Design Contest

Stealth Telephone Screener

Mad Dash for Flash Cash 2002 – Grand Prize

Richard Wotiz, U.S.

Almost everyone has been annoyed at one time or another by an automated telemarketing call in the middle of dinner, to find only silence at the other end of the line. The Stealth Telephone Screener—based on the PIC16F872—answers the phone and determines whether or not the caller is an actual person. If not, it hangs up without disturbing you. Unlike similar commercial products, the caller does not have to know any special code, and is not even aware that they are being screened.

The screener plugs into an ordinary phone line, and has a built-in ringer to alert you when the caller is a person. It works by answering the phone, playing a brief greeting (e.g., “Hello”), and then waiting for a response. If there is no sound for 5 s, or if there is a continuous response that lasts longer than 10 s, then it hangs up. Otherwise, it plays a second greeting (e.g., “Just a minute”), and activates the ringer, while playing a ringing signal to the caller. When you pick up any phone on the same line, the screener stops ringing and releases the line. The ringers on all of the phones are normally kept off, so you would never be aware of a machine-originated call when it comes in.

This technique eliminates both the “predictive dialing” systems (which place multiple calls and only connect to the first person to answer their phone, typically after a delay of more than 5 s, while hanging up on all other calls) as well as automated systems that play a recorded message.

Supporting Files

Keep up-to-date with our FREE Weekly Newsletter!

Don't miss out on upcoming issues of Circuit Cellar.

Note: We’ve made the Dec 2022 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine.

Would you like to write for Circuit Cellar? We are always accepting articles/posts from the technical community. Get in touch with us and let's discuss your ideas.

Website | + posts

Circuit Cellar's editorial team comprises professional engineers, technical editors, and digital media specialists. You can reach the Editorial Department at, @circuitcellar, and

Supporting Companies

Upcoming Events

Copyright © KCK Media Corp.
All Rights Reserved

Copyright © 2024 KCK Media Corp.

Stealth Telephone Screener

by Circuit Cellar Staff time to read: 1 min