NE555 question from a newbie

Discussion in 'General Electronics Chat' started by froggy2684, Mar 2, 2014.

  1. froggy2684

    Thread Starter New Member

    Mar 2, 2014
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    I have a dilemma and a confession (confession is I am a relative newbie to electronics.

    The dilemma is that I want to be able to turn on a light via RF but don't want to continually send RF to the receiver.

    The transmitter is on 443Mhz with a PIR to activate an encoder and send the command to the wall socket to turn on the light, using a NE555 the light stays on for 5 mins.

    What I need to be able to do is have an NE555 timer (or other suggestions) on the transmitter and only send out RF every minute or so to activate the receiver. is this possible?

    Thanks in advance
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Draw a block diagram.
     
  3. froggy2684

    Thread Starter New Member

    Mar 2, 2014
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    After playing about and thinking more, i can maybe "simplfy" the question.

    555's i presume are normally used to turn something on for a period of time. are there any circuit diagrams to turn something off for a period of time.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    It can be done.

    Let say 555 output is normally low, so there is voltage drop across something, that something is ON. Now you trigger the 555, 555 output goes high, so there is no or very little voltage drop across something, that something is OFF. After a user defined period of time the 555 output goes back to low and the something turns back ON.

    That the simple explanation.
     
  5. froggy2684

    Thread Starter New Member

    Mar 2, 2014
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    Thanks for that, I dont suppose you know of an example circuit?
     
  6. shteii01

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    Not off hand. You can try it with LED where LED +leg is connected to voltage source (battery) and LED -leg is connected to 555 output pin. When 555 output is low/zero volt, there will be voltage drop across the LED, LED will be ON. When you trigger the 555, the 555 output will go high/X volts, so the LED -leg now has some voltage on it, the voltage drop across the LED is smaller or zero and not enough to turn LED ON, so LED goes OFF.

    If you have the parts, you can build the above circuit just to demo it to yourself.
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    A 555 configured as an astable multivibrator continually switches its output pin from high to low and back again. The exact ratio of high to low and the time for each can be controlled by some peripheral components. However, the 555 is itself "on" and drawing power while it is performing this function. The easiest way to minimize this power requirement is by using a CMOS version of the 555, such as a TS555.
     
  8. froggy2684

    Thread Starter New Member

    Mar 2, 2014
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    I really need to have a think before posting, I have attached 2 images and wonder if this is what you meant, the first one the led is always off until triggered, i wonder if the second image would work, and the LED is always on until triggered
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    In the second pic you have zero volt on the LED +leg, 6 volt on LED -leg, what you have in effect is reverse biased diode. When you trigger the 555, IF 555 OUTPUT is 6 volt plus forward voltage of the LED, then LED will light up. You can do it that way, but MAKE SURE THAT 555 OUTPUT IS LARGE ENOUGH TO TURN LED ON.

    Let us say that we are dealing with typical LED whose forward voltage is 2.4 volts. This means that LED will turn ON when voltage drop across the LED is 2.4 volts or greater, we don't want to destroy the LED so we keep it at 2.4 volts. So. Initially you have 0 volts on LED +leg, 6 volts on LED -leg. What is voltage drop across the LED? 0 volts on LED +leg minus 6 volts on LED -leg=-6 volts.

    Let us setup 555 so that output pin will produce 5 volts. We trigger the 555, the output pin goes from 0 volts to 5 volts. What is voltage drop across the LED? 5 volts - 6 volts = -1 volt. Will the LED turn ON? No. Why? Because -1 volt is less than forward voltage (2.4 volts) needed to turn LED ON.
     
    Last edited: Mar 2, 2014
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Just reverse the LED in the second pic.
     
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