[sdiy] newbie question: Adding LED to JH Sample & hold schematic

René Schmitz uzs159 at uni-bonn.de
Fri Jun 1 20:42:52 CEST 2012


Hi everyone,

As a first shot, I'd try using a high efficiency LED (say 10000mcd ) and 
put it in series to the Iabc pin.

Sensing the current with an opamp+resistor is probably overkill.

Cheers,
  René

Am 01.06.2012 14:39, schrieb Harry Bissell:
> the idea of looking at the Gm bias is OK, but it may have some difficulty. The Gm bias pin looks like
> about two diode drops above the negative supply, and does not change much with current (it is a current input...
> almost a sort of virtual ground (virtual negative rail).
>
> There will be some voltave variation due to the diode curve with current, but probably only (guess) 0.1V
>
> you could add a series resistor between the current sources and the Gm bias pin without hurting anything (in fact
> most designs have one to prevent possible damage to the Gm bias pin. The voltage at the other side of this resistor
> will be much larger and more positive. It would also be possible to use a differential amplifier (opamp) across the resistor, but make sure the opamp input range is good for operation near the negative rail.
>
> H^) harry
>
> ----- Original Message -----
> From: ganesha<goaqihai at yahoo.co.uk>
> To: Tim Parkhurst<tim.parkhurst at gmail.com>
> Cc: Synth-DIY DIY<Synth-diy at dropmix.xs4all.nl>
> Sent: Fri, 01 Jun 2012 03:22:37 -0400 (EDT)
> Subject: Re: [sdiy] newbie question: Adding LED to JH Sample&  hold schematic
>
>
>
> Hi Tim,
>
> Thanks for your help. This will work fine  if the switch is set to Glide and Hold position, because that is when OSC3 is used as the clocksource. In glide&  Hold mode is especially usefull if you can change the pulsewidth of OSC3.
>
> But if the switch is set to Sample&  Hold mode, the internal clock is used and not OSC3 so if I connect the LED to the OSC3 input, the LED will still blink to the tempo of OSC3 and not to the tempo of the internal clock. That will be confusing.
> So I'm looking for something that will make the LED blink to either OSC3 or the internal clock, depending on the mode the switch is set to.
>
> Best regards,
> Kris
> ________________________________
> From: Tim Parkhurst<tim.parkhurst at gmail.com>
> To: ganesha<goaqihai at yahoo.co.uk>
> Cc: Synth-DIY DIY<Synth-diy at dropmix.xs4all.nl>
> Sent: Thursday, 31 May 2012, 21:05
> Subject: Re: [sdiy] newbie question: Adding LED to JH Sample&  hold schematic
>
>
> Hi Ganesha,
>
>  From a quick look at the schematic, it looks like you could add a simple LED indicator to the clock pulse input (noted over on the right hand side of the schematic as "From OSC 3 Pulse"). This is where you'll send in a pulse from an LFO and so that should be easy to add an indicator there. I put a collection of various LED drivers on the Electro-Music forum here:
>
> http://www.electro-music.com/forum/viewtopic.php?highlight=collection+led&t=49449
>
> You'll be monitoring a square wave, so the first LED driver pictured should do the trick for you. The circuit is very simple and only involves a transistor, a couple of resistors, and the LED. The transistor acts as a "buffer" so that the LED isn't trying to pull too much current from the LFO circuit. Hope this helps.
>
>
>
> Tim (you'd think my spell checker would know not to flag "LFO" as an error by now) Servo
>
> ---
> "Sire, the church of God is an anvil that has worn out many hammers." - H.L. Hastings
>
>
>
>
>
> On Thu, May 24, 2012 at 7:09 AM, ganesha<goaqihai at yahoo.co.uk>  wrote:
>
> Hi,
>>
>> I'm building the JH Synthi Sample&  Hold.
>> http://www.jhaible.de/tonline_stuff/jh_sc_shg.gif
>> Everything works fine but I'd like to add a LED to have some visual feedback of the clock rate if the internal clock is used (timepot on the left), but also if an external clock is used (in the schematic on the right "from osc 3 pulse").
>> So I'm guessing the LED should be connected somewhere in the path going to the amp bias input of the OTA at the top of the schematic.
>> But what is the correct way to add the LED? I want to be sure the circuit keeps working and that the LED doesn't get too much current.
>>
>> Thanks!
>>
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