Switching a non-DC 2-wire signal

Discussion in 'General Electronics Chat' started by MrSoftware, Dec 6, 2013.

  1. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    In my project I will be running 2 groups of 2-wire ultrasonic proximity sensors, and I need an inexpensive way to select which group of sensors are connected to the brain. DPDT reed relays are the most strait forward, but they are too expensive. I'm shooting for 50-cents or less per sensor-pair to be switched, and the least expensive relays I've found are well over $1/ea. Are there any ideas?

    The sensors are basically piezo elements. The brain sends a pulse to the element which then creates a sonar pulse. The piezo element picks up any reflected pressure waves and converts them back to an electrical pulse that travels over the wire back to the brain. A DPDT relay works perfectly to select between 2 sensors, but they are too expensive.

    Here are the sensors:

    http://www.aliexpress.com/item/4pcs...dar-detector-NO-LED-display-t/1280888614.html

    Thanks in advance for any ideas!
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What voltage/current has to be switched?
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Can you connect the low side of each sensor to the common terminal of the brain.

    You likely need to pulse a sensor, then disconnect the pulser to "listen" for the reflection?

    What voltage/current does it take to pulse it?

    What is voltage level during the reflection?
     
  4. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    I am not sure of the voltage or current as I'm still searching for the datasheet for the sensors, and I don't have access to a scope at the moment. The plan is to use an off-the-shelf brain that includes sensors, so the sensor specs are only important for the switch. My guess is the current is miniscule, and the voltage created by the reflection is also tiny. I am not sure of the voltage to create the pulse, but I doubt it's more than a few volts.

    I'm not sure if sharing a low side among the sensors will affect the behavior, so I think it's safest to switch both wires.

    Am I going to need to get a scope to see what's on the wire before I can continue?
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I believe this is the case of zero input, zero output.

    When you get some numbers, then there will be something to talk about.
     
  6. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    Indeed; I was hoping someone was familiar with these sensors.

    I don't have a scope, but reading specs for similar driver circuitry it looks like in the case of sending the pulse, the frequency of the signal will be around 38kHz, current will be less than 40mA and voltage can be as high as 130Vpp if no load is attached (sensor disconnected). When an echo is received the voltage over the same wire will be between 0.05Vcc and 0.9Vcc, no current is specified but since the voltage is generated by a sound echo hitting a piezo element, the current is going to be itty bitty.
     
  7. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    Maybe I'm asking my question the wrong way. Let me try rewording it:

    Mechanical DPDT relays seem to be very expensive. Does anyone know of a PCB mountable inexpensive (under $1/ea) DPDT relay? The smaller the better, the electrical load will be itty bitty.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I'm guessing the only way you'll get the price down below $1 is if you order 10000 at a time :(.
     
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