
F2 4A Poly Fuse 50V
Your choice

Debouncing a Rotary Encoder

N5IB reports, “ALPS, a maker of rotary encoders, recommends 10K pullup to Vcc, then 10K in series with 0.01 uF to ground. The internal pullup in the ATMega is loosely specified – somewhere in the tens of K, max 50K.

Jim Sheldon W0EB responded with, “This settled the really cheap and modified (to take the detents out) encoder on the test set right down. Tuning is extremely smooth and I don’t notice ANY digits showing up and then backing up again as it did before.

“I can highly recommend adding a 10K external pullup to both the encoder A and B inputs as well as an additional 10K in series with .1uF capacitor to ground on both the A and B inputs to the Raduino card.

“It was a nice surprise addition and I won’t leave them out again.”

Hans G0UPL responds, “Debouncing and pullups are also possible in the firmware. This is the method I use in the QRP Labs kits like QCX http://qrp-labs.com/qcx – look at the schematic: no pull-ups, no RC-debounce. Saves 6 components (4 resistors, 2 capacitors). It’s not important in a one-off build or modification but in a kit where you are trying to optimise cost, every resistor helps! The firmware method also gives you more control over how you do your debounce. I prefer the state-machine approach to rotary encoder handling, it implicitly debounces without involving any time constants.”


Guide to Arduino Coding

The best book around for learning how to program your Raduino was written by one of the BITX20 regular contributors Jack Purdum W8TEE.   It is entitled “Beginning C for Arduino”  and can be found on Amazon.

Jack says, “Make sure you get the 2nd edition…it’s a better book and has a chapter on C++ so you can “understand” most library code.”

KF2510 connectors on the uBITx

The connector type used on the main board of the µBITx and the Raduino are of type KF2510.  These were developed by MOLEX and are of the polarised and locking type with 0.1″ spacing between pins.  They are commonly available  everywhere.  In the US they are available from Tayda and Mouser.  In Australia and New Zealand they are stocked by Jaycar.  If you have time for delivery they can be obtained cheaply on AliExpress and Ebay websites, including in sets in a plastic storage box.

The KF2510 comes with both straight pins (for connectors to a board) and with 90 degree pins.  The 16 pin connector between the Raduino and the main circuit board uses the 90 degree type on the Raduino end, and straight female socket on the main board.

Connecting wires to the female connectors is straight forward.  Line up a section of wire (stripped back by around 1/4″) and use needle nose pliers to crimp the bottom-most crimp section first, and then the top one.   Most of us apply a dab or solder between the crimps to secure the wire firmly to the female pin.

The female pins just push in, but must be oriented correctly in order to be held firmly in the socket.  If they come straight out you are putting them in backwards.  The female pins are easily removed by pressing on the back of the metal pin with a flat-head screwdriver by reaching through the little window  and at the same time pulling on the wire.   Rather than curling up your spare wire connectors, it is better to pull them out of their socket for storage  in the junk box.  They can be reinserted when you want to use the connection again later.


Maximum Safe Input Voltage

Paul K0ZYV asks, “What is the maximum safe input voltage to µBITx?”

Paul has lithium ion batteries that provide up to 4.2 volt when fully charged, and he hoped to put four in a pack which could max out at 16.8 volts providing about 2300 maH to power the µBITx.

The consensus seems to be around 15v is the maximum voltage that should be applied to the µBITx.

The reason is that the audio amplifier absolute voltage limit is 15V. All the other components can handle the 15V voltage.  This assumes of course that the 5V regulator on the raduino has a heatsink and better still has a series resistor to limit power dissipation.

Regulating the voltage when using Lithium Ion Batteries

VE7WQ uses a $1.45 Boost Buck DC adjustable step up down Converter XL6009 Module with a 4 cell 18650 Li-ion Rechargeable Battery pack.  This has the following characteristics:

Wide input voltage 5V ~ 32V;
Wide Output Voltage 1.25V ~ 35V
Built- 4A MOSFET switches, efficiency up to 94%.


Faulty TX/RX relay

Glenn  VK3PE suggests, “For anybody trying to get their uBITX to work in Tx mode.  Mine was working then started to become intermittent. I would key the Mic PTT switch and the display shows it’s in Tx mode, but with no power out.

“Tracing signals etc, I found that the K1 changeover relay was not switching 12V to the Tx section. I could hear the relay ‘operate’ but it didn’t actually switch.    I replaced the relay and all is fine.”



Reclaiming Pins on the Raduino

Don Cantrell ND6T has reclaimed two relatively unused pins on the Raduino for reuse.  Check out the details here or on his website.

