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LED Driver

This is the LED driver used on the Oscilloscope Watch. The simulation below shows the circuit:

Sorry, you need a Java-enabled browser to see the simulation.

Right on the input there is a spark gap to help keep ESD events out of the amplifier. The high impedance of the input also helps to keep the circuit protected.

An AC/DC coupling switch is present; the AC path is done with a high voltage 0.1uF capacitor.

Let’s calculate the transfer function. At the V+ pin of the Opamp, we have:

$$V^{+}=V_{in}∙{180k}/(820k+180k)= V_{in}∙9/50$$

This voltage is amplified by the OpAmp, using the non-inverting amplifier circuit:

$$V_{Op}=V^{+}∙(1+{20k}/{180k})= V^{+}∙(1+1/9)= V_{in}∙9/50∙10/9=V_{in}/5$$

Then, the voltage needs to be shifted so it can be applied to the microcontroller, using the superposition theorem:

$$V_{out}=V_{in}/5∙{3k}/(3k+3k)+2.048V∙{3k}/(3k+3k)$$

And finally:

$$\bo V_{\bo \out}=V_{in}/10+1.024V$$

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