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Bypassing a momentary power switch?

keitaro

Hello all! Hope I'm posting in the right place.

I'm currently attempting to bypass the power switch to an NES-on-a-chip style console so that plugging in a power source will automatically turn it on. Originally, the switch was located on it's own board (with some other things like controller ports) and attached back to the main brains of the unit with a ribbon cable. I've attached a picture of this (they were kind enough to provide a pin-out!)

imageimage

The switch in question is a 6-pin momentary switch, the exact same one as seen here and here. If you look at the second picture I've attached, you'll see how this thing is wired to the board. It appears the 5v line from the unit's 7805 is attached to the SW1 pin and not VCC (of the main board) I assumed that to bypass the switch (as I removed the entire board it was located on), I could merely bridge the VCC and SW1 pins on the main board. This did not work.

 

If my understanding of this is correct, this type of six pin switch is known as DPDT. I'm not entirely sure what the benefit or purpose is of using this kind of switch in this application would be. Following the video I linked previously, I used my continuity checker to see if the pins were the same as demonstrated. For whatever reason, every single pin of the switch returns continuity with each other, regardless of the state of the switch. Further compounding things is the fact that SW1 and VCC also appear to returning as having continuity with one another---which leads me to think my theory of simply bridging these lines should have worked! If you look at the picture I have attached, you can see how this switch is wired to those lines. Am I missing something really obvious, or is there more to this? Thank you all so much for your help and suggestions!

 

EDIT: I attached a picture of the actual board that I removed (which contains the switch circuitry in question), for reference and context, hope that helps image if needed, I can post the main board it attaches to as well, but this is the meat of it. There's a set of pads corresponding to these pins, the SW1 pad on the main board connects to the 7805 like I mentioned, as well as obviously the SW1 line on the board pictured here.

Attachments:
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  • Hi Tony,

     

    Are you sure it is a momentary switch, or could it be a latching switch? They look, identical, but the feel will be slightly different between switching it on, and switching it off.

    The two middle pins (marked SW1 at the top and VCC at the bottom in your drawing) each connect on the same row to either the left side, or to the right side, depending on the switch state being on or off.

    So, it seems incorrect that the centre pins would be SW1 and VCC, because in both states they would be connected together. You're doing the continuity check on an unpowered board, right? Please make sure you discharge any capacitor on the VCC or SW1 lines to the GND pin before doing the continuity test.

     

    Sometimes a DPDT is used because it could be cheaper - also having two both of them connected allows for the switch to handle a bit more current - not elegant, but considered acceptable by some for low-cost consumer equipment with a limited product life.

     

    What happens if the power is not connected and the switch is in the on state, and then power is applied?

  • in reply to shabaz

    you may be correct about it being a latching switch, my mistake! There are no caps on this break out board, but that's good advice to keep in mind--I was in fact doing the test on an unpowered board. As for what happens when applying power to the unit with the switch engaged on, the unit turns on right away. Disengaging the switch will then turn it off as normal.

  • in reply to keitaro

    Hi Tony,

     

    In that case, bridging wires will work, but it is likely the wrong wires are being bridged if that didn't work. I don't have a suggestion except to carefully follow the wires if the continuity check method is not working for some reason (like charged capacitors). Also a photo of the switch and wires to the board would help, and a photo showing what you bridged and how.

  • in reply to shabaz

    hopefully these pictures help clarify what I'm working with! Basically, on the main board with the cartridge connection, I bridged the two pads that I've labeled (the ones which attach to SW1 and VCC as they're labeled on the switch's board) using a lead I snipped from a component.

  • in reply to keitaro

    That should work, but there could be some other signal on that board that isn't present if it is removed, causing the in-built firmware to keep the board from operation as expected. There is a reset signal for example. But I can't see an issue to be honest. I suppose the way to test is to re-solder that board on, confirm it functions, and remove a wire at a time. Not a great way to do it, but there are not many connections.