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Help needed with a dark activated switch

Discussion in 'General Electronics Discussion' started by Tom_L, Feb 6, 2014.

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  1. Tom_L

    Tom_L

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    Feb 6, 2014
    My first post here - and a request for a bit of assistance.

    I'm in the process of assembling a night vision device based on a cascade image intensifier tube. The tube itself runs just fine off 4x AA Eneloop rechargeable batteries, right now the output voltage is at 5.2V. But I also wanted to add a control circuit that would only allow the tube to power on in darkness (to prevent accidental damage of the II tube due to overexposure). I basically copied this schematic that someone had apparently tried already:

    http://www.hvlabs.com/Images/nightviscircuit.JPG

    Unfortunately, it does not work as intended. The dark-activated switch performs OK - when adjusted properly, the NPN transistor switches on when it's dark enough and I get full 5.2V at the collector.

    However, as soon as I attach the tube between the collector and Vcc the output voltage sinks to about 1.2V and the tube does not power on.

    I assume the output current is insufficient to power the tube - which only needs 30mA. What should I do?
     
  2. Tom_L

    Tom_L

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    Feb 6, 2014
    [​IMG]
     
  3. dustin02rsx

    dustin02rsx

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    May 18, 2011
    When you say your tube is functioning just fine off ~6V when out of the circuit, do you have the potentiometer shown in your schematic in series with it at this point? Are you keeping the same value of resistance on the pot as you are in the circuit? If not, test it with the potentiometer in series.

    The transistor should be able to drive 30mA no problem (the part number in your schematic is rated for 6A.). You may have your potentiometer resistance too high causing a lack of current to go through your tube.


    I'm assuming the batteries are fully charged and the rest of the circuit works and it is in fact switching the transistor.

    Also, what does your supply voltage read when hooked up to the circuit?
     
  4. Tom_L

    Tom_L

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    Feb 6, 2014
    No, I omitted the final potentiometer ("tube brightness") - it's irrelevant because the tube itself has a voltage stabilizer of some sort and the performance is pretty much the same within the 4.5-7V range.

    The rest of the circuit is definitely switching the transistor on/off depending on the light level. With no load applied to the collector the output voltage reads at 5.2V (=Vcc). But as soon as I connect the tube (between Vcc and transistor collector) the voltage drops to somewhere like 1.2V.

    The batteries are not fully charged... But there is still some juice left. If I connect the battery pack directly to the tube it works just fine, so I don't think it's an issue.
     
  5. Tom_L

    Tom_L

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    Feb 6, 2014
    Quick update if it makes any difference - I tried substituting a 9V alkaline battery and it works, powers up the tube just fine, but 9V is too much voltage for the tube itself and I'd far prefer AA's.

    Trying 4x freshly charged Eneloops didn't work, though. Fresh alkalines didn't work either, despite the slightly higher voltage.
     
  6. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    When you say with no load you get 5.2V. How are you measuring that.
     
  7. dustin02rsx

    dustin02rsx

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    0
    May 18, 2011
    What is your measured current when your load is in?
     
  8. Tom_L

    Tom_L

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    Feb 6, 2014
    Oh well... Looks like I did figure it out after all after all. I reduced the 4K7 resistor at the base to 2K. Works like a charm :)

    Many thanks to everyone chiming in! Even though I do feel a bit embarassed not having spooted such an obvious flaw right away. Maybe this thread will be of some use though if anyone else tries to play with the P8079HP cascade night vision tube.

    (Well worth the effort BTW, it's really a great way to get near Gen 3 night vision performance for something like 200$ altogether.)
     
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