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Sensor Used to Light up an LED

Discussion in 'Sensors and Actuators' started by jeff77789, Feb 23, 2013.

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  1. jeff77789


    Feb 23, 2013
    So you guys have to pardon my ignorance since i am new here...and new to general electronics as well but hopefully what i will say will make sense once you get to the end of it

    Basically, I have this sensor that takes a reading. This reading is a voltage and it can range from 0 mV to 2490.2 mV. This reading is going to be transferred wirelessly over 20ft to a receiver in which it interprets the data and acts as a switch to turn an LED on or off. The threshold voltage for this is going to be 1171.9 mV. When the voltage drops below 1171.9 mV, the light needs to turn on. (In the image, i wrote 2 volts but it is actually 1172 mV)

    The sensor/transmitter unit is powered by 14v DC and the receiver/LED unit is powered by 9v DC. There is actually 2 sensor units and 1 receiver unit. I plan on using RF links with different frequencies for the 2 Tx/Rx pairs to transfer data.


    What I circled in black is what I am having an issue with. I need components that will encode the analog output into something that the rf link can send (i also think that the AN output needs to be grounded but i am not sure about that)

    on the receiver side, i need something that can interpret the signal being transferred over, set a threshold of <1172 mV, and then act as a switch for the LED

    it would be nice if the LED's would start flashing if the voltage was <586 mV
    it would also be nice if the voltage threshold could be variable by a potentiometer

    ^those would be nice, but i guess that's out of my knowledge to implement

    thanks in advance everybody!
  2. Electrobrains


    Jan 2, 2012
    Your application is clear. The solution will probably be rather complex if you don't have much experience.

    I would involve some small micro controller, both on sender and receiver side. Then you can implement flashing LEDs, thresholds, delays and many other functions. One such function would be to code/encrypt the RF signal, increasing noise immunity and allowing you to use only one receiver and one frequency for the two transmitters.

    Such solution would be very good, but would need much time to engineer (my experience is that the software would take most time).

    On the sensor side I would use an analog comparator or window amplifier to switch at exactly the levels you specify. It seems you want high accuracy on that/those threshold(s).

    You could make the circuit without uC, just using comparators and cheap transmitters/receivers. That would be a quick-and-easy solution, but would be susceptible to disturbance if other RF sources would come close.

    Of course you could buy some finished product that would include the finished RF link with several channels etc. and interface it with the comparator circuit and LED driver.
  3. jeff77789


    Feb 23, 2013
    in addition to the AN voltage pin from the sensor, there is also a "Tx" pin in which:
    delivers asynchronous serial with an RS232 format, except voltages are 0-Vcc.
    The output is an ASCII capital “R”, followed by three ASCII character
    digits representing the range in inches up to a maximum of 255,
    followed by a carriage return (ASCII 13). The baud rate is 9600, 8
    bits, no parity, with one stop bit. Although the voltage of 0-Vcc is
    outside the RS232 standard, most RS232 devices have sufficient
    margin to read 0-Vcc serial data. If standard voltage level RS232 is
    desired, invert, and connect an RS232 converter such as a MAX232.
    When BW pin is held high the TX output sends a single pulse, suitable
    for low noise chaining. (no serial data).
    [this is an ultrasonic sensor]

    how would this change things? would i still need a microcontroller on the sensor side? can i just plug this output directly into the RF transmitter?
  4. Electrobrains


    Jan 2, 2012
    The serial port output of your sensor gives you another reason to use a micro controller on the transmitter side. The 0V/Vcc level would probably allow you to directly interface it with a uC that has an integrated UART, thus totally avoiding going analog.

    If you would choose the less advanced "cheap-quick-and-easy way" described above, without uC and coding, you would probably be able to send over the serial signal itself.
    But that solution is no longer very attractive, because now you would need two uC's on the receiver side. Your RF links would need to be able to handle the speed of 9600 baud.
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