Faster Than A Patch

's Soeed Dialer

Dial home while driving by simply pushing a single button on the microphone.

Craig Oichton K7UKW 2540 Esst 12th Street The Dalbs OR 97058

Its Friday afternoon You're caught in heavy rush-hour traffic. You're going to be late getting home. It would be nice to punch up the patch and advise your wife to adjust dinner time, but do yon dare risk dialing in this traffic? Sure! If your rig is equipped with thh speed-dial circuit, your eyes never have to feave the road This article describe* circuitry which allows a telephone number to be dialed at high speed by simply pushing a single button on the microphone.

Most hams that operate FM mobile are sooner or later faced with a dilemma—we feel the need (or urge) to use the autopatch while driving. The need is not urgent enough to pull off the freeway or busy street and parkr and trying to manipulate a DTMF pad while driving is certainly not worth the risk And yet it really would be nice to make that call I have found myself in this situation many times. This was the reason behind the development of this speed-dial circuit

There is another benefit gained from using this speed-diai circuit When the dial button is pushed, the phone number is transmitted at a rather fast rate. The entire seven-digit sequence takes only about one second This means that each burst of tones lasts only about 65 to 70 milliseconds (there is a silent space between tones). The shorter the burst length the lower

Photo A. The speed-dial switch mounts on the side oí the microphone near the area where the remote volume control used to be. A protection ring around the dial button eliminates any accidental dialing sequences while handling the microphone.

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Fig:l Block diagram of the speed-dial circurt.

Photo B. In the newer-style microphone (ModeI PÇM-369), capacitor C3 must be changed to a physically smaller tantalum type. Leave its leads long and let it hang over the edge of rfie board. The older-style PCM-2000 microphone does not need this change.

Photo G In the PCM-369 microphone, the row-and-column leads are connected to the existing solder pins of the plastic bus connector. Route the wires underneath the flat printed-bus cable. Be careful not to burn or melt this printed bus or the connector when soldering in this cramped area.

the probability that it flutter or "picket-fencing' will cause the telephone equipment to see a double digit Hence, this speed-dialed number tends to be more accurate than a hand-dialed number lasting hundreds of milliseconds per digit


Unfortunately, in order to fit the parts inside the already-stuffed microphone case, certain sacrifices had to be made, (here just wasn't enough room for memory chips and their associated parts, so this dialer can hold only one phone number —presumably your home phonep or one that you call regularly. This number is programmed with tiny wire jumpers and is changeable in about ten minutes with a soldering iron. An advantage of this type of "memory/' in addition to saving space, is that it is guaranteed not to forget when powered down!

A preconditioner for the PT I would have been nice — a circuit that brings up the transmitter and gets the squelch open on the repeater receiver before the phone number is dialed. Again, space limitations won out over bells and whistles, and because of that, I recom mend that vou squeeze the PTT bar just prior to pushing the dial switch I dtd design in some PTT preconditioning, using what limited space 1 had. The circuitry keys the transmitter about 200 milliseconds prior to the actuaf dial sequence. Sometimes, however, that is still not quite enough time to bring up the transmitter and open the receiver squeich circuits in time to hear the first digit dialed As a general rule I would rec ommend thai llie PTT bar be squeezed first, just to be safe. This is really much easier to do than it sounds, and can be done quite nicely with just one hand while leaving your eyes on the road and traffic.

This circuit was designed and physically tailored to fit inside my A/rien DTMF microphone However, the circuit is electrically compatible with any DTMF encoder using row-and-cotumn inputs. This includes virtually all of the current encoder circuitry on the market today. If you happen to own an Azden microphone, then you Ye home free, as I have included a full-size printed-circuit-board layout for this project which will fit into either oi the Azden microphones supplied with their earlier transceivers.

It is important to point out that although this article is concerned with mounting circuitry inside a microphone, this is only necessary when the DTMI encoder and touch pad are also mounted in the microphone. The speed-dial circuit must l>? installed where it has access to the row-and-column inputs of the DTMF encoder chip. In cases where the touch pad is panel-mounted on the radio, the speed-dial circuit would be installed inside the transceiver.

Not Disturb Circuit Diagram



Tig, 2. Schematic of the speed-dial circuit,

73 for Radio Amateurs * March, 1986 21

Photo D. In the older- sry/e PCM-2000 microphone, the row-and-column leads are soldered to existing PC pads on the end of the DTMF board. The crystal must be bent up s/ig/if/y while soldering. Use ¡ust a small amount of heat and do not disturb the existing wires which are poking through these pads from below the board,

I have installed this circuit inside two different Azden microphones One was supplied with the Azden PCS-2000, and the other was supplied with the PCS-3000. These two microphones are physically the same basic unit. Note that Azden radios having serial numbers prior to 53752 require microphones with a battery inside. Because of the limited space in this case, the circuit will probably not fit inside that microphone without changing the physical layout of the parts. I have not attempted to install this circuit in one of those early mn crophones.

One of the microphones I

did modify uses a DTMI encoder, model CSTTK-1, made by Communications Electronics Specialties, Inc. The encoder comes fully assembled in a replacement back for the microphone, and uses a chip, part number S2559C, made by American Micro-Systems, Inc In general, if your Azden transceiver is a PCS-2000, you will have this type of microphone.

The other microphone I modified included the DTMF generator as a kit (included in the purchase price of the Azden PCS-3000). This unit has a differently-shaped encoder board, a smaller touch pad, and uses a chip

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