Electret Microphones

The Electret Capsule or "Condenser (Mike) Insert" is a high quality and very convenient microphone element which lends itself to a wide range of home-brew or bespoke accessories. While the exact dimensions of the transducer capsule may vary from one supplier to another, 9mm (Ø) x 6mm is quite typical and is considerably smaller than most other accoustic transducers. Thus, its most likely first use, by a new Cairo-ist, is to bring back into service an old microphone assembly that previously housed a now-damaged or insufficiently sensitive insert (e.g. an electrodynamic transducer). A simple means to achieve this up-grade is to mount the electret capsule in a semi-rigid spider.

Items Check List : Electret Capsule Insert = Maplin FS43W

Wiring Note

The Electret principle is that of a pair of very small capacitive plates, one of which varies its position with respect to the other, in response to small variations in incident air pressure. Being capacitive, the plate impedance is very high so an FET is integrated into the assembly to act as an Impedance transformer. As noted elsewhere, this FET needs an excitation voltage (4.5V is optimum, but they 'tolerate' the range 1.5V to 10V d.c.) and the external connections may be presented as either a 2- or 3-terminal device.All CAIRO applications only require a 2-terminal device.

The 3-terminal versions are wired as FET Source-followers and may be reduced to 2-terminal equivalents, either with a de-coupling capacitance between 'O/P' and '0V' (if space allows) or a direct short (e.g. a solder bridge), so that the drain ('+V' pad) is Mic-High, and the bridged pads are Mic-Low. (In the CAIRO scheme, the impedance defining Drain-load resistor is in the rig-adaptor.) On the 2-terminal versions (often called "PCB-mounts") the Source terminal pad (Mic-Low) may be identified by a track to the capsule body. Such capsules must be isolated in metallic housings to comply with the CAIRO mike isolation rules.Only the mike cable screens must be connected to the metallic housing.

Paper-clip Spider

Unfold two standard paper clips and straighten any severe kinks.(Only one is illustrated, left.) Select a pencil (or similar 'rod') with a diameter that is approximately the same as the electret capsule. Place it in a vice and bend the central portion of each paper-clip wire to form a 4500 loop that has two straight ends emerging at right angles from the overlap (a one-and-one-quarter turn).

Meanwhile, wrap the electret capsule in one complete turn of draught-excluder tape; available from any DIY store as a self-adhesive strip of PVC-foam. Adjust the two paper-clip loops over the foam so that there are four arms, protruding aproximately at right angles to each other. This forms a convenient spider for locating the electret in the void in an accessory that previously housed a discarded microphone element. The draught-excluder tape provides electrical isolation (necessary in metallic housings) and cushions the capsule against mechanical vibration.


In some cases, where the void in the original microphone housing is quite spacious, it may be possible to also include a miniature loudspeaker, behind the electret spider, to create a complete speaker-mike accessory. The well-known tulip mike (from Pye) is a good candidate for this significant 'improvement'.

Long-Reach Microphone

In some "cluttered-desk" circumstances it is convenient to use a microphone that has a long arm that stems away from a firm base to 'float' in the free space near the user's face. Certain cantilever desk lamps (e.g. Anglepoise) can be modified to achieve this.

Unthread the mains wiring and remove the lampshade 'trumpet' to replace the latter with a small module of approximately matchbox proportions. (Some styles of in-line mains cable (10A) connectors may suffice.) Install an electret capsule at one end or forward-facing surface of this 'module' and route a two-core individually-screened cable through the other end or rear surface. This feed cable is then threaded through the cantilever assembly to follow the same route as the previous mains cable.

Before closing and fixing the module to the cantilever, weigh the discarded lamp-shade assembly (with a bulb fitted) and weigh-out an equivalent mass (approx. 200 grams) of lead shot (as available to anglers, for example). Enclose the shot in a small polythene bag that can be adjusted around the inside of the module to maintain the physical balance.

Gooseneck Mike

Removal of the magnet from the 'head' of the RS magnetic retrieving tool, allows an electret capsule, with one turn of insulation, to fit the collar. Slender twin-core overall-screened cable will then pass inside the gooseneck helix. Heat-shrink sleeve adds extra rigidity to the stay-put helix particularly when the original full length (= 13", 33 cms) is retained. Heat-shrink might also be suggested as a means of adding colour (for labelling purposes) in some (multi-mike) circumstances. If so, it should be heat-shrunk into place, before the Electret capsule (and cable) is installed. (Excessive heat 'discharges' an Electret.) In place of the original handle, a DIN-5 plug may be installed with the gooseneck helix emerging directly from the plug, thereby creating a one-piece, plug-in accessory.

(These can be made to look as good as any 'professional' equivalent and I've used them in formal Conference and Stage settings before now, where they are visible to delegates or audience.

Illustrated : A gooseneck with black heat-shrink sleeve in a base with Red-button PTT-switch
and two more gossenecks; chrome, i.e. no heat-shrink, and red sleeve.)

