Comments please.

Discussion in 'The Projects Forum' started by nerdegutta, May 12, 2013.

  1. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
    2,515
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    Good morning.

    I'm building a wireless thermometer with a LM34. Attached is the transmitting circuit, and I'd like some constructive comments.

    Transmitter and Receiver modules: 433MHz RF Wireless Transmitter/Receiver

    Transmitter:
    JP1: 12v car battery.
    JP2: Picaxe programming socket.
    JP3: Transmitter power jumper. I'd like to have the option to choose between 5v and 12v for the transmitter. With 12v the range increases.
    JP4: The TX module.
    JP5: LM34 temperature sensor.

    C1-C6: Bypass capacitors

    R1: Value from datasheet (Picaxe manual)
    R2: Value from datasheet (Picaxe manual)
    R3: IC out=4.85v -> (4.8-2.1)/0.02=135.7Ohm -> 130 Ohm

    Receiver:
    JP1: 12v wall adapter
    JP2: PICAXE programming socket
    JP3: The RX module
    JP4: 16x2 LCD module, with backlight

    C1-C5: Bypass capacitors

    R1: Value from datasheet (Picaxe manual)
    R2: Value from datasheet (Picaxe manual)
    R3: Contrast pot, value from datasheet

    TP1-TP9: Testpoints for future use.

    The plan is to get a reading from the LM34, and calculate the value from Fahrenheit to Celsius, and display it on a different circuits LCD. I got it working on the breadboard.

    Any obvious errors/faults?
     
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    Last edited: May 12, 2013
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    If this is a small board and regulator and the Picaxe chip are in close proximity. You may skip the C4. Or perhaps better replace it with a small tantal in range of 1 to 10uF. However it is not by any means anything wrong in keeping your current setup
     
  3. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
    2,515
    785
    Ok, thanks.

    I haven't started on the board design/layout yet. I'm trying to make it all SMD, except the electrolytic capacitor.
     
  4. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    C2 seems a bit oversized, the datasheet only mentions a 0.33μF capacitor at the input side. If the 12V supply is really noisy I think you could use a resistor in series with the 12V line then a much smaller capacitor to ground on the input of the 78L05.
    The resistor would be sized so the voltage drop across it at the maximum current drawn doesn't take the input of the regulator below the 7 volts that it needs, maybe 10 ohms would be good for 100mA peak load.
    [​IMG]
     
    Last edited: May 12, 2013
  5. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    785
    When I placed the electrolytic cap, I also thought that this was i bit high. Perhaps 100uF is more suited?
     
  6. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    I think 10μF would be fine with the resistor, and they are available in 1206 packaging as surface mount 16V ceramic capacitors.
     
  7. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Great!

    I'll check my supplier.
     
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
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    Considering your location, I am curious as to why you are using an LM34 instead of an LM35. I assume you are avoiding the DS18B20 for the same reason?

    Nice schematics! DipTrace?

    I once breadboarded a time and temp unit using a PICAXE; a photo is attached. (It did not include any RF links.)
     
  9. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    I figured it would be easier to calculate C from F, rather than getting negative values from the LM35.

    Thanks. I don't do DipTrace. I'm a Linux guy, and I find Eagle easy to use. But that's me...;)

    Do you remember which uC it was?
     
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    It was a 20X2. The schematic is attached.

    I wish I was a Linux guy; every time I buy (or rent) a piece of overpriced bloatware from the world's richest guy, I retch.
     
  11. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
    2,515
    785
    Thanks.

    Well, the first thing I do when I get some new stuff, is to remove all the bloatware. I did that on a Samsung Galaxy SII, and it seemed to works faster.

    Most of my computers have Linux installed.
     
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