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Tony - LU7DTS - Campana - Argentina
My friend Ermes, IK4OLY from Carpi (MO) Italy has made me know a circuit that he developed
based on an earlier project aimed to control an UHF repeater. I thank Ermes to allow me to
publish his electrical circuit and picture of the project. While the original circuit operates at 12
volts, Ermes modified some component values in order to be powered with 5-volt aiming to use
with portable PC (notebook or netbook) from their USB port can be obtain +5-volt to power the
interface
Circuit details
The project includes a vox control, just to avoid using serial port. A very common integrated
circuit like a LM358 dual op amp and the Optocoupler 4N32 were used. Many amateurs prefer a
total isolation between the PC ground and the transceiver VHF / UHF ground . So you will see
that the printed circuit board ground will be connected only to PC's ground, that ground will be
completely isolated from the TRX ground mass.
To achieved this objective two audio transformers (600 ohms 1:1 ) which maintain isolated. the
ground masses. These transformers similar to those used in earlier version of telephone
modems.
The resistor R9 value should be between 330 ohms to 1.2 Kohm; a larger value reduces the
brightness of the LED and therefore the power consumption during VOX activation.
The VOX time delay is controlled by the transistor Q1 (BC337), which loads the electrolytic
capacitor C2 through variable resistor VR4. The delay will result between 0.2 and 2.5 seconds.
To increase this delay time you can change the variable resistor VR4 from 500 Kohm up to 1M
and selecting C2 with a capacity of 10 µF. These changes should allows you to control the
VOX time delay between 0.4 and 5 seconds.
In order to facilitate the
construction of this interface I
designed a circuit board using
PCBWizard software. Fig 2 to
the left shows components
mounted on PBC .
On upper part of PCB find the
VOX control circuit. Below is the
audio circuit using two audio
transformers in order to isolate
the PC and TRX grounds
The 4N32 is an Optocoupler with Photodarlington Output transistor. In case you have in stock a
Photoisolator like 4N26 with transistor output I designed the PCB taking into account both
possibilities: the 4N32, which includes a Photodarlington output and 4N26 type that has only a
single NPN transistor as output. On the left find the figure where you can see the circuit using a
4N26, in this case a NPN transistor BC337 should be added.
To the left figure shows how to wire
the PCB when using Optoisolator
4N26 wich includes Q2, BC337.
To the right you can see how to wire
the PCB when using a 4N32
Optocoupler. In this case, instead of
using an additional BC337 (Q2)
transistor, a flying wire must be
place between the base and emitter
pads as shown.
If the transceiver PTT requires less than 100 mA, you can use the 4N26 Optoisolator alone,
without BC337 (Q2) and put the flying wire as it was explained in 4N32 PCB wiring.
Fig 3 shows the mirror image of copper side of this printed circuit board (PCB).
By clicking on the Fig 3 image you can
download a PDF file containing this design.
Please use this PDF file without reducing or
enlarging when printing. It must be printed
actual size (100%).
IK4OLY Protopype
Fig 4 shows the project prototype made
by Ermes, IK4OLY on a single side
perforated board. As mentioned before
this prototype was made to be used with
laptops, notebooks, or netbooks that
have no serial port. The interface is
powered from USB port (+5 V)
Fig 5 shows the same
interface project done
on a printed circuit
board (PCB).
PCB was design using
PCBWizard program.
Nella Fig 6 si può vedere il circuito stampato
(PCB) lato rame. La Fig.7 invece mostra il
“layout” dei componenti sul circuito stampato.
.
Fig 8 shows the preset calibration of control circuit VOX: VR3 controls audio input level, VR2
controls VOX gain, VR4 controls VOX delay ("tail"), between 0.2 and 2.5 seconds.
Eventually you can manage to increase the VOX delay time by changing VR4 value up to 1 M
ohm and C2 up to 10 uF as explained before.
EchoLink will continue sending audio to the station on the Internet for a short amount of time after
the signal stops.
Consumption:
This current in standby is less than 1 mA while during activated VOX the current should rise up to
approximately 10 mA. As you can see this interface is very suitable for portable PC do to its very
low consumption.
Additional information:
Those of you who wish to modify the design of this interface can download a compressed zip file,
where you will find the PCBWizard file for PCB design as well as the LiveWire file for electrical
circuit design.
To download this compressed file click here
In case you prefer not to use the PCB boar along with all connection cables permanently attached
to it, is possible housing this project in a suitable box. The alternative is to use a box with panel
chassis connectors to avoid connection cables to be attached permanently to the box.
Obviously this is not the only possibility left to the reader
investigate other valid alternatives.
For my project I made the box using PVC free foam sheets
which are easy to handle, cut and drill using conventional
tools. Also very easy to glue with cyanoacrylate glue or similar
type.
I put a 4 pin mini-USB (type B) chassis connector in order to
use a standard USB to mini-USB cable (Type A to Type B) to
get the +5 V form. Fig 9 shows details of USB connectors A
and B.
.
Fig 10 shows the front side of the
box where I put a potentiometer
(VR4) to control the VOX delay, D1
yellow LED to indicate when VOX is
activated and an additional red LED
(with a 1.2K resistor in series) to
monitor the +5 V.
Fig 11 shows the back side of the container box where I put the mini USB type B connector and
two 5-pins male panel chassis connectors, one for the TRX and the other one for the PC.
Fig 12 shows the top view.
EchoLink Sysop Setup
If you are going to use a EchoLink Sysop Fig 14 shows
how to Setup the TX Ctrl .
You must activate “External VOX”
Acknowledgment
I wish to thank Ermes, IK4OLY in giving me the opportunity to publish his project on this site, as
well as the spelling of the page in Italian language.
Final Words:
This interface is very simple to make and setting up. Very versatile for digital modes and for those
who want to install an EchoLink Sysop.
I encourage you to make this project and I wish you an excellent construction.
Tony - LU7DTS
Fig 2 Printed Circuit Board Components side.
Fig 3 Mirror image of copper side
Fig 4 IK4OLY Interface prototype
Fig 5 PCB finished: Component’s side
Fig 6 PCB finished: Copper side.
Fig 8 VOX circuit setting
Fig 7 Components layout
Fig 9 USB connectors A and B
Fig 11 Back side of the box
Fig 12 Upper side of the opened box
Fig 13 Finished VOX Interface
Background
Fig 14 - EchoLink Sysop Setup
Fig 1 Electrical circuit of this VOX interface