How to estimate Capacitor C1 value for 433Mhz Remote control

Discussion in 'The Projects Forum' started by JeffTanth, Jul 13, 2014.

  1. JeffTanth

    Thread Starter New Member

    Jul 13, 2014
    4
    0
    Hi all,

    I am trying very hard to resuscitate my car remote alarm. The remote is able to lock and unlock the car when I activate it in the car; very near distance. But it is not able from further distance.

    A marking on the capacitor, C1 is missing. Is there anyway to compute the range of the values for this capacitor and I use a variable capacitor to tune it. I had read much from the internet and think it is about 9 to 10 pF; but it only extend the distance very little.

    I am playing with a field strength meter and testing many capacitor values and they seemed not to work properly. RF electronics is really tough especially if one does not have the oscilloscope to view it's workings.

    I would appreciate if someone can explain to me how this circuit works or direct me to places that I can learn more about this sort of circuit.

    Many thanks in advance.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,974
    744
    Looks like its an Hartley oscillator transmitter centered at 433Mhz.The loop on the pcb is an inductor tapped off with the antenna, the two caps in series form a tuned circuit with the inductor, one is variable giving an overall range of approx 1 to 5 pf, the "unknown " cap i would put a 1-47pf variable in and try for best result, also Q1 is an NPN.
     
    Last edited: Jul 13, 2014
  3. JeffTanth

    Thread Starter New Member

    Jul 13, 2014
    4
    0
    Thanks for the fast reply. Will need to go to the shop tormorrow to get a variable cap. Will post the result in the next few days.
     
  4. JeffTanth

    Thread Starter New Member

    Jul 13, 2014
    4
    0
    Sorry that the remote is still not working outside of the car.

    Have tried all discrete capacitors from 1p to 20p. Testing with capacitor 10pf to 47pf when the one wire broke. So I stop there and will continue next week. The response distance is better by 1 foot when the wire is left dangling from the circuit board. It is quite difficult to adjust two cap trimmers and stay consistent.

    I suspect the transistor Q1 is not work well, will desolder C1 and Q1 to test as well later. Also waiting for my USB SDR to arrive to read the codes off the remote control.
    Also did some scope reading with my
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Working on receive antenna placement might have more effect.
     
  6. JeffTanth

    Thread Starter New Member

    Jul 13, 2014
    4
    0
    Thanks for suggestions. It did helps a little; not much.
    I think my circuit diagram has some errors and will be rechecking it again later in the week. Also will add some information taken off my old oscilloscope which reads up to 15mhz).

    Got to work so will be late in reply.
     
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