Hi, I'm a novice in circuit design. Here is what I'm trying to accomplish. I have 2 sets of leds. One is white and on as soon as the switch is on. The second set is red and is on when a signal wire as around 80 ma and turn off the white set. Then when the signal wire drop to <80 ma then it returns to the original white set on.

Can anyone help me find a way to accomplish this.
Thanks for all your help.

 

A lot depends on how you can interact with that signal wire.

 

Think of the source of the current in the signal wire. How would you measure the current? Is there something besides current that is measurable and will give you the result you want?

A second question is how are you going to compare the sensed values?

Both questions may point you in a direction to follow.

By the way, is this part of schoolwork?

 

A lot depends on how you can interact with that signal wire.

I'm not sure I understand what you mean.
But that signal wire come from a CB radio and is for the external RF meter

 

Think of the source of the current in the signal wire. How would you measure the current? Is there something besides current that is measurable and will give you the result you want?

A second question is how are you going to compare the sensed values?

Both questions may point you in a direction to follow.

By the way, is this part of schoolwork?

Nope, no school work for me I'm a hobbyist and I'd like to add LEDs to my CB external analog meter and I'd like to have the LEDs to switch to the red one when the signal is strong. Witch mesure about 80ma

 

I'm not sure I understand what you mean.
But that signal wire come from a CB radio and is for the external RF meter

Whenever you measure something, you change what is being measured. That has two primary effects -- it makes the measured value different than it would be without the measurement being taken, and it affects the circuit's operation. The goal is to make both of these impacts small enough to be acceptable.

The normal way of measuring current is to put a resistor in the path and measure the voltage across it. But the presence of the resistor changes the current flowing in the wire being measured. But the impact can be far worse because that additional resistance can cause the circuit to behave differently, perhaps to the point of not functioning at all.

You might try putting different resistors in series with that signal wire and seeing how much resistance you can add before the behavior changes enough to be unacceptable. Then figure that you want to add no more than about half that resistance.

If this won't get you a reasonable voltage at 80 mA, there are other ways to measure current, such as using a current mirror or a hall-effect sensor.

You can also try to not measure the current, but instead measure the voltage somewhere along that wire, such as at the input of the meter you mentioned. It is usually far easier to measure voltages without disturbing the system.

 

I am trying to imagine how any portion of a legal CB radio has a current that goes above 80 mA to drive a meter. I have repaired and modified quite a few CB radios and never seen an external meter to read received signal strength. So it would be useful for the TS to explain how that part is working. because there may be another way to trigger the red LED.

 

What power sources are available to drive the comparator circuit and the LEDs?

ak

 

What power sources are available to drive the comparator circuit and the LEDs?

ak

I have 12 volts DC to drive it as it's in a car.

 

I am trying to imagine how any portion of a legal CB radio has a current that goes above 80 mA to drive a meter. I have repaired and modified quite a few CB radios and never seen an external meter to read received signal strength. So it would be useful for the TS to explain how that part is working. because there may be another way to trigger the red LED.

Well, it's an analog signal meter connected to the original Meter in the CB radio. It's also commonly called a "Jumbo S meter".