9
\$\begingroup\$

The Project

So I have a project idea. (groan)

I want to monitor multiple motion and/or ultrasonic sensors to build a simple proximity alert. I want to "deploy" sensors at key points along the perimeter of my home (outdoors). Later, I'd like to send z-wave notifications to my home controller and play a gentle alert tone for large signals but ... baby steps... For now I'll settle for a simple output (a lit LED will do) just to get things working.

The Problem

I'm aware of the components I'll need and have an idea for the sketch to drive it (either with one or as many sensors as an Uno 3 can support after shields) but I'm at a loss regarding distance. I have a choice between multiple Arduinos with their own sensors, each communicating back to home base OR one Arduino in the loft with multiple sensors and a hydra-like set of cables to each, as well as a SINGLE hardwired power draw. I like the latter approach as it's less wasteful in a number of ways and is certainly cheaper (especially considering radio shields).

The Actual Question

What I want is recommendations on how to approach (if it's even feasible) long-distance connection of the sensor components to the Arduino. Specifically:

  • Might some of the extra Cat-5 I have laying around be a good match for this?
  • If not, why (I'm trying to learn - a reference to good reading is all I ask)?
  • Is there an alternative I hadn't considered?
  • Am I trying to run before I can walk?
  • Mother will they tear your little boy apart?*

*Okay, so that's just a Pink Floyd reference. I'm just getting started with the Arduino as a new hobby (I'm a software engineer by trade, no EE background or experience). I'm currently facing the problem of "not knowing what I need to know to ask a good question." Please forgive any perceived laziness and feel free to school me. :-)

Update

Further research did turn up this thread where someone suggested this is feasible but the OP said one sensor wouldn't respond. A responder said one sensor's manufacturer suggested a low pass filter on the sensor side of the connection, as close as possible. Thoughts?

Also, it occurs to my network engineering background that if multiple cat-5 runs in different directions wastes at least 2 of 8 strands, maybe a 2-pair telephone data cable would also work just fine, assuming the tips are soldered to make them solid leading into the breadboard. Again, thoughts?

\$\endgroup\$
  • 3
    \$\begingroup\$ Z-Wave is a home automation radio implementation around 900MHz; for those not familiar with it. \$\endgroup\$ – JYelton Jan 5 '13 at 0:00
  • \$\begingroup\$ @JYelton Thanks - I should've linked that out. The notification layer is not important at this stage, though, only the viability of my "sensor at the end of a CAT-5 run" idea (or wired alternatives). \$\endgroup\$ – Joshua Nozzi Jan 5 '13 at 0:02
  • \$\begingroup\$ Depending on the flexibility you have in designing your sensor modules, a common and ancient signaling / sensor approach traditionally used with long cables in industrial applications is 4-20 mA or 10-50 mA signaling - the cable and sensor module make up a current loop, the module regulating the current through it, from 4 mA (analog minimum, or digital LOW) to 20 mA (analog full-scale or digital HIGH). Open circuit = 0 mA = sensor offline alarm. Short circuit = Current limit = sensor shorted alarm. \$\endgroup\$ – Anindo Ghosh Jan 5 '13 at 5:10
  • \$\begingroup\$ Using a current loop signaling will thus provide the additional benefit of identifying security breaches that bypass a given sensor. Would you like me to make this into an answer? \$\endgroup\$ – Anindo Ghosh Jan 5 '13 at 5:11
  • \$\begingroup\$ @AnindoGhosh Very informative. Yes, from another StackExchange user in another community, this sounds like an answer waiting to be posted. :-) Thanks. In the meantime, I've got some googling to do. A circuit diagram of how this might be accomplished (Arduino or no) would go a long way as I'm just starting to be able to read them. \$\endgroup\$ – Joshua Nozzi Jan 5 '13 at 5:27
8
\$\begingroup\$

Paraphrased from my comments above

Depending on the flexibility available in designing the sensor modules, a common signaling / sensor approach traditionally used with long cables in industrial applications is the 4-20 mA (or 10-50 mA for longer throws or EMI-intensive environments) current loop signaling standard.

