Specifications

BASIC Stamp II Application Notes
Parallax, Inc. • BASIC Stamp Programming Manual 1.8 • Page 359
2
3: Phoneline Interface
Introduction. This application note shows how to interface the BS2 to
the phone line in applications that use the DTMFout instruction.
Background. The BS2 instruction DTMFout generates dual-tone, mul-
tifrequency signals—the same musical beeps used to dial the phone,
activate pagers, and access repeaters in ham-radio applications.
Commercial designs that interface electronic devices to the phone line
normally require the approval of the Federal Communications Com-
mission (FCC) to ensure the quality and reliability of telephone service.
Manufacturers of phone accessories often take the shortcut of using an
off-the-shelf interface, known as a Data Access Arrangement (DAA).
Since the DAA has already been checked out by the FCC, it’s generally
much easier to get a DAA-based design approved than a from-scratch
circuit. Unfortunately, DAAs tend to be somewhat expensive in small
quantities ($25+ each), and are sold primarily through high-volume
distributors geared toward serving manufacturers.
Where does this leave experimenters, hobbyists, and one-off instru-
ment makers? Pretty much on their own. For them, we present the
circuit below. It’s not a full-blown DAA suitable for production de-
signs, but it is a good starting point for prototype DTMF-transmit
Schematic of the phone-line interface.
Jameco (JC), 1-800-831-4242
or 415-592-8097
Parts Sources
600-600Ω
transformer
(JC: 117760)
280V “Sidactor”
(DK: P3002AB-ND)
10Ω
(both)
3.9V zeners (both)
DK: 1N5228BCT-ND
phone line
(red and green)
0.001µF
0.1µF1k
P0
BS2
connect switch
(or relay contacts)
Digi-Key (DK), 1-800-344-4539
or 218-681-6674
P3000AA61-ND