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LED Indicator for GATE voltage drop
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author LED Indicator for GATE voltage drop
ESSY
Hey All,

Probably a newb question, but I am building a GATE out via an extra MCP6002 op-amp, and wanted to add an indication LED when the GATE goes high.

How do I add an LED indicator when without affecting the GATE out voltage? Is this another op-amp use case?
chloeprice
I am not really an electronics professional myself, but I think an opamp voltage follower like this could work:

NANO-Jorge
Yes, if you have an opamp you could do a buffer for the gate signal, then you don't have to worry about voltage drop.
latigid on
From what I've read, the LED is better placed inside the feedback loop.

https://www.allaboutcircuits.com/technical-articles/the-basics-behind- constant-current-led-drive-circuitry/
ESSY
thanks for the feedback everyone we're not worthy . I'm learning stuff every day on here.

chloeprice circuit was really close but I ended up (through trial and error) putting the resistor in the feedback loop and everything was awesome - meaning there was no voltage drop on the GATE signal.


Synthiq
The resistor has no function in this configuration and the current in the LED can't be adjusted without a resistor in series and will now only be limited by the internal current limit in the TL074 output stage.
av500
ESSY wrote:
Hey All,

Probably a newb question, but I am building a GATE out via an extra MCP6002 op-amp, and wanted to add an indication LED when the GATE goes high.

How do I add an LED indicator when without affecting the GATE out voltage? Is this another op-amp use case?


just wire a resistor and a LED in parallel with the opamp driven output. use a low current LED so you can use a 1k resistor. at your typical 5V gate voltage that's a max current of 5mA
ESSY
Synthiq wrote:
The resistor has no function in this configuration and the current in the LED can't be adjusted without a resistor in series and will now only be limited by the internal current limit in the TL074 output stage.


Is this a bad thing? It appears to be working as expected.
Synthiq
ESSY wrote:

Synthiq wrote:

The resistor has no function in this configuration and the current in the LED can't be adjusted without a resistor in series and will now only be limited by the internal current limit in the TL074 output stage.


Is this a bad thing? It appears to be working as expected.


The output current of a TL074 is limited to around 10mA so you were lucky this time. If you had used an amplifier without current limiter you could have fried both the amplifier and the LED.
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