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Author Topic: Detect and ignore 5 volts not 12 question.  (Read 543 times)
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Pa, USA
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Anyone know of a ciruit, with or with out the Arduino unit, that can detect voltages between 5 and 12 volts, and only output if 12 volts is detected.

Basicly i have a wireless device that has an output that varies in voltages, The manufacturer says it will be floating voltage when idle, and 12volts in alarm. I tested with my meter, 5.5 volts and 12.8 volts. If i attach a 12volt relay directly to the circuit, it holds down the device and wont allow it to operate properly. Im thinking a Transistor is the answer, but im not sure on detecting only 12 volts. Any one have an idea i can try???
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floating voltage when idle, and 12volts in alarm
If this is the case use a voltage divider set to give 5v when the device is producing 12v. Then you will get a signal that is either 0v or 5v.

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« Last Edit: August 31, 2011, 06:26:55 pm by Graynomad » Logged

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Would a voltage divider put any load on  the lines like a relay?
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None worth mentioning, if you use large(ish) values in the 10-100k range. A relay is more like 50-200R range, big difference.

A 9k1 and 5k6 resistor should work, even 91k and 56k.

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« Last Edit: August 31, 2011, 06:50:39 pm by Graynomad » Logged

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Is it possible to use a voltage divider with a relay directly (without Arduino), i need to Minimize the circuit and size of circuit. Thanks for any suggestions.
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What about a Zener diode?  Pick one with a voltage near your desired trigger voltage.
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Yep a zener and resistor will do a similar job.

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Is it possible to use a voltage divider with a relay directly (without Arduino)
No, but if that's all that needs to be done a transistor should work.

Transistors aren't my forte but the signal should be able to drive one easily enough.

In general though use a resistor from your device to the base of an NPN, another pulling the base to GND, collector to the relay coil -, emitter to GND, coil + to 12v and a diode across the coil.

But it will depend on the ability of your device to drive something, Do you have a data sheet.

There are many examples on the web an din the Arduino playground I think.

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Yes you can get a transistor to do this. You need a potential divider to reduce the 5V to just less than 0.7V. Feed that into the base of a transistor, put the emitter to ground and the collector to one end of the relay coil. Put the other end of the relay coil to the positive of your supply.
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