I want to add a very simple input signal indicator LED to my amp. This is just to show if signal is present or not. I'm sure the attached circuit will work. But I worry that the LED switching on and off will introduce noise into audio portion of the circuit. I'd like to use a dual op amp where one half is the audio input buffer and the other half drives this LED.
Should I worry about the LED generating noise? Is there a better way? Maybe a shunt capacitor across the LED, a buffer transistor or both?
Thanks!
Should I worry about the LED generating noise? Is there a better way? Maybe a shunt capacitor across the LED, a buffer transistor or both?
Thanks!
Attachments
Assuming this is not a mic or phono preamp (or the like) I probably would not worry too much, but if you are really concerned, one trick is to have a constant current source feeding the LED (returned to -VE, with a transistor bypassing the LED (also returned to -VE). That way the switching current does not get imposed on the audio reference....
This is also a good trick when implementing bargraph metering on sensitive electronics like the aforementioned mic preamps.
Regards, Dan.
This is also a good trick when implementing bargraph metering on sensitive electronics like the aforementioned mic preamps.
Regards, Dan.
Maybe use the second OpAmp as a "sample and hold" and drive the LED across a diode bridge so either polarity is indicated. A high brightness LED will operate at only a couple of miliamps so there should be no problems there.
Loses it's simplicity though 🙂
Loses it's simplicity though 🙂
one led has 1.5 Volt drop
one transistor has 0.7 Volt drop
is there any special level - to define for you a signal present?
- one differential input ac voltage amplifier (using op-amp)
- with fullwave rectified output and
- some delay function
- to feed one LED indicator
would be the most flexible and versatile solution
not too difficult to design with ordinary standard compoments
but way to complicated & advanced
for to be a pleasure to put into your circuit 😀
----------------------------------
from your examples
I would take:
-> transistor follower buffer -> LED + Cap across
this would give some delay filtering of output from amplifier
at the same time the small base current for transistor
would not effect the amplifier output, hardly anything
one transistor has 0.7 Volt drop
is there any special level - to define for you a signal present?
- one differential input ac voltage amplifier (using op-amp)
- with fullwave rectified output and
- some delay function
- to feed one LED indicator
would be the most flexible and versatile solution
not too difficult to design with ordinary standard compoments
but way to complicated & advanced
for to be a pleasure to put into your circuit 😀
----------------------------------
from your examples
I would take:
-> transistor follower buffer -> LED + Cap across
this would give some delay filtering of output from amplifier
at the same time the small base current for transistor
would not effect the amplifier output, hardly anything
There is absolutely no reason to worry about any extra noise, unless you do something really wrong, like messing with grounds.
- Status
- Not open for further replies.