Tech's Files: More Electronics Basics

Dec 1, 2009 12:00 PM, By Eddie Ciletti



Education Guide

Mix is gearing up to present its longstanding annual Audio Education Guide in its November 2014 issue. Want to have your school listed in the directory, or do you need to update your current directory listing? Add an image, program description, or a logo to your listing! Get your school in the Mix Education Guide 2014.

A quad comparator drives this simple LED ladder. Three series resistors across the bottom set the threshold for each LED.

A quad comparator drives this simple LED ladder. Three series resistors across the bottom set the threshold for each LED.

Whereas a single “throw” allows a single option — on or off, connect or disconnect, make or break — a two-throw switch allows an either/or option. Polarity-reverse switches are more likely to be double-pole/double-throw (DPDT). Toggle switches can typically have one to four poles, beyond which the physical size becomes unwieldy.

Rotary switches can have multiple poles and position/throws, and options like break-before-make (non-shorting) and “shorting” for jobs like input select or EQ boost/cut, respectively. Often, rotary switches (and pots) are customized (more costly), and sometimes they need to conserve front panel real estate or have remote-control capability, the latter being our focus.

Modern “soft” power switches — classified as “momentary” SPST — are at the front of more complex control circuitry that allows almost no current to pass when the mechanical switch is closed (conducting). Here, all the heavy work is done by “remote control,” using either electronic (FET) switching or relays. A relay is a switch mechanism with a magnetically conductive pole piece that can be opened or closed when a current is passed through a coil that, in turn, generates a magnetic field. (The coil is described as “energized.”)

A relay can be a single- or double-throw switch, with one, two or four poles being very typical. A single-pole/single-throw switch could control multiple relays; a single-pole/multiple position switch could select one relay from a group of several. There's a relay flavor to suit any in a large variety of applications.

The coil in this telecommunications-grade relay draws about 11 mA, power consumption being critical if the relay is electronically controlled by a microprocessor or other integrated circuit. Conversely, switch mechanisms for high-voltage/high-current applications require greater distances between switch contacts (to prevent arcing) and higher tension to keep the contacts closed (and low-contact resistance); therefore, greater current is flowing in the relay coil to create a more substantive magnetic field. Here again, a transistor buffer driver can do the heavy lifting.

The coil driver detailed in Fig. 2 was used in a custom 5.1-channel mute box. Rather than drive all relays from a single switch, I isolated each relay from the master switch with a transistor, allowing future options such as individual mutes or linking to the existing monitor controller.

Look at Dem Lites!

While a single LED does a decent job of indicating level changes, a bar graph display can expand the resolution to provide as much precision as necessary. In this application, a simple Class-A DI/mic preamp, I wanted four LEDs to indicate signal presence at four levels starting at instrument level (-20 dBu), nominal (+4 dBu), followed by the highest clean level (+15 dBu) before going into soft clipping (+20 dBu). In class, a comparator was chosen to teach my students one of the ways to get the job done, the alternatives being a dedicated LED meter driver chip and the more challenging micro-controller, with the latter requiring programming skills.

The circuit is fairly simple if you look at one comparator at a time (see Fig. 3). The LM-339 is a 14-pin IC with four comparators, enough to drive four LEDs. A comparator is like an op amp configured for a differential (balanced) input signal, but instead of looking for balanced audio, one input sees the AC (audio) signal while the other sees one step in a DC voltage divider/ladder. When the audio signal crosses the DC threshold, the comparator output swings in the direction that turns the LED on.

In addition to the comparator is a PNP transistor current source. Note that the 2N3906 emitter arrow is opposite that of the 2N3904 in Fig. 1. Its job is to keep LED brightness constant, no matter how many LEDs are on. Similar to an op amp running on a single-voltage supply, the comparator is given a reference DC voltage via the trim pot. Once the ladder is optimized for each LED, the trim pot serves as an overall calibration control. In the classes I teach, each student calculates the steps in the voltage ladder, in both directions; each DC step roughly corresponds with the RMS voltages associated with the dBu levels.

Familiarity with schematics often expedites the design process. You'll find lots of help from manufacturers' data sheets and application notes at their respective Websites, as well as through parts dealers such as Digi-Key, Allied and Mouser. A Web search for a 2N3904 or an LM339 should put a manufacturer's page at or near the top — look for National, Fairchild or ON semiconductor. Just beware the unscrupulous Website that asks you to pay for data sheets!

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