Help needed with LED Bar Current Meter!

[blue7hand]

I would like to add a LED current meter to the output of a squarewave ozcillator.

I found loads of designs to display voltage, done with a row of Opamps or with the LM3915.

But how do i adopt this to meassure current?

About 8 LEDs should display 1mA to 15mA.
Supply voltage is 18V.
The output ozcillates between +/- 13V.

My thinking sor far:
I could put a resistor of upto 10 Ohm in line, which would then have upto 150mV.
But how can I feed this to an opAmp? Because the voltage of the two connections of the resistor have no relation to any other point of the circuit, I don't see how to create a reference voltage for the opAmps.
I hope it makes sense what i'm saying.

Maybe my thinking is all wrong and there exist completely different solutions...

Anybody can come up with one please?

Thanks a lot!

 

[Russlk]

Your circuit can be connected to the oscillator output as long as you are not grounding any part of it, and I see no reason to do it.

 

[blue7hand]

Hi Russlk,

I don't exactly understand what you mean.
But maybe I should mention the following:
LED current meter will be part of the ozcillator circuit and therefore has the same supply and ground, which is different from the signal outs.

blue7Hand

 

[Russlk]

The signal out has to be somewhere between VCC and GND unless the output has a transformer, in which case there is no problem anyway. This circuit will work with a floating signal source because it does not amplify the common mode voltage. The output DC level is adjusted by the ratio of R5 - R6.

 

[mettula]

that LM3915 is a logarithmic v-u meter specifically (think so) desinged for audio. pins 1, 10-18 are LED's , one could search for total specification by searching for the LM3915

 

[jrz126]

You'll probably want the lm3914 V-U meter IC, it's similar to the 3915, but its linear instead of logarithmic

 

[audioguru]

Hi Blue,
Don't you want to measure the current through the oscillator's load?
Then since the oscillator has a dual-polarity supply, isn't the load connected to ground (the connection between supplies)?

Simply put your current-measuring resistor in series with the grounded terminal of the load, and amplify its small voltage with a normal single-input opamp circuit that also has a dual-polarity supply.

Why are you using a square-wave? Most AC current measuring is done with sine-waves.