the sPARC gap

  January, 2004

Part 3:  Loose Ends

o sooner had I sent last month's issue for reproduction than the January issue of QST came, with an answer to the appeal I'd put in the last paragraph of Part 2 of this series.  Using the references on page 53, I was able to download a couple of schematic drawing programs, one of which gave me just about all of what I needed to do up a much more presentable diagram that I stuck you with, last month.

There are some variables that will need to be dealt with, before a standard circuit board layout can be produced, all of which have to do with which parts are used in the circuit.  I have one layout which is a revision of the one I used previously, but a different set of interstage transformers would trash my layout completely.  Also, with the abandonment of trim pots for R1 and R2 in favor of ones that can be adjusted outside the box there are some other issues to resolve.  My board layout appears with this article, but there are likely to be others, especially if we go to different parts.

The complete schematic for the interface (see page 3 in last month's sPARCgap for the basic diagram).  U1, U2 pinout:  1:  R5 or R6, 2:  J3 frame  5:  switched lead (collector), 4:  switched lead common (emitter).  J5 pin assignments appear in text.  Square pin on J5 is pin 1.

Also, with this installment comes a complete parts list (which is the authoritative one, rather than the markings on the schematic).  The cabling will be revisited as well.  Unfortunately, the price quote I gave of under $20 is a bit on the optimistic side.  With all the extra jacks (specifically J4 and J6),  and the cabling, it's closer to $30 to build, but that's still a bit cheaper than most of the commercial units on the market.

Parts List:

C1:  1 µf capacitor
D1, D2:  1N914 or 1N4148
J1, J2:  3.5 mm. stereo jacks
J3: 2.5 mm. stereo jack
J4, J6:  Switched jacks (mono will do, since only the tip is used)
J5:  RJ-45 jack
R1, R2:  1K potentiometers (audio taper preferred)
R3, R4:  51 ohm 1/4 watt
R5, R6:  2.2K 1/4 resistors
S1:  SPDT center-off toggle switch
T1, T2:  600 ohm 1:1 interstage audio transformers (RS 273-1374 used on my version)
U1, U2:  4N33 or NTE3083 Opto-isolators (can be had at Norvac or Fry's)

Cabling:

Sound Card audio connections:  6' stereo cables, 3.5 mm plugs on both ends (Radio Shack stock item)

Serial port to optoisolator circuits:
DB9F connector (with hood), 6' stereo cable (plugs removed), 2.5 mm stereo plug
Pinouts: Pin 4 (DTR) to Ring, Pin 5 to cable shield and sleeve, Pin 7 (RTS) to tip.

Radio interface cable:  RJ-45 plug and short piece of Category 5 cable (no longer than 18 inches).  Plug on the radio end will either mate with the accessory jack on the back of the radio, or the microphone jack, if there is no other way to get the audio to and from the radio; consult your owner's manual.
Pinouts:  1 (wht/org)  PTT common; 2 (org)  PTT hot; 3 (wht/grn)  FSK common; 4 (blu) Line In or Mike jack (xmt audio); 5 (wht/blu) Xmt Audio common;  6 (grn)  FSK hot;
7 (wht/brn)  Rcv Audio common;
8  (brn)  Line Out (Rcv Audio hot).

Circuit board layouts had not been finalized at press time. I am working on one that uses the Radio Shack audio transformers. With this information, the parts and the circuit board, anyone can concoct a well-working sound card interface of his or her own.  See you on the digital modes!

-AL7W