  • The cable and sensor module make up a current loop, module regulating current through it
  • A current of 4 mA indicates analog minimum, or digital LOW
  • 20 mA indicates analog full-scale or digital HIGH
  • Open circuit = 0 mA = sensor offline alarm
  • Short circuit = Current limit = sensor shorted alarm

Industrial sensor modules are often designed to be powered by the same current loop, thus eliminating the need for local power supplies. This is feasible, of course, only if the sensor module does not require greater than 4 mA drive current.

Various options exist for signaling current regulation, such as using BJTs, MOSFETs or complementary TrenchFET parts.

At the data collection end, voltage generated across a shunt resistor is amplified using an op-amp, for analog sensors. Digital signals can be captured using a suitably trimmed comparator circuit designed with some hysteresis.

Depending on any lightning or other high voltage risks perceived along the transmission cables, isolation amplifiers may be recommended instead of conventional op-amps for amplifying the shunt voltage. This ensures that the data collation devices are protected from potential differences that may creep in through induction, ground potential differences, or other causes.

For example, TI's AMC1100 Fully-Differential Isolation Amplifier is designed specifically for current-shunt sensing with HV isolation.


An added advantage of using a current loop signaling approach is that security breaches to the home security system implied in the question, can be detected if any sensor is either shorted out, or disconnected.

\$\endgroup\$
  • \$\begingroup\$ Thanks for this. It gives me a lot to study. Would you mind clarifying something for me? I'm reading the phrase "the sensor's power supply" a lot. This leads me to believe the sensor itself must have its own supply separate from the Arduino. Is this the case or did I misunderstand (ie, "power supply coming from the Arduino can be 'the power supply'")? \$\endgroup\$ – Joshua Nozzi Jan 5 '13 at 16:23
  • 1
    \$\begingroup\$ @JoshuaNozzi That depends on what sensor we are talking about. For something like a light-dependent resistor, no additional supply is needed. For an electret microphone, a low bias voltage is needed, no significant current. For an ultrasonic echo-sensor, a few milliwatts at a few volts are needed. Some cases, Arduino supply is enough, other cases, additional power is needed. Many sensors = total supply needed overshoots what the Arduino's regulator can supply. \$\endgroup\$ – Anindo Ghosh Jan 5 '13 at 16:27
  • \$\begingroup\$ For now I would be happy to get this working (not yet delivered): amazon.com/gp/product/B009BTHQ26/ref=oh_details_o02_s00_i00 - appears to be the same basic component as this (connector notwithstanding): sparkfun.com/products/8630 - a walk-through can be found here: bildr.org/2011/06/pir_arduino This is probably worth a separate question, but I'm trying to work out how your answer affects me with this relatively inexpensive component in particular. Up to the community whether I post it separately. \$\endgroup\$ – Joshua Nozzi Jan 5 '13 at 16:41
  • \$\begingroup\$ Another aside for anyone following, I'm not so concerned about sabotage since this isn't so much a security system as an extension of the senses. It'll never trigger an alarm, only a notification, and attempts to blind it would trigger it, so there's no sneaking up on it to cut the wire in the first place if it's positioned and aimed properly (but again, it's not for alarm, just notification). That's my theory anyway, so the tamper detection aspect is not a concern. \$\endgroup\$ – Joshua Nozzi Jan 5 '13 at 16:50
  • 1
    \$\begingroup\$ @JoshuaNozzi I have the exact same PIR sensor module, bought off eBay for $1.90. It uses 5 volts at around 7 mA to power it, and the output is a 5 Volt signal. Output can be wired up to a MOSFET at the sensor end for current loop signaling: Gate low = Rds high, so around 7 mA drawn. Gate high = Rds low, so limit it to 20 mA or so and you're good. Deserves a separate question for more detail, I suggest. \$\endgroup\$ – Anindo Ghosh Jan 5 '13 at 16:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.