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

Oren Leavitt obl64 at ix.netcom.com
Thu May 31 09:53:16 CEST 2012



On 5/31/2012 2:17 AM, ganesha wrote:
> Hi,
>
> I didn't get any replies to this question.
> Is it more complicated than it looks at first sight?
>
> Can someone perhaps point me in the right direction?
>
>
> Thanks
>
>

Try this:

Since the voltage on Iabc (pin 5) of the sampling 3080 will at or very 
near V- (on pin 4) when off (in the "hold" state), you could use a 
comparator to monitor this for any rise above V- (say V- + 0.5V) and 
drive an LED.
Any current turning on that 3080 will show a voltage there of about a 
diode (Vbe) drop above V-.
Make sure the input impedance of the comparator is high enough not to 
load the small currents driving the 3080.

Just a thought...

- Oren

>
> ----- Original Message -----
> From: ganesha<goaqihai at yahoo.co.uk>
> To: Synth-DIY DIY<Synth-diy at synth-diy.org>
> Cc:
> Sent: Thursday, 24 May 2012, 16:09
> Subject: [sdiy] newbie question: Adding LED to JH Sample&  hold schematic
>
> 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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