Specifications

BASIC Stamp II Application Notes
Page 360 • BASIC Stamp Programming Manual 1.8 • Parallax, Inc.
3: Phoneline Interface
applications using the BS2. It’s based on a circuit presented in Encyclo-
pedia of Electronic Circuits, Volume 5, by Graf and Sheets (TAB/McGraw
Hill, 1995; ISBN 0-07-011077-8). We’ve filled in specific component
values and sources, added parts for coupling the BS2, and tested the
circuit’s ability to dial the phone.
How it works. Starting at the phone-line end of the circuit, a double-
pole single-throw (DPST) switch or set of relay contacts isolates the
circuit from the phone line when the circuit is not in use. Closing the
switch puts the phone into the “off-hook” condition, which causes the
phone company to generate a dialtone. Although a single set of contacts
would be sufficient to break the circuit, a tradition of robust design in
phone circuits makes it normal for a hook switch to break both sides of
the circuit.
After the switch, a Sidactor surge-protection device clips large voltage
spikes that might result from nearby lightning strikes. Its voltage rating
is selected to let it do its surge-protection job without interfering with
relatively high ringing voltages or phone-company test voltages. Note
that nothing can provide 100-percent lightning immunity, but the
Sidactor is cheap insurance against most routine surges.
A 600-to-600-ohm transformer isolates the BS2 from the line’s DC
voltages. On the other side of the transformer, a pair of zener diodes
clips any voltage over approximately 4.6 volts. The remaining resistors
and capacitors couple the DTMF tones from the BS2 into the trans-
former. They also work together to smooth the ragged edges of the
DTMF tones, which are generated using fast pulse-width modulation
(PWM). Before filtering, these tones contain high-frequency compo-
nents that can make them sound distorted or fuzzy. With the circuit
shown, the tones come through crystal clear.
Programming. You’ll be amazed at how easy it is to dial the phone with
the DTMFout instruction. Suppose you want to dial 624-8333—one line
will do the trick:
DTMFout 0,[6,2,4,8,3,3,3]
where 0 is the pin number (0-15) connected to the interface and the