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Some might remember a childrens game where you have a sheet of symbols, holes in it that lead to contacts, and then you have two wires and whenever you have two symbols that match you put some cables onto those two, and a light shines that tells you, you were right. In my childhood this game was simply called "electric" and I want to create a similar game for my kids too, that doesn't fall apart as soon as you look at it.

My goal for the design were

  • Will fit into an approximately test tube sized transparent plastic tube
  • For the looks is a freeform circuit
  • Does not use anything more complex than a transistor (i.e. only three pin TO devices, no more pins or flatpacks or so)
  • When the contacts are closed, will light an LED for about 2 seconds
  • Will draw very low (ideally none) power when not active
  • LED will not go out while contacts are closed
  • LED will turn on to full brightness, and when it goes off it shall not dim, but instantly go down to zero brightness. Some not really visible brightness changes (i.e. current fluctuations) are ok though.
  • Amount of "P " transistors (PNP, PMOS) or other uncommon ones is low or zero. Ideally would be NPN only.

My design started out with some simple latching switch, adding timeout and other requirements as I went along, and what I came up with in the end is (ignore V2-V4 and S1, they are for testing the leads actions):

enter image description here

Basically around Q1 and Q4 is the latching switch, C2 is charging up until M1 causes the switch to latch off again; Q5 is there to hinder charging of C2 as long as the leads are connected.

Unfortunately that feels like too much components for this task. Also -- while the LED stays on while leads are connected -- it does not really do retriggering, at least not for very short contacts. This might be fixed by adding yet another transistor in front of Q5 to increase the current it shunts C2 to ground with, but that would mean: yet another component.

So the question is:

Can this be simplified to less components, or at least NPN only? Maybe at the same time solving the retrigger problem, or can that at least be solved without adding transistors?

Note: I might have forgot to mention some requirement because it is evident for me how the game should work, but not for you if you are unfamiliar with it. If you have the feeling that it would be the case with your suggestion, I would appreciate if you ask if "behaviour XYZ" is ok too.

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  • \$\begingroup\$ Could you review everywhere you've written PNP in the text and decide if it should have been NPN? Some of this doesn't make sense. \$\endgroup\$ – user1844 Oct 22 '15 at 11:50
  • \$\begingroup\$ @WillDean: Done, you were right, a little disagreement between my brain and my fingers it seems... \$\endgroup\$ – Ppup Oct 22 '15 at 11:51
  • \$\begingroup\$ Can this be described as "LED should light when contacts are closed, on-time of LED is 2 seconds minimum"? \$\endgroup\$ – user1844 Oct 22 '15 at 11:55
  • \$\begingroup\$ @WillDean: The values are not critical, but the LED should lit when closed, then stay lit between 1-3 seconds after the contacts are opened again, no longer. So you can try two other contacts again, and not have to wait or do something like taking off the batteries or so... \$\endgroup\$ – Ppup Oct 22 '15 at 12:04
  • \$\begingroup\$ This looks like a job for the classic two-transistor monostable multivibrator to me. \$\endgroup\$ – pjc50 Oct 22 '15 at 12:40
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This sounds like one transistor to me - pull-up a large delay cap through the contact and a small resistor, and connect it to the base of a NPN transistor with a larger resistor.

Have the NPN transistor turn on the LED.

When the contact is closed, the cap will be charged and the base of the transistor will be supplied through the contact. When the contact opens the cap will supply the transistor for a while.

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  • \$\begingroup\$ I don't think that this would look nice enough. I think the LED brightness would quickly drop to something dim and stay there for a while. Anything than a more or less hard on/off of the LED would likely confuse the kids. \$\endgroup\$ – Ppup Oct 22 '15 at 12:29
  • \$\begingroup\$ Fair enough. I guess 'looks nice enough for my kids' is not a parameter easy for someone else to measure. There are obviously things you could do with a zener which would give you a harder cut-off. \$\endgroup\$ – user1844 Oct 22 '15 at 12:44
  • \$\begingroup\$ Indeed it isn't, so I will add some hard on/off to the requirements list. The point is for them to get some clear bright feedback and not having to guess if the LED is still on from the last time, or the batteries are empty or so... \$\endgroup\$ – Ppup Oct 22 '15 at 12:54

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