Specifications

BASIC Stamp II

Page 258 • BASIC Stamp Programming Manual 1.8 • Parallax, Inc.

a noisy phone line or radio link, you could use the optional ontime and

offtime values:

DTMFOUT 0,500,100,[6,2,4,8,3,3,3] ' Call Parallax, slowly.

In that instruction, ontime is set to 500 ms (1/2 second) and offtime to

100 ms (1/10th second).

Technical Background

The BS2’s controller is a purely digital device. DTMF tones are analog

waveforms, consisting of a mixture of two sine waves at different audio

frequencies. So how does a digital device generate analog output? The

BS2 creates and mixes the sine waves mathematically, then uses the

resulting stream of numbers to control the duty cycle of a very fast

pulse-width modulation (PWM) routine. So what’s actually coming out

of the BS2 pin is a rapid stream of pulses. The purpose of the filtering

arrangements shown in the schematics of figures I-2 and I-3 is to smooth

out the high-frequency PWM, leaving only the lower frequency audio

behind.

Keep this in mind if you want to interface BS2 DTMF output to radios

and other equipment that could be adversely affected by the presence

of high-frequency noise on the input. Make sure to filter the DTMF

output thoroughly. The circuits shown here are only a starting point;

you may want to use an active low-pass filter with a roll-off point

around 2 kHz.

Demo Program

This demo program is a rudimentary memory dialer. Since DTMF

Figure I-2

10µF (both)

≥40Ω Speaker

(or 8Ω in series

with 33Ω resistor)

I/O pin

C1 C2

Notes:

C1 may be omitted for piezo speakers

C2 is optional, but reduces high-frequency noise

I/O pin

Driving a Speaker

0.1µF 0.01µF

Driving an Audio Amplifier

Amplifier

(e.g., Radio Shack

277-1008C)