Parasitic power from buttons

SuperSprint

Well-known member
Joined
Jun 29, 2010
Messages
2,753
Reaction score
664
Location
Georgia
Has anyone ever tried stealing power from the button drivers on a PCB? I'm working on a project that needs to go inside the control panel, but I don't want to run separate power up there. It's a low draw application, so I'm thinking a 1/2 dozen diodes tapping the button sources and dropped onto a +V buss would allow me to "steal" 5v from the PCB without affecting control functionality.
 
The circuit isn't expecting ANY load; depending on how the board designers approached it, it may work fine, or you may get nasty input glitches or fry something on the board. Or worse, it may work fine at first, but slowly cook the input. If this is gonna be a universal cabinet, one board may work just fine, another will just let out magic smoke.

Power ground and button ground are the same thing, so you only have to run one wire. I'd just run the one wire to a punchdown block beneath the CP; having Vcc there is useful for digital active controls (e.g. newer trackballs and spinners) and illuminated buttons anyway.
 
Last edited:
I don't want to say too much because this is a simple project and I want to solve it myself. If I toss it out, it'll be completed in about 3 posts and that would be no fun.

Anyway, think retrofit "rapid fire mod". That's why I want parasitic power. Two male spade lugs, one jumper, and it's installed even if you're a novice. The "load" isn't much, roughly the equivalent of a single 74LSXXX chip. It isn't driving any secondary loads either, only switching the i/o to gnd to trigger inputs.

But since these aren't being driven by the chip, but rather the i/o pull up power/resistors, I don't think the input encoder will source any power. The bigger question is a voltage drop, but since the TTL lower threshold is 2V, I'm hoping that won't be a problem if I gang 4-6 of them together.

Make sense? I'm much more worried about funky signals than smoke.
 
Last edited:
The more I think about it, the more I think it just wouldn't work. I doubt the input pin is going to provide enough current to power your chip -- you'll get a stuck closed reading in input test and your circuit won't run because its mere presence pulled down the input pin to next to no voltage.

Adding a power wire up to the CP is the way to go.
 
Maybe make an adapter that steals power from a coin door lamp? Just have to regulate it to your 5V. At least it would still mostly be plug and play
 
The more I think about it, the more I think it just wouldn't work. I doubt the input pin is going to provide enough current to power your chip -- you'll get a stuck closed reading in input test and your circuit won't run because its mere presence pulled down the input pin to next to no voltage.

Adding a power wire up to the CP is the way to go.

No, the input pin isn't an issue. Usually these things are done with a resistor to +5 pulling up the line, and a CPU input. Drawing current on that line (or frankly, even driving that line at +5) is NOT going to damage the inputs - the input isn't sourcing any current. The pull-up may be on the chip, or separate and on the board), but the actual input line isn't doing the work.

The thing is that if you pull any significant amount of current you WILL lower the voltage, and there's a good chance that you'll take it down far enough that it will make the input unstable.

Here's a better question: does whatever you are doing have to work when the buttons are NOT pressed? Because if it doesn't, then you can hook it to the ground side of the buttons, using the buttons as +5 when they are pressed. If it's an auto-fire or something, then it could pick up current from the button and start working as soon as the button is pressed. Using a capacitor it could hold enough charge to make it through whatever cycle it's doing.

Good luck.

EDIT: looking at the Missile Command schematics, they use 470 ohm resistors on the button inputs. 470 ohms, 5 volts V=IR thus I=V/R means that you can get a maximum of 0.01064 amps (= 10.64 milliamps) through that line (and that WILL trigger the input). If you needed to parasite without triggering the input, you'd need to draw a lot less, perhaps 2 milliamps (that's a TOTAL guess, you'd have to play with it). Not saying it can't be done, but it doesn't look at all easy.
 
Last edited:
I hear what you are saying, but I'm curious why you feel that way. The input pin doesn't provide the current. Current is provided by a resistor tied to 5V Vcc that is shorted by the button. The input pin just measures the logic level on that circuit. Additionally, I wouldn't be tying off one input, but several. And TTL circuits are high from 2-5v, so a fair amount of voltage drop is acceptable.

As for tying off a lamp, that's easy but I could just as easily tap 5V and skip the VR
 
Back
Top Bottom