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433MHz RF Receiver Issue

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Discussion in 'Radio and Wireless' started by jhest1976, Sep 7, 2018.

  1. Sep 7, 2018 #1
    jhest1976

    jhest1976

    2
    0
    Sep 7, 2018
    I am having an issue with an RF receiver being used with an HT12D decoder IC. When I power the circuit with 5V (recommended for the RF receiver), it works with the transmitter as it should to control a small LED bulb. However, the LED light I want to control needs closer to 12 volts to illuminate.

    I modified the circuit (see attachment) and it is now powered by approximately 10.86V (8 rechargeable AAs w/ full charge). A 7805 voltage regulator decreases voltage to the RF receiver to approximately 4.99 volts. The HT12D is now powered by the 10.86 volts, and is no longer functioning properly.

    My question is, since I have increased the voltage to the HT12D, do I need a different resistor value on pins 15 and 16 of the HT12D? If so, how do I calculate the value?

    Please let me know if further details are necessary, as I am sure I have left something out.

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  2. Sep 7, 2018 #2
    Harald Kapp

    Harald Kapp Moderator Moderator

    9,055
    1,818
    Nov 17, 2011
    The HT12D is a CMOS chip. CMOS inputs, unless stated otherwise, typically have an input threshold at ~ 1/2*Vdd. Unfortunately the datasheet of the HT12D states only the logic level input voltages for 5 V, but a minimum of 3.5 V for high with Vdd = 5 V supports this hypothesis.
    Therefore when operated from 11 V (I think I read 10.89 V in your schematic, it's a bit hard to see) the threshold will be at about 5.5 V. The 5 V logic levels from the receiver cannot be detected in this configuration. Even a full 5 V from the receiver will be detected as low by the HT12D in most cases.

    My recommendation: operate both circuits from 5 V, use a simple LED driver (see e.g. our resource) to power the 12 V LED.
     
  3. Sep 8, 2018 #3
    jhest1976

    jhest1976

    2
    0
    Sep 7, 2018
    Thank you for the information and suggestion, Harald. Hopefully I will have time to work on it this weekend.

    My plan was to use one of these LEDs in my project as an indicator. Unfortunately there don't seem to be any specs for the lights, but I have seem others stating '12v, 0.6W'.
     
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