Tekay

Signal present indicator circuit
« on: January 13, 2005, 05:44:26 AM »
Hi!
Needed a signal present indicator for the inputs in my summing unit but couldn't find anything on the web, so this is what I came up with!

Lights up full when a signal is present at the input. Working fine from +12 to +24V

http://www.vintagedesign.se/diy/signal%20led%20indicator.pdf

Simpler suggestions are welcom!
Thomas "TK" Kristiansson
----------------------------------
"The Sound Is In The Iron"
www.vintagedesign.se  www.tkaudio.se


FrankSL

Signal present indicator circuit
« Reply #1 on: January 13, 2005, 02:22:23 PM »
Shouldn't the peak hold cap need a full discharging resistor?

Frank

Tekay

Signal present indicator circuit
« Reply #2 on: January 13, 2005, 02:58:10 PM »
The schematics is exactly my set up and it works fine feeded with a signal from my tuner!
Thomas "TK" Kristiansson
----------------------------------
"The Sound Is In The Iron"
www.vintagedesign.se  www.tkaudio.se

mkruger

Signal present indicator circuit
« Reply #3 on: January 13, 2005, 03:23:57 PM »
it will still work fine without the discharge resistor,, also known as a bleeder resistor... what he was suggesting is more for safty and common practice to fully discharge the circuit when the unit is off.

btw- i really like your idea! i will find that very usful.  :thumb:

dukasound

Signal present indicator circuit
« Reply #4 on: January 13, 2005, 03:59:49 PM »
Hi Thomas
Can we use this like overload indicator?
Duka

FrankSL

Signal present indicator circuit
« Reply #5 on: January 13, 2005, 04:09:09 PM »
Quote
it will still work fine without the discharge resistor,, also known as a bleeder resistor... what he was suggesting is more for safty and common practice to fully discharge the circuit when the unit is off.


Yes, my suggestion was only to not let the ~0.6V on the cap when the unit is disconnected.
Thanks for the circuit, it will come useful for sure,
Frank

Tekay

Signal present indicator circuit
« Reply #6 on: January 13, 2005, 04:49:49 PM »
Next project is to make the overload circuit!
In the LA-4 they just take the signal from one of the op's to a forwarded 1N4148 then a backwarded 10V zener and finaly a LED to ground.
I guess a comparator is the best solution for an overload circuit.
Thomas "TK" Kristiansson
----------------------------------
"The Sound Is In The Iron"
www.vintagedesign.se  www.tkaudio.se

PRR

Signal present indicator circuit
« Reply #7 on: January 13, 2005, 07:05:57 PM »
This requires Q1 Beta over 200. Won't work with just any transistor.

Input sensitivity seems to be 0.5V peak. Tuner output will always trigger it, but in recording we sometimes have weaker signals. If it works for you, fine. I have done whole movements that would read "dead" on this indicator.

> Shouldn't the peak hold cap need a full discharging resistor?

Why? Base-emitter of Q2 will discharge the cap. Rapidly over 0.6V, slowly below 0.4V, but this is very commonly done when you need a lowest-parts solution and don't have exact critical timing specs.

Tekay

Signal present indicator circuit
« Reply #8 on: January 14, 2005, 02:45:00 AM »
Thanx PRR!
Any suggestions to make it more sensitive?
Thomas "TK" Kristiansson
----------------------------------
"The Sound Is In The Iron"
www.vintagedesign.se  www.tkaudio.se

Nasse

Signal present indicator circuit
« Reply #9 on: January 14, 2005, 03:29:05 AM »
How about an amplifier and buffer stage in front...

I was looking for the simplest signal detector that I could find for a twin noise gate type thingie I had to do few years ago. I have one mixer project which I maybe never get done ready so I know I might need such detectors anyway, so was looking for these.

