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Microphone switcher

 
Microphone switcher

Here is the description of a realization to switch a headset, a telegraphy key, an external speaker to four transceivers.

It also includes a switchable compressor, powered by the transceiver used.

On the front panel, there is an SP/MUTE switch, a volume control for the headphones, an ON/OFF switch for the compressor followed by its output level setting and finally the switch to select the transceiver.

On the side, the output to the external speaker, the input for the telegraphy key, the output to the headphones and the microphone input. These last two are side by side to allow to easily connect a headset.

On the rear, there are the microphone, SP, CW outputs for four transceivers.

Obviously this configuration corresponds to what I wanted for my station. The advantage of building something to his idea is to have the choice of ergonomics -especially when one is left-handed-, functions, aesthetics.

 

The schematic diagram

Microphone switcher

For practical reasons, the pins corresponding to the UP, DOWN and FAST commands are not switched and therefore not connected. Only MIC, MIC ground, ground, PTT and +V pins are used and all microphone 8 pins connectors are wired for YAESU.

Important, the grounds are switched and never connected to each other or to the box. The shield of the compressor part is only connected to the microphone ground line.

A small precision concerning the +V presence LED. It is curiously powered by a FET transistor whose reference is of little importance elsewhere. In this configuration, the transistor is a constant current generator and provides a few mA, enough to power an LED. It can thus be applied to the drain from 3 to 30V without the brightness changing. As the +V varies between 5 and 8V depending on the transceiver used in my station, the LED always lights up in the same way.

 

The wiring plan

Microphone switcher

It has proved indispensable in order to ensure that we do not forget anything. As the chosen switch has three stages of three circuits, the last connected wire necessarily cover the previous ones and it is better not to be wrong.

The loop around some wire evokes the shielding of a coaxial cable and the wire that comes out, its ground braid.

 

Supplies

The switch
Microphone switcher

The first difficulty is to find the switch. Before even going further, it is the one that conditions everything else. There is a commercial product that uses straight switches and you can also find old push button selectors.

For my part, I wanted a rotary model and with the number of circuits necessary, necessarily a wafer switch. Well, as surprising as it may seem, it is very easy to get wafer switches at a very reasonable price. These are "NOS" products for "New Old Stock", in short articles never used or mounted, dating sometimes from the 50s or even before!

So I ordered in Ukraine a switch of Russian origin, guaranteed any climate, airtight, supplied with its button and fastening screws. It has four positions for nine circuits, exactly what I needed. In addition, it is tiny, only 55mm in length with regard to the part that will be in the case for a maximum diameter of 30mm contacts included. If you make the same choice, be aware that the screws provided are M2.5 with rounded head. It will be necessary to obtain countersunk heads if you wish that they do not exceed the frontage.

Microphone switcher
Microphone switcher

Small precision on its wiring. Each wafer has twelve pins and the switch interconnects the wafers in pairs. Here we have four positions, so three circuits per pair of wafers. Thus, when the switch is in position 1, the pin No.1 of the first wafer is connected to No.1 of the second. Since there are three four-position circuits on a wafer, No.5 is also connected to No.5, No.9 with No.9. If we go to position 2, it will be No.2 of the first wafer that will be connected to No.2 of the second and we will also have the pairs 6-6 and 10-10. It is therefore necessary to connect together Nos. 1,2,3 and 4 of the first wafer to make the common of the first circuit, the same for 5,6,7,8 and 9,10,11,12.

Same thing for the second pair and for the third. This configuration is different from conventional switches which have a single pin -sometimes decentered- to form the common of each circuit.

 

The case
Microphone switcher

Its dimensions are mainly conditioned by the number of connectors to install, in all eleven jack and five microphone connectors. I finally opted for a small model 16cm wide, 4 high and 8 deep, aluminum with the advantage of having sides joining the two facades. This makes it possible to remove the covers without having to disconnect anything and thus to intervene easily for the wiring or later possible maintenance.

 

The compressor
Microphone switcher

It is not essential but if it interests you, it is a well known circuit, the SSM2167. This circuit is already easily mounted on a small module containing the CMS components necessary for its operation. You can set the noise threshold and the compression ratio by setting the values of two resistors.

Since I only use a Heil-Sound headset microphone equipped with an HC-4 cell and therefore without preamplification, I did not put any input level adjustment, only one output level.

This module is powered by the transceiver permanently, even when not in use. But we can replace the bypass switch by a triple inverter and thus add the power cut.

I mounted this module on a portion of printed circuit with a 5V regulation in order to feed it correctly. Indeed, the +V can be 8V on some devices. When the +V = 5V, the regulator does not work anymore and leaves a little more than 4V. Nevertheless one can just as well put a 3,3V regulator, the SSM2167 working perfectly from 2V.

 

List of supplies

Here is the list of supplies used with their cost. It is a project that comes to about sixty euros without counting the cords that will be needed to connect each transceiver.

list of supplies

 

Construction

some pictures are better than a long talk.

The case is fixed under a shelf of the station by a "dual lock" type velcro, which has two identical faces, which makes it easy to remove it if necessary. This model is very effective because much stiffer than the classic velcro, the action on the switch does not even move the case, it looks like it is fixed by screws!

 

The front panel

serigraphy

Here is how I practice to realize my facades of projects.

First, I draw a plot using a drawing software that manages the layers. Here, it is about Fireworks, an old program dating from 2006. It is enough to choose the layers necessary to print only the marks of drilling for example or to print the final serigraphy.

I chose a resolution of 150 DPI ,so dots per inch.
Each real measure is thus converted, starting from its value in cm: First divided by 2.54 then multiplied by 150 and this gives the number of necessary pixels. This gives you the certainty of printing the project to the right dimensions.

Then comes the printing on photo paper and then its lamination. All that remains is to make the holes in the punch and cut precisely the facade thus produced. It will be held at the same time by the nuts of the switches, the potentiometers and by stickers sticking double face for the angles.

During the first tests at the time of redoing the facade of my first kit, Miss Mosquita -40m QRP CW-, I tried to laminate matte but the result is unfortunately dull, the plasticization forming a visible veil over the screen printing. It's finally quite standard laminating sheets and printing on glossy photo paper that make it better. There are of course reflections when a light source is in front of the facade, but the graphic rendering is much better, the serigraphy coming out very well.

A curious effect occurs regarding the color. As the serigraphy is well printed with the background color of the above preview (# 272F3A) which is dark gray pulling towards black with very little blue. But the combination between the photo paper and the lamination brings out the blue according to the brightness as can be seen even more in the photos. Surely a question of absorption of light as a function of wavelength.

 

In use

I plugged three transceivers on this case:
a small QRP 40m CW -The Miss Mosquita II- which however has only a headphone output but is largely powerful enough audio to power the external SP of the station while allowing to use the headphones of the headset. And no need to unplug the CW key to switch from one transceiver to another.

The FT450D with which I use the headset alone, but the compressor can be useful.

and another QRP, the ILER40 with which I engage the compressor to optimize its small 5W. Again, for the CW, no need to disconnect/reconnect the key to switch from one device to another.

 

Conclusion

It really is a practical tool that brings real comfort in the station. As the various devices are connected in the same order as they already had on the antenna switch, it's a breeze to switch from one transceiver to another by simply selecting the same number on two switches.

It is already essential and moreover, there is no longer a cable that hangs on the tablet of the station.

 

Resources

 

 

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