RF detector with my comparator

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Thanks a lot Dana.
And how about keeping this LED at constant brightness independent of the VCC?
A constant current source?
Can this one be used for the purpose instead of the anode resistor?
 

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Here is what solution would look like with ATTINY85 -



Regards, Dana.
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neazoi said:
Thanks a lot Dana.
And how about keeping this LED at constant brightness independent of the VCC?
A constant current source?
Can this one be used for the purpose instead of the anode resistor?
Click to expand...

Personally I would use a processor, sample a current sense R in series with
LED, and drive the series string with a onboard PWM whose duty cycle I change
in code to implement a brightness control loop.

But yes another bunch of discretes to implement a current source OK.

But your implementation no, there is a direct function of Iled and Vcc -




Regards, Dana.
 
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Simple current regulator diodes -


You can also use LDO regulators in constant current mode (note the 2 in4148's useds to sim a led) -




Regards, Dana.
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The reg I show in prior post is not LDO, just adjustable. LDO a better choice as
it will improve compliance range of current source.


Regards, Dana.
 
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I think we are talking about uA constant current sources here. A 1.5A rated regulator is 'overkill' to say the least and the current at its ADJ pin alone is probably more than needed.

Possibly a JFET or even a constant current diode would be a suitable device to use.

Brian.
 

    neazoi

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No he was referring earlier to a LED drive current source.

So and adj or LDO in a TO92 package or sot would do. If high power
LED then TO220 or DPAK...


Regards, Dana.
 

Hi neazoi,

Say buddy, to bias the q2 at 90mV always, maybe you could make q2 input a non-inverting summing input kind of thing, or if that can't work, use a non-inverting summing amplifier at the q2 input, and bias the circuit to this permanent 90mV you need with a resistive divider from V+ as one of the summing amp inputs to generate the summing amp permanent output to keep q2 biased?
 

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