May 1999, Issue 106

Dallas 1-Wire Devices, (Part 2):
All on One

DON'T TOUCH THAT

Most 1-wire devices come packaged in a variety of ways. Sometimes, a device is permanently connected to the rest of your circuitry, so the plastic-encapsulated through-hole or surface-mount package is the style of choice.

But, some applications require semipermanent or momentary touch-memory packaging. Touch memory for these apps is encased in a metal can about as thick as a nickel. Although difficult to see, an insulator between the lid and the body of the can provides two isolated contacts—ground and data.

Dallas has a variety of receptacles for use with touch memory—for example, keyfobs, plastic cards, and even jewelry (e.g., as the stone of a ring).

Dallas has been pushing touch memory for all sorts of security products. Let’s look at how these devices might be used. For example, a touch receptacle at your front door can be locally monitored by a micro.

When your unique ID touches the receptacle, the micro polls the device, recognizes your ID, and commands a DS2407 addressable switch to energize an electromagnetic door strike, which unlocks the door. The micro can directly control the electromagnetic door strike, but when we expand coverage to multiple doors, you’ll see how the addressable switch becomes an integral part of the circuit.

Imagine having multiple entrances monitored on the same 1-wire bus. When you touch your ID ring to the front door, how will the 1-wire bus know you’re not at the back door?

Because the addressable switch has two switch outputs, one switch can control the door strike and the other can connect the receptacle to the bus. By turning on only one receptacle at a time, you know where the touch ID is coming from.

This setup can be expanded to include many doors, such as an apartment building where the main door must give access to many occupants and each apartment’s door must only give access to its owner.