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Part substitution.

Discussion in 'General Electronics Discussion' started by Old Grey, Feb 20, 2016.

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  1. Old Grey

    Old Grey

    Feb 5, 2016
    Last edited: Feb 20, 2016
  2. Bluejets


    Oct 5, 2014
    Did you notice the pin configuration between the BC517 and the MPSA14 is different?
    davenn likes this.
  3. Old Grey

    Old Grey

    Feb 5, 2016
    I remember changing the part in Fritzing but still went back and checked, and it seams right. I'm using PCB prototype board so ignore the 2 layers on the pic. The resistor coming off the B of the BC517 is supposed to be 10M, not 1M - I already posted before noticing -.
    Also I was given a pack of diodes that I used that had BA216 = 1N4148 = 1N914 written on the bag, so I hope they are right.

    The circuit is supposed to add 0.1V to the output of a narrowband lambda o2 sensor for each position of the rotary switch so as to fool the car ECU into thinking the car is rich, and therefore leaning the mixture.

    After some fiddling with the VRs 3 switch positions added voltage, but now none do - all around 0.050V now-, so I was thinking maybe the parts aren't quite right.
    Last edited: Feb 20, 2016
  4. Old Grey

    Old Grey

    Feb 5, 2016
    I put the LM3914 into another circuit and it was fine.
    I touched up each solder joint, and nothing.
    Then I changed the BC517 and it now put out 0.5V. After a while it went to 1 V and I adjusted the trimpots correctly, but when I turned the rotary switch it stopped working properly. The LED also stopped working.

    Then I found a proper explanation of the circuit. It's detailed but weird, because it looks like they were modifying the old circuit as they were explaining each section.

    The 470uF acts as a delay so the engine will start on cold cycle before this circuit takes over. You calibrate the circuit without C1, and since I don't need cold start I chucked it.
    It works on a threshold system where as the O2 sensor voltage goes up (0-1V), successive pins on the LM3914 go high (0.05V steps). And depending on the rotary switch position, when the PN200 is low it switches the high VR2 pre-set voltage to the output.
    This part of the circuit seams to work, I put switch on pin15 and it goes low to high at 0.1V, and pin10 goes low to high at 0.5V

    The problem now is that the high VR2 pre-set voltage won't go over 0.43V, and I need 0.98V output - it did work for a brief time but now doesn't -. I've got VR2 pegged to one side and the most it puts out is 0.43V.

    I got the BC517 from France as new/old stock, now I'm wondering if they aren't any good.

    EDIT : I left it on 1V input for a long time and the output sometimes goes to 0.7V, then back to 0.4V.
    Last edited: Feb 21, 2016
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