LEDs connected to two devices (PROP-1 and ST-300)

[BigRez]

I'd like to be able to control a couple of LEDs with both a prop-1 and an ST-300 device but not simultaneously.  While the prop-1 is waiting for a trigger, the LEDs would occasionally blink/fade.  Then after triggering the start of the ST-300, they would blink in sync as controlled by the ST-300.  And if they did get used simultaneously, what could happen?

Can I wire them up to both devices directly or should something (diodes?) be placed in line similar to the following:


prop-1 ---------|D>------+
                         |----------> LED
ST-300 ---------|D>------+

[JonnyMac]

What you have is a diode OR gate so if either device is active the LED will be active.  I am assuming, of course, that you're using TTL outputs for the LED control.

[BigRez]

Thanks Jon.  Yes, they'd be connected through TTL on the prop-1 and the LED connections on the ST-300.  So for the diodes, I have some 1N4001 left over and assume these will work?

[JonnyMac]

Any general purpose diode will be fine.  You'll get about 0.7v drop across the diode, so you'll want to account for this when sizing your current limiter.

[JackMan]

I'm assuming your Prop-1 output goes low when the ST-300 triggers? How do you get an LED to fade with a Prop-1?

[BigRez]

It doesn't have to go low.   But yeah, "fading" probably isn't correct.  It'll be on, off, or some level of blinking.  Haven't figured it out exactly what it'll do but wanted the ability to control the LEDs from two different devices.

[JonnyMac]

Hint: Make sure that the LED and the boards have a common ground.

[JackMan]

It doesn't have to go low.   But yeah, "fading" probably isn't correct.  It'll be on, off, or some level of blinking.

If the Prop-1 output doesn't go low, isn't there a possibility of the Prop-1 lighting the LED's when the ST-300 is driving them?

[BigRez]

It doesn't "have to" but can and that's part of the main question...  Can I have both devices activating the LEDs at the same time? .

[JonnyMac]

Yes; with a two input OR gate, if one input OR the other is active, the output will be active.

[menehune]

If you use an regular "OR" gate and not an "Exclusive OR" gate, any (or all) active input will activate the output.
http://www.kpsec.freeuk.com/gates.htm#or

Diode logic OR gate:
http://www.play-hookey.com/digital/electronics/dl_gates.html

[BigRez]

Yes, the OR condition is what I want and not the XOR.  (XOR would not light the LEDs if both the prop-1 and the ST-300 are active.)  thanks.

[JackMan]

I guess I was just referring to your initial post:

While the prop-1 is waiting for a trigger, the LEDs would occasionally blink/fade.  Then after triggering the start of the ST-300, they would blink in sync as controlled by the ST-300.

They would not exclusively blink in sync with the audio if the Prop-1 output didn't go low.

[BigRez]

Understood.  Thanks all.  The plan is that the sequence will be able to define how the LEDs should be controlled; by the ST-300 (prop-1  low), full-on (prop-1 high), or blinking/st-300 sync.

It would be better if I could turn off (disable) the st-300 sync when I use the controller to light the LEDs.  Could I wire them up so that they both connect to the LEDs but when the prop-1 is high the st-300 is made low?

I know I could use relays inline with the st-300 connection and flip them on or off with the prop-1, but perhaps there's a better way to wire them up so that when the prop-w goes high, the st-300 connection is set low (effectivly ignored)?

[JonnyMac]

Can you point to docs for the ST-300?