I have an old circuit somewhere, it is just one cmos 555 monostable and few passive parts, treshold is set with multiturn pot and audio signal is taken with a cap to the chip, and if reference voltage set with pot plus signal peak is enough, even short peaks are easily seen because you can tailor the monostable time long enough. I did not build it because the project text told that you can zap the cmos 555 if you feed signal in when the unit is not powered. I think it can not detect unsymmetrical signals not so accurately but who cares if it does the job.

For my opto gate project I used TLC271 comparator and put 555 retriggerable monostable after that, worked like dream. Maybe I could have done it with just one 555 but I did not want it to stop working...

If I remeber well there was a peak detector circuit in Cristmas (or was it Summer) Elektor done with cmos gate, tuned near changeover with a trimmer and signal applied there with a cap...

I have old Alesis Microverb somewhere, it has nice tricolor led input level indicator. Both leds light when thre is no signal or it is low level (I dont have the words for the colour, sorry my english) and goes green when signal level is "normal" level, and finally bright red when input is overloaded. Could be done with few opamps...


Tekay

Signal present indicator circuit
« Reply #10 on: January 14, 2005, 04:02:13 AM »
Changing R3 to 6k8 makes it start to light up at -50dBm! This was tested with BC547C, more sensitive with BC184C. Changing R1 to 100K for less loading on the signal lower the sensitivity to -40dB.
Thomas "TK" Kristiansson
----------------------------------
"The Sound Is In The Iron"
www.vintagedesign.se  www.tkaudio.se

daArry

Signal present indicator circuit
« Reply #11 on: January 14, 2005, 04:06:48 AM »

Signal present indicator circuit
« Reply #12 on: January 14, 2005, 04:47:37 AM »
Hi,
here is a circuit i found a couple of month ago. I havenĀ“t tried it. It has 3 Inputs (e.g for 3 different stages in a circuit, but you can also use only one) and a LED for signal and one for overload.




Regards
Leslie...

clintrubber

Signal present indicator circuit
« Reply #13 on: January 14, 2005, 05:24:22 AM »
There's also the circuit in the Trace Elliot bass-amps - has some kind of traffic-light setup with three LEDs. if I remember it correctly it was a few NPNs. Can look it up.

   Peter

clintrubber

Signal present indicator circuit
« Reply #14 on: January 14, 2005, 05:28:27 AM »
TraceElliot-circuit can be found here:
http://www.britishaudioservice.com/manuals.shtml

Like for instance on page 3 of this file:

http://www.britishaudioservice.com/schem/SMX/SMX%20PRE-AMP.pdf

Bye,

  Peter

gyraf

Signal present indicator circuit
« Reply #15 on: January 14, 2005, 05:41:26 AM »
What would be really usefull would be a circuit like this:

Red/green dual-colour LED.

- Green starting glowing at some -45dB - gradually brightening up untill 0dB
- Red starting glowing at some -10dB, gradually brightening untill 0dB. This makes a green->yellow transition from -10dB to 0dB.
-Green cutting out at a set threshold, 0dB, which instantly changes yellow to red.

I've seen circuits like that in action, and they work very well indeed - both for general overview and for precise overload indication.

But I've never come across a schematic..

I think we'll need some primitive form of logging for the green part of the signal. Comparator to switch out the green. I'm pretty sure the red can be driven linearily.

Does anyone have a guess at a schematic?

Jakob E.
..note to self: don't let Harman run your company..

PRR

Signal present indicator circuit
« Reply #16 on: January 14, 2005, 05:38:06 PM »
> But I've never come across a schematic..

I posted one here some months back, and everybody yawned......

http://www.groupdiy.com/index.php?topic=877

That one is hard-switch. If you want a semi-linear (or semi-log) response for "normal" signals, you want an op-amp or a crippled comparator in the middle range.

gyraf

Signal present indicator circuit
« Reply #17 on: January 16, 2005, 11:49:53 AM »
Thanks, PRR - I'll have a closer look at that one..
..note to self: don't let Harman run your company..


 

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