Tech's Files: More Electronics Basics

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



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Figure 1: A positive moving input signal turns the transistor

Figure 1: A positive moving input signal turns the transistor "on," so the collector pulls the LED's cathode toward ground via the 47-ohm resistor.

This month, “Tech's Files” will stray from the conventional audio signal path and venture into some über-basic examples of analog control and display circuitry. All of us are familiar with LEDs as visual indicators. As I tell students in the classes I teach, Electronics 101 defines a diode as a device that allows current to flow in only one direction, which can enable rectification, commonly used to convert an alternating current into direct current. In the 1920s, diodes were found to emit light in certain applications. Years later, experiments with diodes made of exotic materials (such as silicon germanium and gallium arsenide phosphide) spawned commercial LEDs in various colors.

A single LED can translate amplitude variations into a corresponding brightness range — not very precise, but definitely operator-intuitive. An audio signal can drive an LED to moderate brightness, and by exploiting its ability to rectify an incoming signal, two LEDs in parallel with the signal path can create a simple fuzz box. If the design goal is to make the LED bright enough to get attention, then a transistor (or op amp) must be inserted between the LED and the audio signal to act as a buffer/driver.

Positive DC turns this transistor

Positive DC turns this transistor "on," so the collector pulls the relay coil cathode toward ground via the emitter resistor.

Figure 1 shows a simple buffer/driver option. Swapping out the LED for a load resistor (on the collector) yields the more familiar single-stage voltage amplifier. Like a transistor, an LED is a semiconductor that has polarity and must be oriented accordingly. Notice that the LED “arrow” is pointing in the same direction as the NPN transistor's emitter. This particular circuit is being used to indicate the threshold and degree of processing for a simple optical limiter.

Regardless of whatever “load” (a resistor, LED or relay coil, as shown in Fig. 2) is being used, the resulting headroom, LED brightness or reliable relay latching can easily be optimized if the user tweaks the bias resistor on the emitter and the voltage divider (the two resistors connected to the base).

Either/Or and Other Options

Before diving into relay-driving circuits, let's review what switches do. Switches fall into many categories based on the number of circuits (poles) and the number of connections or positions. An old-fashioned power switch — toggle, rocker or push-and-latch type — is often single-pole/single-throw (SPST) or double-pole/single-throw (DPST). All of these types have a maximum of three positions (center OFF) and two “throws”; just think of Dr. Frankenstein “throwing” that giant switch lever.

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