LED / HDD Disable Circuit

[flashjazzcat]

This should be easy for anyone who knows the first thing about electronics. Having fitted an IDEa interface in my 1200XL, I've wired the IDEa's 2-pin "OFF" header to 2 pins of the old channel selector switch (one pole tied to ground, which basically means the IDEa's "off" pin is connected to GND when the switch is in the left position). Great.

 

Now, I thought it would be nice if the last unused LED on the 1200XL (there being three, the other two used for power and HDD activity) was ON when the channel selector switch is in the "ON" position.

 

I'd like to ensure I don't blow anything up, though:

 

 

This is the kind of scheme I had in mind. Middle switch pin tied to GND, +5v on the other pin, forming the LED circuit. However, I'm a total noob when it comes to circuitry... just wanted a second opinion.

[+GroovyBee]

Something like this :-

 

   +-----+           A     B     C
+5 -----|  R  |----->|-----O     O     O------ IDEa
   +-----+       LED        GND

 

So the switch has GND on "B". If it's closed between "B" and "C" IDEa is off, If closed between "A" and "B" it completes the LED circuit.

[flashjazzcat]

Makes sense - many thanks indeed!

[BillC]

Something like this :-

 

+-----+ A B C
+5 -----| R |----->|-----O O O------ IDEa
+-----+ LED GND

 

So the switch has GND on "B". If it's closed between "B" and "C" IDEa is off, If closed between "A" and "B" it completes the LED circuit.

Something like this :-

 

+-----+ A B C
+5 -----| R |----->|-----O O O------ IDEa
+-----+ LED GND

 

So the switch has GND on "B". If it's closed between "B" and "C" IDEa is off, If closed between "A" and "B" it completes the LED circuit.

It's not necessary to switch the LED, it can be connected to always monitor the status of the HD disable pin. In the following schematic I show both possibilities for the LED.

[+GroovyBee]

It's not necessary to switch the LED, it can be connected to always monitor the status of the HD disable pin. In the following schematic I show both possibilities for the LED.

 

If you omit R2 and LED2 from your circuit diagram then you are expecting the IDEa disable line to power the LED with the switch open. I'm not familiar with the IDEa's design but I suspect that signal is an input of some kind with a weak pull-up so trying to power a LED from it isn't a good idea.

[flashjazzcat]

Thanks for all the ideas (no pun intended), but scheme a) is most suitable, since I don't need (or have space for) an "IDEa OFF" LED. The IDEa disable pin is pulled low when the header is jumpered. Pics when it's working, of course.

[flashjazzcat]

Right this is working:

 

 

Only problem is that since I wanted to keep the HDD activity light on a completely isolated circuit (since the resistor for that light is on the IDEa board), I kind of ran out of pins on the 1200XL's 5 pin LED header. I had to assign one pin to GND for the enable light, 2 pins for the activity light, which leaves only GND and +5v for the power light. So +5v for the HDD enable is taken straight from +5v for the power LED, which naturally means when the HDD is switched on, the power light dims slightly.

 

If I could figure out how to make the HDD activity light share the common ground pin on the 5 pin ribbon connector we could keep all the power lines for the LEDs separate, but I'm really not sure (how) that will work.

Edited April 22, 2013 by flashjazzcat