Electronic Design

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 components 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 shows the (Spice simulated) timing diagram of the voltages on the relay coil and on the I/O pin of the DS2405. There's a negligible delay between input and output.

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