0
\$\begingroup\$

I want to build a centralized system which will be getting values from many sensors around an agricultural field.

The field's large area leads to long wires going from sensors to the central controller. I do not wish to use repeaters in between. How far (in metres) can those typical 5 V cheap sensors (specifically PIR sensor) transmit without the output fading too much?

I understand that this distance of signal travel depends on current delivering ability of the sensor and also on the resistance of the transmission wire. So say if I use a high impedance op-amp at the input, hence solving the current problem, how far would I be able to sense the signal from sensors considering that much electromagnetic interference is not around?

EDIT: Sensor Link: https://m.alibaba.com/product/62366227634/Free-shipping-Motion-sensor-Human-Body.html

\$\endgroup\$
  • 2
    \$\begingroup\$ "... those typical 5 V cheap sensors ..." Oh, those ones? Which ones? Please link to a datasheet (not an Amazon / eBay / AliExpress ad) for what you are talking about. What is the big problem you are trying to solve? Intrusion detection on a farm? Why would you not use radio? \$\endgroup\$ – Transistor Apr 11 at 9:10
  • \$\begingroup\$ @Transistor can u please tell me what 'radio' are you specifically talking about? Is it a better alternative? Yes intrusion detection is the problem! I choose PIR because of the cheaper ones available online. \$\endgroup\$ – Bhuvnesh Apr 11 at 9:22
  • 2
    \$\begingroup\$ For long distances you would switch to using a current mode circuit, not voltage. Also how are you planning to power your IR sensor? What you need is a complete system solution which is not what we provide here. By the way the question "How far can the signals travel?" falls in the same category as "How long is a piece of string". \$\endgroup\$ – Oldfart Apr 11 at 9:23
  • \$\begingroup\$ @Oldfart ok so can you tell me what maximum length of wire can I connect to the sensor output (in the link) and still get measurable output at the other end? Am I asking the right thing now? \$\endgroup\$ – Bhuvnesh Apr 11 at 9:29
  • \$\begingroup\$ No, you are not asking a question that can be answered. First of all you need to choose a sensor that has a decent pdf data sheet and not link to some unreliable and un-reputable site like alibaba. Then you need to state what power supplies you are proposing to use and give some kind of indication what amount of electronics can be used at the sensor end. Then it might start to become answerable. \$\endgroup\$ – Andy aka Apr 11 at 9:34
2
\$\begingroup\$

This isn't a sensor, it's a component. It outputs a 3.3V pulse which lasts several seconds after last detection. The pulse is intended to drive a logic circuit, microcontroller etc. There's no spec of the output current capability. It can be less than 1mA. In theory the pulse can be filtered in the receiving end with a lowpass filter which has BW less than 1Hz. I guess with it the pulse stays detectable with a comparator in cables which are hundreds of meters long.

The max 3m detection range isn't much for large area surveillance. You need numerous sensors. To keep cables simple you very likely want to use some kind of daisy chaining. Ideally one 2...4 wire cable should feed and read as many detectors as wanted, the signal and power lines would be ideally the same and every detector should still be identifiable when an alarm occurs. Designing robust enough system for this is a challenge. Another challenge is the weather. Lightning creates easily high, say 2000V surges without a need of straight hits. Water as rain, as condenced and as ice must be kept out in every situation.

A substantial amount of tests and design is needed. The given data says that human motion can be detected at 3m. But how are you going to handle swinging plants, sunshine and moving shadows? I guess your actual problems are totally different than "could the 3.3V output be detected".

| improve this answer | |
\$\endgroup\$
0
\$\begingroup\$

Given that it's a pir sensor we can assume that the output signal won't fluctuate frequently, so I wouldn't expect bandwidth issues. You may get noise problems if the cable very long and it is in a noisy environment, however you can solve that with a low pass filter.

What I would worry about is the power supply, if your device uses many mili amps and the cable is very long and thin you may experience a voltage drop, maybe below the operating range of the circuit, also if it has consumption peaks the inductance of the cable may generate a voltage drop, that is fixable with a decoupling capacitor close to the load.

Now the actual calculations are for you to do unless you provide us with the resistivity of the cable you are using, the length, the current and preferably the datasheet of the sensor

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ This is just a long comment. \$\endgroup\$ – Solar Mike Apr 11 at 9:52
  • \$\begingroup\$ The module seems to be a HC-SR505 "The module has a very low power draw of about 70uA even when active." protosupplies.com/product/… \$\endgroup\$ – Jasen Apr 11 at 9:59

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.