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RF switch transmitter pls help

Discussion in 'Electronics Homework Help' started by max_torch, Mar 13, 2015.

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  1. max_torch

    max_torch

    98
    1
    Feb 9, 2014
    In my class we were tasked to construct the project called 'Remote Control Doorbell' from Alexan Projects and Circuits Volume 2, which are actually a bunch of projects compiled from electronics enthusiasts magazine. Below is a bad copy of some of the pages (The copy our teacher gave us to work with was already a bad copy, I could not find an original copy).[​IMG]
    [​IMG]
    [​IMG]
    [​IMG]

    For now, let's focus only on the transmitter first:
    Currently among 10+ groups in my batch that are working on this, all of us have the same problem with the transmitter: We can observe the modulating signal (somewhere between 26-40KHz) being output by the multivibrator using a frequency counter and oscillator, but there is no frequency(300MHz) being generated by the LC oscillator, even if we isolate the LC oscillator from the multivibrator by connecting the emitter of the feedback transistor Q2 straight to ground. All I can see when I try to read the output of the oscillator is a fixed straight line at 9V, no matter what the magnification and time/div I set the oscilloscope at, not even a faint trace of any kind of oscillation going on.. My group has tried reading if the the feedback amplifier transistor of the LC oscillator is properly biased so we first bypassed Q1 by connecting the emitter of Q2 to ground and this is what we read:

    Base Voltage of Q2 is 1.25V
    Collector Voltage of Q2 is 9V
    Emitter Voltage of Q2 is at ground potential

    So I'm not sure but maybe this indicates that the feedback transistor Q2 is properly biased and in the correct operating mode, which leaves me with the only culprit for the problem as to be the rest of the oscillator circuit components. I tried to check if the terminals of the component parts have continuity with the copper tracks on the bottom and they all seem to be properly connected (I placed one probe on the terminal of the component from the top side of the board, and one probe on the copper track at the bottom of the board to check this). I do not know anymore what option to explore please help us.

    Also, I bought a wireless doorbell cheap from CD-R-King so I can open it and see how a properly functioning wireless doorbell should operate and maybe I could compare it with what we are making.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,663
    2,019
    Nov 17, 2011
    This sounds not reasonable. A NPN transistor typically has Vbe ~ 0.6V...0.7V. A reading of 1.25V looks like you used a darlington transistor (which has 2*Vbe in series, hence twice the voltage of a single transistor). A darlington is usually not optimized for operation at 300MHz.
    Tell us which transistor type you used.
    Try a transistor designated for HF operation.
     
  3. duke37

    duke37

    5,297
    739
    Jan 9, 2011
    Will the scope respond to a frequency as high as 300MHz?
    You could build a simple diode detector, using a fast diode such as a 1N4148 to detect if the oscillator is working.
     
    max_torch and Harald Kapp like this.
  4. max_torch

    max_torch

    98
    1
    Feb 9, 2014
    Hmm good point... I guess I'll have to check if 300 MHz signal is within the range of the scope or the frequency counter. That could be why no one is getting an output signal. And this is the transistor Q2, it is indeed designed for HF operation: http://pdf.datasheetcatalog.com/datasheet/toshiba/1222.pdf
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    9,663
    2,019
    Nov 17, 2011
    O.K., but according to the datasheet you should see a mx. Vbe of 0.8V (page 3, top left). Your measurement of 1.25V doesn't fit in here.
    Also note that the 2SC1923 is specified at 100MHz. Its transit frequency (gain=1) is 550MHz. The datasheet gives no information on gain vs. frequency, but at 300MHz it may already be considerably less than expected. You could try to make the circuit work at 100MHz to verify the basic functionality of your setup.
     
  6. max_torch

    max_torch

    98
    1
    Feb 9, 2014
    There is no problem with the components or the circuit design, it has already been constructed before and proven that it works.. However our teacher who made it lost it and he's making it again when we requested him to make it again to see if he can help us..
     
  7. max_torch

    max_torch

    98
    1
    Feb 9, 2014
    Turns out that the instruments we were using could not detect 300 Mhz..
    Thank you really this advice was spot on!
    Okay, so I wonder how do I make a diode detector with the 1n4148?
     
  8. duke37

    duke37

    5,297
    739
    Jan 9, 2011
    Menu - diode detector.
    Take a fast diode, connect one side to ground.
    Connect a small capacitor (100pF) to the free side. This is the input.
    Connect a high value resistor (1M) to this junction
    Connect a small capacitor (100pF) from the other end of the resistor to ground.
    This is the output. Load it with more than 1M, a lot of meters have an input resistance of 10M.

    Keep leads short. It can be built dead bug style on a piece of plain PCB.
    Use a 1N4148 (1N914) or a small Schottky diode. You will need about half a volt to turn on the 1N4148 and somewhat less for the Schottky.
     
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