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/images/learn_more/low_ed/types/mechhand.gif

 

[3no]

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]

proposed circuit

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?

 

[AndrewT]

Hi 3no,
there are at least two previous threads that looked into making ordinary relays become latching. One specifically to be operated from a push/push momentary make switch.

Glad to see you are still smiling and particularly for volunteering all your research to the cause.

Does post2 not work? It seemed so simple. That was what prompted me to ask.

 

[lgreen]

Or you could get yourself a "latching" relay.

A 1 coil latching relay will latch on/off (change states) every time it gets a pulse.
1 coil latching relay

A dual coil latching relay will latch on when that coil gets a pulse, no big deal if its on already and gets another pulse as it just stays on. It will latch off when the other coil gets a pulse.
dual coil latching relay

Both types remember and stay in their last state unless jarred physically (a lot).

the examples are low pwr relays but these could switch a higher power one or the logic on your krell.

Not sure this helps but who knows, it might give you assistance in some way. You are right, this is a tough crowd!

 

[moamps]

A 1 coil latching relay will latch on/off (change states) every time it gets a pulse.

"In one–coil latching, the direction of current flow determines the position of the relay."

 

[cowanrg]

i was actually trying to do this exact same thing with my xbox. it has a push-button switch on the front for on/off. i wanted to control it with a 12v trigger (i can assing 12v triggers from my processor).

i never really came up with anything, but the latching relay idea sounds decent.

 

[3no]

Thanks for all the interest and advice - I'll take all the help I can get!

My concern, and the reason for going this direction, is not latching but preventing the preamp and the power amp from getting out of synch. For example: preamp is off (so 12v trigger level is low), power amp is off, and I (or more likely someone else) turns on the power amp by pushing the front panel PB. Now when the preamp is eventually turned on the 12v trigger will go high and the relay interface module will send a "push" pulse to the power amp turning it off. Out of synch.

The robust solution needs to account for the state of both the power amp and the 12v trigger level from the preamp to decide whether or not to pulse the power amp PB. It's either that or rip into the existing circuitry in the amp which is latching the power on/off from the momentary PB on the front panel, and I don't want to do that - I want a clean, simple interface to the Krell.

 

[lgreen]

Hey, good point.

sound like you want to connect a 2 coil latching relay to both the on/off switch and to the 12v trigger, that way it cannot lose sync.

If pwr switch is "on" and somone turns on the pre to trigger the 12V trigger, it will simply re trigger the latch but since the latch is already on nothing will happen. Same thing if its manually turned off and pre power is unapplied.

of course the question is still what circuit do you use??? hmmm...

note- sorry about that, moamps is right on those 1 coil dealies

 

[cowanrg]

could you have the 12v trigger ALSO trigger another relay that was on the power supply end? for instance, the trigger would not only send a pulse to the main on/off switch, but also control a relay that controlled power running into the amp maybe?

 

[3no]

Great suggestions! I think it's time to breadboard a few things and see what works. That may take a while by the time I get parts and time to do it, but I'll report back when I have some results (or a smoked amp).

 

[cowanrg]

i would love to see what you come up with. i think a few people could find it helpful! keep us informed.