THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

LOOKING THROUGH THE INTERNET

A Prototype Security System
by Skylar Lei, Jim Haslett, and Mike Smith

Start • Web Server Choice • Interactive GUI • Video System • Remote Camera System • Control RF Link • Solar Power Module • • Sources

CONTROL RF LINK

Besides the video link, the system requires additional RF links to send the base station’s commands to the remote sites. A simple logic high/low signal is all that is needed to turn the equipment on and off. This signal requires a narrow bandwidth signal carrier with a frequency specific to each camera control.

To keep costs low, we used wireless door chimes ($20 at Canadian Tire) for our control RF links, using custom digital code settings on the chimes. We reverse engineered the chime and found the pins that give us the desired signals. The door-chime signal can only go for 30 m, which is satisfactory for the prototype. We could either experiment with a directional antenna to improve the transmission range or get the customer to pay for more expensive equipment.

As we mentioned previously, some of the bits from the COM8017 chip on the video switching module are sent to control the remote camera system. A pulse generator recognizes the level changes in the 8017 control signals (see Figure 3). This circuit generates pulses of 0.85-s duration to activate the RF link via the press-button control logic of the door chime.

(Click here for figure)

Figure 3—The pulse generator converts the video-switching module control signals to a level capable of activating or deactivating the RF links.

When the control pulse reaches the remote site, a simple T flip-flop is used to reverse the pulse back to levels that are capable of switching the power transistor needed to activate the remote equipment.

PREVIOUS • NEXT

Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles.
For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ęCircuit Cellar, the Magazine for Computer Applications. Posted with permission.