Drive A Relay Using A 1-Wire Addressable Switch

The 1-Wire Net, or MicroLAN (by Maxim-Dallas), is a simple way to connect slow devices (such as sensors, relay drivers, switches, and so on) using simple components. These components contain the 1-Wire protocol handler, some type of interface to the external world, and often a parasitic power supply. You may build a very long string of 1-Wire comp
Drive A Relay Using A 1-Wire Addressable Switch - schematic

onents by just distributing them in a string over a good quality twisted-pair copper wire. The controller connected to one string end can supply the power for the entire string. A common use of such a string is to control relays to activate some electrical loads. For this purpose, the DS2405 (1-Wire flip-flop with open drain output) is a good choice. Of course, the parasitic power you can steal from the 1-Wire cable isn`t enough to operate some power-hungry load, like a relay solenoid (or a photocoupler LED). Obviously, you don`t want to connect the 1-Wire reference to the relay driver reference because 1-Wire can be very touchy. You could use a dc-dc converter to galvanically insulate the relay driver, but these converters are expensive. The circuit shown in Figure 1 uses a single transistor and a transformer to supply the required galvanic insulation to operate a relay without disturbing the 1-Wire Net. Q1 and T1 form a Hartley oscillator. And, the load on the transformer affects the oscillator operation. It stops oscillating when the DS2405 is closed due to the excessive load on T1`s secondary coil. The presence of oscillations, detected by D4, D5, and D6, increases the voltage on Q1`s base, engaging the relay. Therefore, when the DS2405`s output is open, the relay is engaged. Conversely, when its output is closed, the relay is re-leased. With the indicated values, the oscillation frequency is about 300 kHz. Figure 2...

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