Add 12V trigger input?

[3no]

I have an early model Krell KAV-500 which does not have a 12V trigger input. It has a push-on, push-off power button (electronic state sequencing, not mechanical).

Any suggestions on a clean modification to add a 12V trigger input? I came up with zilch in a search...

[sreten]

Hi,

unless I totally misunderstand :

You need a reed relay across the momentary on/off button.

The 12V trigger pulse only has to be long enough to operate the relay.

If the 12V trigger is say OV for on, 12V for off, then you can convert
this to pulses to operate the relay with a capacitor in series with
the relays coil.

/sreten.

[3no]

Thanks, sreten. It's the latter. I'll bet that it's possible to calculate the cap value from the inductance of the relay, impedance of the 12V trigger source and the desired on time, or is it easier just to try a few caps and see what happens?

I think I need a nonpolar cap, right? Suggestions for a starting value?

[AndrewT]

Hi 3no,
when you have a successful mod, please post the circuit around the push/push switch.

[3no]

This is a tough crowd I post looking for a circuit and end up volunteering to experiment and post the circuit I was looking for.

I will post the circuit, if I tackle it. I may breadboard it for grins, but hesitate to modify the Krell for this approach as it is not very robust -- it can get out of synch with the preamp.

[thylantyr]

Or design a mechanical hand

http://www.robotics.utexas.edu/rrg/i...s/mechhand.gif

[3no]

Originally posted by thylantyr
Or design a mechanical hand

I thought about that - using the "hammer" from a doorbell chime. It has the advantage of no mod to the amp. But it too suffers from the synch-loss possibility. And my friends would laugh at me.

[mazurek]

I was looking at solid state relays at mouser, for about 30 dollars, it seems like this problem can be solved very easily. Otherwise you need a transister turning on a power supply to a relay.

[3no]

I see the major issue is that the pushbutton is presenting a "toggle" command to logic on the power supply circuit board. What I need to do is present a discrete "go to state x" command (represented by the 12V trigger level), so I can't do that just paralleling the PB, but would have to get further into the PS cicuit. And I don't have a schematic for the unit.

Alternatively, I need logic that takes into account the current state of the unit and decides whether to "press" the PB if and only if the state needs to change.

[3no]

OK, consider the attached circuit for your entertainment .

The concept is to "press" the power push button on the amp by energizing a reed relay whenever the 12v trigger and the 12v supply in the amp are in different states. The NO relay contacts connect in parallel with the PB. The "different state" detection logic is formed from two additional form C reed relays. The whole mess is energized from either the amp's +12v or the trigger +12v. This assumes that the 12v supply in the amp is down when the amp is in standby mode (yet to be verified). Requires only 4 connections to the amp: +12v, Ov, PBx2.

Any comments (will it work, improvements, ...)?

PS please excuse the hand drawn diagram. Can anyone suggest a low-end CAD app for doing this on a PC?