When the μBITX was manufactured as a “semi-kit” the Raduino control module was designed to simply plug into the main transceiver circuit board. This made wiring easier and also permitted close and controlled connection for the high-frequency synthesizer outputs. Much more reliable than extra cabling and shorter runs, too. The downside is that more of the input/output pins from the Nano micro controller are dedicated to native functions of the radio, and only two are available for additional modifications by the user.

One of those pins is already used as the key input for the CW mode. That left just one pin, an analog capable one, to be used for such purposes as monitoring supply voltage, an S meter, an RF power meter, or VSWR indicator.

The two available “spare” pins are the only two that do not have an internal “pull-up voltage” option available on the Nano processor. These pins are designated A6, and A7. In the initial design the A6 pin is used as a CW key input and so requires an external resistor to the +5 volt supply in order to detect key closures. That detection requires an Analog to Digital Conversion (A/D conversion) and consequent decoding and processing of the resultant measurement in order to determine each key closure.

It worked, but was highly prone to error, especially if key, cable, and connectors had anything but excellent low resistant contacts and connections. I found myself regularly burnishing key contacts and having to solder across every press-fit junction in both key plug and jack. Add to that the added shielding and bypassing of the key line and there still remained an annoying glitch or two at high speed. Ah! Opportunity!

Digital pins D0 and D1 are used for serial communication. D0 is labeled “RX0” on the board, D1 is labeled “TX1”. If you were to use an RS-232 arrangement, these would be the pins that you would have to use. When you use the serial monitor (like my pocket generator uses) then these are the two pins used. Even when you program the Nano, the on-board USB controller circuit uses those pins. However…

When you are not using a serial communication function, these two pins are “up for grabs”!

Don ND6T opted to use D1 for a key input. As long as I don’t have a key plugged in and  pressed, the program loads nicely into the Nano and no one is the wiser. Since this pin has pull-up voltage capability, I don’t need an external resistor. Without A/D conversion the key is sampled in microsecond intervals. Since it uses TTL logic levels (0 and +5 volts) it is not prone to poor resistance connections, bypass, or shielding problems. Works like a charm.

Additionally, these two pins have their own little LED indicator lights on board the Nano. When one of the pins is taken to a low state (close to ground potential) the corresponding LED lights. Those nifty little indicators are usually hidden since the Nano is placed between its circuit board mounting and the display, but if you position yourself just right then you can see it light up. Nice little trouble-shooting extra!

Use the attached photo to locate the two pins on the back side of the board. The end pin is D1 and the next one is D2. Yes, they are out of order with the remainder of the pin numbers in that row. Cut a two pin section of right angle .1” spaced header block. Use alpha cyanoacrylate (Krazy Glue®) to mount the body of the plastic block to the printed circuit board, with the pins snugged up next to those two on the circuit board (D0 and D1). Then solder the new connector to those pins. That makes it convenient and easy to connect to your key jack or whatever else you want to sense or control.

The remaining digital pin is a good one to use for an added Function switch. Don uses it as a convenient access to a menu for changing sideband modes, switching VFOs, activating RIT, or adjusting my IF band pass.

That leaves two analog inputs. Hmmm!


Adding an RF Gain control

Don Cantrell ND6T has posted details of how to add a simple RF Gain Control mod to the uBITx on his website.

Almost any potentiometer, from 1 Kilohm to 5 Kilohm, can be used to add an RF Gain control.  This will make a nice addition to the transceiver. If you have an audio taper pot (logarithmic pot), that’s even better.

Locate the trace (assisted by the photo above). Use a knife ( e.g. X-acto number 5 ) to carefully scrape the coating from the trace and to cut a small 1mm (or so) separation where the connector header will go.  Cut off a section of two pins from some .1” right-angle header stock and use pliers to form the short pins to contact the newly bared copper trace while the plastic portion of the block was flat against the printed circuit board.

Carefully tin the trace where the header pins will connect to it. A bit of alpha cyanoacrylate gel glue will help fix the block in place in a few seconds. Then solder the pins to the board. This makes a very convenient access point, one that can be easily disconnected just as you would the other plugs that connect this board. If you don’t want to use it, just place a standard shorting plug on it and you are back to normal operation. Want to add an AGC circuit? This would be a place to plug it in.

Use small shielded coaxial cable to connect the control to the board. I use common RG-174 type. Tie the shields together at the control end only, not the plug end.

Use a 2-conductor section of female .1” spaced header stock for the plug. I strip the shield on each of the cables back about half an inch at the plug end and bare about an eighth of an inch of center conductor. I slip an inch length of heat shrink tubing over that end of the two cables and slide it back out of the way for the moment. I then solder the bared center conductors to the plug pins, test them, and then cover the plug and cable end with the heat shrink. I apply heat to shrink the tubing, making a nice form-fitting cover and strain relief.

About 26 dB of control is achieved with this arrangement.