Monday, August 18, 2014

Experiment 13 Part 1 (Chapter 13) -- Noisy Circuit

I'm breaking Experiment 13 into parts... and there's no video for this first part, sorry to say. The first half of this experiment involved replacing some of the components in the Experiment 12 circuit, namely the 100K resistor with a 1M potentiometer and the 10k pot with a flat 10k resistor. I left the 10k pot in and just cranked it to its maximum value (and checked it with my meter). The 1M pot allows for some tweaking, but I found in my experiments I had to dial it down quite a bit to the lower range (around 200-230k).

Before diving into the experiment, however, I want to address one of the goals of this chapter -- designing a circuit. Charles opens the chapter with an example description of the final circuit (an alarm will sound if the input -- your voice or other noise -- exceeds a threshold) and then proceeds to ask the question - how do you go about designing a circuit knowing the end result you desire?

The key statement IMO is "so long as a circuit can be broken down into sections, and you can make them communicate reliably with each other, and you can test them one at a time, the design process doesn't have to be too difficult."

I've built a few circuits over the past few years where I stole a piece from here and another piece from there... I wasn't designing the schematic and circuit from scratch, but instead using pre-existing circuits that I understood. And that's what's going on here... Charles is pulling bits and pieces from earlier experiments to create one final circuit... and it's pretty slick and easy to follow if you take your time.

For the first part, I just wanted to recreate the input half of the circuit -- the electret must receive input and an increase in voltage needed to be detected from the LM741 with a meter. Figure 13.1 provides four different locations in the circuit to take some readings... I'm including my results below:

These values won't mean much to you if you haven't read pages 91-93 in the book. The takeaway was to notice an increase in the input voltage reading (AC)...  my tapping on the electret with a specific metal pen (voice wouldn't cut it) was providing 0.004V (40mV) and the LM741 was outputing 2.1V at Point C in the circuit (and AC voltage -- remember, the op-amp outputs AC). Point D in the circuit, however, is where the second half of the experiment will continue, and it needed to be at least 2.5V (AC) to trigger the eventual 2N2222 transistor that will be added (in Part 2 of my Experiment 12 post).  I was getting 2.6... so everything is good.

One troubling part to me is the sensitivity of the electret. My voice just doesn't trigger it... even when I'm speaking right into it. Only tapping on the shell with metal pen would get me the upper voltage I needed. I had to practice a bit to get a consistent tap strength, too.

But... it works. I'm getting over 2.5V at point D in the circuit (referencing Figure 13.1) and am now ready to move on to the next half of the circuit...

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