A similar version of this article appeared in the June 31, 2008 issue of PET
One-wire switches such as the DS2413
allow you to control remote loads via the versatile 1-wire network. The load can be connected to the open-drain switch directly, or indirectly through a relay. By monitoring voltage at its PIO pin, the DS2413 also lets you read back the state of the load.
The PIO pin of that device is limited to 6V or 4mA, which restricts the type of load or relay it can drive. Other devices with higher ratings (13V/50mA for DS2406
; 28V/20mA for DS2413
) can solve the problem for some applications. You can use a relay for loads that exceed those ratings, especially for applications that feature high current or high voltages such as 120VAC (Figure 1
Figure 1. These are common load-control applications for a 1-wire switch (DS2413).
On the other hand, a discrete-component circuit such as Figure 2
may be more practical and cost effective. Q1 acts as an open-collector switch connected to the load. The state of the load, however, cannot be read because Q1's base-emitter voltage limits the voltage swing at the PIO pin.
Figure 2. Using a 1-wire switch to drive an external transistor lets you control high-voltage loads.
The circuit of Figure 3
solves this problem. R1 and R2 form a voltage divider that provides an adequate signal swing at the PIO pin. Diode D1 limits the PIO-pin voltage to a safe level, and transistor Q2 provides an additional logic inversion that makes the state of the DS2413 internal pull-down correspond to the state of the load. (The PIO pin is off by default, so Q2 is included for convenience and safety.)
Figure 3. This circuit allows a 1-wire switch to drive a high-voltage load while monitoring the status of the load.