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

ganesha goaqihai at yahoo.co.uk
Fri Jun 1 09:22:37 CEST 2012

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,
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:


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:

>I'm building the JH Synthi Sample & Hold.
>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.
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