Items Check List : magnetic pick-up tool, [542-144] ; twin screened cable, [367-224] ; 9.5 mm heat shrink [398-177]

Gooseneck Base

The above Gooseneck Microphone is a particularly convenient item that may be used in a wide variety of circumstances. As an alternative to the cantilever or long-reach microphone, it may be simply plugged in to a stable base for desktop use.

A variety of household items can be pressed into service as a base; e.g. an old saucepan lid of diameter between 4"-6" or 10-15 cms. Select one that previously had a knob that was secured by a single, central bolt (rather than a two-bolt handle). Remove the knob and punch a single DIN cut-out in readiness for a circular ('professional') DIN-7 socket. Also drill a hole for a PTT button to be installed on the lid surface, 'in-front' of the mike socket. (Check that the intended position has enough room for the switch body to stay within the lid's void.) Drill a cable departure hole in the 'saucepan-sealing' rim of the lid, to the perpendicular rear of the PTT and socket alignment. (In some cases, this hole may be drilled through the 'shoulder' where the rim turns (out) to become the lid surface.) Before fitting and wiring the electrical items, consider spraying the entire lid for the finished item to have a smart appearance. Matt-black aerosol paint is a general-purpose finish, but other colours may be preferred to match a coloured-stem Gooseneck Mike. (The base, illustrated above, was rescued from an old fireside companion set.)

Items Check List : Matt Black Aerosol, [496-783]


In use, the Palm-Springs desk accessory resembles a computer mouse. For an operator working at a computer, the actual computer's mouse would be positioned to the right of the keyboard and worked with the right hand, as normal, while the Palm-Springs accessory, in a mirror position to the left of your keyboard, is operated with the left-hand.

(This arrangement assumes right-hand dominance and might be reversed by the left-handed operator. In this respect, the Palm-Springs is an unhanded accessory.)

The base unit is an ABS box that fits the palm of the hand, and has a gooseneck mike emerging from the rear of the top surface to pass between the 'third' and 'ring' fingers. The box has a pair of genuine feet at the rear and a pair of PTT push-button switches as the front "feet" . Thus it is palmed down for transmission and springs back up when released; as befits a "palm"-"springs" accessory !

For reception listening, two versions are suggested.

One version may have a DIN-7 socket installed on the rear side panel, as a convenient receptacle for (lightweight) headphones. If so, the socket may be accompanied by a 'pre-set'-type cermet volume control, mounted in the centre of the front side panel. This position ensures its body does not foul the bodies of the PTT switches inside the module.

The alternative Palm-Springs has a small loudspeaker installed above a simple array of holes drilled into the base. These are set out in the central space between the pair of real feet and the PTT feet. In use, the sound is emitted towards the desk surface from where it diffuses around the operator's hand. It may be accompanied by a 100R cermet potentiometer installed in the rear side panel (where the DIN-7 socket would be in the headphones' version).

In both versions, the wiring of PTT feet should be such that the press-to-make contact pairs of both switches are in parallel. Thus, either 'foot' will engage the PTT circuit even if the operator's palming action is unevenly applied. In the speaker version, it is useful to select push switches that have an additional pair of press-to-break contacts. If so, these are wired in series with each other, between the potentiometer wiper and the loudspeaker element. This arrangement ensures that the speaker becomes disabled during transmission, so that the item could not be used in full duplex circumstances (where the close proximity of the speaker and microphone could lead to 'howl-round' ).

Items Check List : ABS Box, ~ 85 x 55 x 40 [502-348];
PTT 'feet' mini push-button,1 n/o, 1 n/c [320-657] ;
Feet Stick-on, square 12.7 x 12.7 x 5.6 [543-327];
Speaker Miniature, paper-cone ~ 40mm [248-476]

Lavalier Microphones

These accessories hang by a neck-chord at a mid-chest position from where they may be lifted to a near-mouth position for transmission. Few commercial sources (if any?) supply these but the Author commends them as a particularly versatile accessory.
Use some form of simple 'tube' (e.g. a single-battery torch) with an electret capsule in its 'top' face, a PTT button across the body, and a spectacles' neck chord anchored to the upper 'half' (above the centre of gravity). A convenient length of CAIRO Line enters the 'base' together with the hard-wired tail to a single earphone.
They handle very well in Control stations, but are equally suited to outdoor operations, e.g. hiking or biking, when worn beneath outermost garments for reasonable protection during very inclement weather. (The transceiver itself, e.g. a hand-held, would be installed inside one's rucksack or saddle-bag in such conditions.)






Supplementary Material


Remote Operations

CAIRO Basics

CAIRO-8 Basics

Raynet Packet

CAIRO Installations

CAIRO-8 Relays

CAIRO Relays

Relay Installations

Summary Configuration

Data Card

Handy Hints