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  1. #41
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    Default Re: Active low transmitter question

    Hi Richard / Everyone

    OK - So I managed to get a call into the salesman today and he said:

    First off the video. The module in the video is a Bravo module, very similar to the Gamma module (the ones I have) in fact they are the forerunner on my module but a difference is the video module (Bravo) has 'latching and momentary' outputs the video shows a momentary button press onto a latched output on the Rx, so that explains the Led staying on.

    On to my module (Gamma). As I understand it now, these don't have latched outputs on the Rx. And the Tx / Rx cycle is this:

    1/ The default module setting is for a 720ms Tx set on mode5.

    2/ The Tx occurs when a Tx pin is pulled Low and must be held low for more than 720ms.

    3/ If the Tx pin changes state (goes high) within the 720ms time frame, then the transmitter will automatically resend another two data packets*.

    4/ The Tx cycle is: Tx pin is set Low - stays low for greater than 720ms - Tx pin goes High, ends Tx.

    5/ The corresponding Rx pin would then go High for greater than 720ms - Then go Low again when the Tx pin goes High.

    *Just as an aside, I really don't want false triggering on the Rx output pin as this will be attached to a pic input pin then output to a relay alarm buzzer.

    Any way a little clearer I hope?

  2. #42
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    Default Re: Active low transmitter question

    As an aside. Has anyone used a 2N7000 Mosfet attached to a 3.3v pic output to drive a relay?

    The schematic (link below) is exactly what I want to do from a pic output pin rather than the Receiver output pin.

    http:///www.rfsolutions.co.uk/downlo...elayOutput.pdf

    Any thoughts appreciated as ever.

    Dave

  3. #43

    Default Re: Active low transmitter question

    Never tried 2N7000 but for 3V outputs, we use an IRLZ44N. It's a power mosfet that we use to switch up to a couple of amps without needing a heatsink.

  4. #44
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    Default Re: Active low transmitter question

    Thanks for the reply peterdeco1 - So would that be the same wiring / resistor configuration as the 2N7000 (see link diagram) a straight swap in other words?

  5. #45

    Default Re: Active low transmitter question

    Yes, although I use a 47K across gate to source. Either way it will work. EDIT Note there is a L in the part number for Logic Level. It is NOT the standard F.
    Last edited by peterdeco1; - 15th July 2021 at 18:10.

  6. #46
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    Default Re: Active low transmitter question

    Thanks again peterdeco1 - If there's two parts to order from, I'm the guy who orders the wrong one (but not in this case thanks to you).

  7. #47
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    Default Re: Active low transmitter question

    As an aside. Has anyone used a 2N7000 Mosfet attached to a 3.3v pic output to drive a relay?

    what relay ? specs ?


    2N7000 specs are admirably suitable for low voltage / low coil current relays

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    an IRLZ44N to drive a smallish relay is way overkill
    This is more entertaining than Free to Air TV

  8. #48

    Default Re: Active low transmitter question

    I agree Richard. It's like putting a Ferrari engine on a lawnmower. At work we have a small PCB with an 8-pin PIC and 4 of these mosfets. It replaces about a dozen different subassemblies in products all governed by the PIC program. These transistors sometimes simply switch a string of LED's on and off, a 2A DC motor or a combination of the two. That's why I am "programmed" to using them.

  9. #49
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    Default Re: Active low transmitter question

    "what relay ? specs ?"

    Hi Richard - I haven't got around to choosing one as yet.

    I need a relay activated by the pic (3.3v) via a transistor/mosfet to switch an alarm sounder on (fairly loud sounder) probably 12v (I haven't chosen the sounder yet either).

    I think I need to try and do a better schematic to show what I'm trying to achieve - I'm a carer, really busy right now but will try and do a more appropriate diagram over the weekend and my thinking on the components.

    Thanks for your help as ever.

    Dave

  10. #50

    Default Re: Active low transmitter question

    I'm sure by "spec's" he means how much current does the relay draw.

  11. #51
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    Default Re: Active low transmitter question

    Ok - Instead of a schematic (can't find all components in menu I need to make drawing), have decided to talk through each section of the project as there isn't really that much to it if that's ok?, so:

    Section one / Transmitter Side (Battery Operated): Through beam sensor (which I have bought along with the transmitters) http://www.alibaba.com/product-detai...2abf516dBZICkn Half way down page under 'Retro Reflective Type'

    The sensor is 12v and operates over a distance of 5m in daylight, beam gets broken and outputs 12v (obviously too high for a input into the pic) so to drop the voltage down to 3.3v I'm thinking of a Voltage divider Vout = Vs x R2 / (R1+R2). Vs = 12v R1=1k R2=390 Ohms giving Vout of 3.367v or a 12v to 3.3 Zener cct or using a http://uk.rs-online.com/web/p/voltag...4923/?sra=pstk (not sure on which way to go).

    So the beam gets broken - 12v from sensor down to 3.3v to the pic, pic reads the input pin as high and makes an output pin change state and causes the transmitter to Tx.

    Section two / Receiver side (mains operated): The receiver receives a Tx input, pin on pic reads the Tx input and sets an output pin high onto the gate of a 2N7000 http://uk.rs-online.com/web/p/mosfets/6714733 which then drives either a http://uk.farnell.com/axicom-te-conn...55?st=3v relay or a http://uk.rs-online.com/web/p/solid-...relays/8887619 which in turn drives a loud sounder http://uk.rs-online.com/web/p/piezo-...E&gclsrc=aw.ds

    I have been drawing a pcb layout for these cct's using Diptrace as I'm going to make my own / first pcb's too.

    Any thoughts on the above as ever very much appreciated as ever.

  12. #52
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    Default Re: Active low transmitter question

    which in turn drives a loud sounder
    why a relay at all ? the 2n7000 could drive that alarm directly with ease
    Last edited by richard; - 22nd July 2021 at 03:52.
    This is more entertaining than Free to Air TV

  13. #53
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    Default Re: Active low transmitter question

    why a relay at all ? the 2n7000 could drive that alarm directly with ease
    I never thought of that, great idea, cheers Richard.

    What do you think the best way to drop the 12v from the sensor output to 3.3v into the pic, would the voltage divider be ok/work?

  14. #54
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    Default Re: Active low transmitter question

    works for me. you could protect against divider failure with a 3 to 5v zenner if you are paranoid
    the first pic may be unnecessary too if the tx works like i think it does
    This is more entertaining than Free to Air TV

  15. #55
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    Default Re: Active low transmitter question

    works for me.
    Excellent, voltage divider it is then.


    you could protect against divider failure with a 3 to 5v zenner if you are paranoid
    Yes I do fit into that category (mildly at present), will look at at a backup with Zenner.


    the first pic may be unnecessary too if the tx works like i think it does
    I hear what your saying Richard, so I need to built the Tx cct on the bench now and test the Tx driven directly by voltage diver (no pic).

    Thanks for your help, appreciated.

    David

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