help with transciever.

Discussion in 'General Electronics Chat' started by dray, Apr 29, 2006.

  1. dray

    Thread Starter New Member

    Apr 13, 2006
    5
    0
    Hello out there,
    I'm building a unit which uses a micro-controller to take acceleration data, and then transmit the data back to a ground station. I'm using aerocomm 4790-1000 trancievers. I'm having a problem with interfacing the transciever with the micro-ontroller. I have tx, rx, and the command/data pin hooked up via diodes to regulate voltage from 5 to 3.3. I'm using and seperate power source to run the transciever. When I plug in the power source I get three volts on the rx,tx and command/data lines. As far as I am to understand this should not happen unless directect by the micro-controller to do so, or at the very least I'm pretty sure I'm not suppose to have voltage coming out on the rx line.
    Any help would be greatly appreciated.
    Thanks
    D.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,176
    1,799
    My standard request -- post a schematic
     
  3. dray

    Thread Starter New Member

    Apr 13, 2006
    5
    0
    Here is my schematic.
    Any help would be appreciated thanks.
    d
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,176
    1,799
    I don't think you can do what you are trying to do the way you are doing it. Whoever told you you could hook up 3V and 5V parts this way has misled you.

    Here is what I would do if I were you. Stop messing around trying to save a few pennies with discrete parts. Investigate the LVC family of logic devices. They can work from a Vcc at 1.65 Volts up to 5.5 volts. The cool part is that the inputs will tolerate voltages higher than Vcc.

    Use a buffer powered from 3 Volts to run the 5V Tx signal to the trnasceiver. The buffer will tolerate the 5V input and produce a 3V output. Use a buffer with TTL thresholds( low < 0.8 Volts, high > 2.4 volts) powered from +5V to bring the 3Volt Rx signal fromn the receiver into the microprocessor. Similarly for the command data line. Something like this should be the ticket

    Level Shifter

    The following is no reflection on you, but I have to say:

    I have seldom seen such an awful schematic. Please tell me that you wasted hours creating this schematic with a general purpose drawing package that knows nothing about schematic entry and cannot produce a netlist. Nobody, but nobody, uses the semicircular bridges, and your ground next to the +3V was very hard to figure out. Put the Ground symbol at the bottom of the page. Wires can cross each other without a junction and nobody thinks they are connected. Most reasonable schematic entry packages have a junction symbol for this purpose. With free schematic entry pacages available there is just no excuse for using a method that is not designed for what you want.

    As a final point, why is the schematic showing up as a WORD file? Downloading WORD files can be very risky. I consider it rude to subject us to the risks of downloading WORD files.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    While what you say is true, why didn't you recommend to the poster an inexpensive schematic capture program?

    Now, for my recommendation ... you can print your schematic to a pdf file. One free pdf distiller is Cutepdf which you can download at http://www.cutepdf.com You could also post your schematic as an image file.

    Schematic capture programs are numerous. Everyone has their favorite.

    Searching Google gives you a starting point for Schematic capture with netlists.

    Good luck.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,176
    1,799
    TinyCad and ExpressPCB are two that come to mind. Try the following links

    TinyCAD WebPage at the sourceforge

    ExpressPCB
     
  7. Nirvana

    Well-Known Member

    Jan 18, 2005
    58
    0
    Hi there could the voltage decrease result from the junction voltages of the LED's?
     
  8. dray

    Thread Starter New Member

    Apr 13, 2006
    5
    0
    I don't follow. What do you mean by junction voltages?
     
  9. Nirvana

    Well-Known Member

    Jan 18, 2005
    58
    0
    Hi there, the junction voltage without going into any detail is the voltage the semiconducting material takes in order to conduct sufficiently. For example if you had a resistor in series with a silicon diode and this connection is supplied with 4 Volts then the diode being a semiconductor and silicon in this case takes (by the nature/physics of the device material itself) about 0.7V. Therefore there will be 4- 0.7 = 3.3 Volts dropped across the resistor. :) These semiconductor devices all have junction voltages, silicon about 0.7 V and Germanium about 0.2 V. :)
    Nirvana.
     
Loading...