Apple II game-port 100mA current limit

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Apple II game-port 100mA current limit

According to the Apple II reference manual (page 100) the Apple II game port only allows a current drain of no more than 100 mA.

Many years ago I made a 6 way switchbox which allowed for 6 joystick/paddles etc. to selected simply by rotating a switch (I'll post DIY instructions on how to make one if there's any interest). To indicate which port was in use I connected an LED (in series with a 100 Ohm current limiting resistor) between pin 1 (+5V) and pin 8 (GND) for each port. 

 

I didn't know what I was doing back then, and the switchbox has worked fine and (as far as I know) hasn't caused any damage to the computer, but ....is having an LED/100 Ohm resistor along with a joystick (or paddles etc.) connected at the same time OK or not? Should I remove the LED for each port or change the resistors to a different value?

 

cjs
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After the 1.7 V drop of the

After the 1.7 V drop of the LED itself, the voltage drop across the resistor will be 3.3 V or so; I = V/R = 3.3/100 = 33 mA, so even with what the pots are pulling, I'd imagine that should be ok for the Apple. However, normally you don't want to run LEDs at more than about 20 mA, so it would probably be a good idea to replace the resistor with a 220 Ω one anyway.

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cjs wrote:After the 1.7 V

Thanks. I'll make that change.

And I would guess that 80's LEDs didn't light up near as much as the ones available today, so if there's any truth to that the more reason for a higher value.

Time permitting I hope to be posting a "how to" for my switchbox as I think it's something many Apple II enthusiasts might have a use for.

I noticed that I had put a 100 Ohm resistor between each LED and +5V from the socket next to it (via the switch), but I figure it's a better idea to have a single, common 220 Ohm resistor to ground as in this revised version I did today (see schematic below) as opposed to the original solution from around 37 years ago (see photo above).

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For those interested: I've

For those interested: I've just posted a DIY blog article on how to make the above box.

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Nice...

Very nice!  Thanks for sharing!

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The resistor for the 20mA LED

The resistor for the 20mA LED is ok. If you use modern 2mA-LEDs 1k5 is ok.

 

When you connect several joysticks or paddles to your box, the potis are connected parallel to the mainboard. Means: a single poti has 150k ohm, but six connected parallel are 25k ohm. I think that your rotary switch must disconnected the unused potis, probably the unused buttons too.

 

 

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