on this page:
http://www.allaboutcircuits.com/vol_3/chpt_4/2.html

this paragraph needs correction:
If the switch is open as in (Figure above (a), the base wire of the transistor will be left “floating” (not connected to anything) and there will be no current through it. In this state, the transistor is said to be cutoff. If the switch is closed as in (Figure above (b), however, electrons will be able to flow from the emitter through to the base of the transistor, through the switch and up to the left side of the lamp, back to the positive side of the battery. This base current will enable a much larger flow of electrons from the emitter through to the collector, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated.

it should read:
If the switch is open, as in [delete this parenthesis ( ] Figure above (a), the base wire of the transistor will be left “floating” (not connected to anything) and there will be no current through it. In this state, the transistor is said to be cutoff. If the switch is closed, as in [delete this parenthesis ( ] Figure above (b), [delete: however, ]electrons will be able to flow from the emitter through to the base of the transistor, through the switch, and [delete (erroneous): up to the left side of the lamp, ]back to the positive side of the battery. This base current will enable a much larger flow of electrons from the emitter through to the collector, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated.

a couple of paragraphs later:
.... More important[ly], the current-controlling...
.... Consider [the] Figure below, where a pair of solar cells provides 1 V to overcome the 0.7 VBE of the transistor to cause base current flow, which in turn controls the lamp.

in all cases, "Figure" probably shouldn't be capitalized, but that's a matter of style.

then:
Or, we could use a thermocouple (many connected in series) to provide the necessary base current to turn the transistor on [as] in [the] Figure below.

picture caption:
A single thermocouple provides 10s of mV. [what is "10s" of a millivolt?]Many in series could produce in excess of the 0.7 V transistor VBE to cause base current flow and consequent collector current to the lamp. [in good conditions, a thermocouple provides 50mV at very low current. an array of 14 pristine thermocouples seems like a bit much. really, that's a lot of flame. i'd change the example to some other sensor. but that's just me. i'd hate to see something catch on fire. ;-) ]

picture caption:
Amplified microphone signal is rectified to DC ["in order to" or "to"] bias the base of the transistor providing a larger collector current.

lastly, one of the bullet points:
When a transistor has zero current through it, it is said to be in a state of cutoff (fully nonconducting). [i'd dispute that "cutoff" is occurring when a lead is "floating". (that's in this page's text.) true cutoff happens in reverse bias, doesn't it? maybe a caveat can be offered here. but that's up to you. i see that you don't want to be too technical on this particular page.]

well, anyway, that's my 2 cents.

thanks
john

 

John, your input is appreciated. I have translated some of your suggestions into a format that is much easier to read and understand. The ones I left off I either disagree with or didn't understand.

transistor: (a) cutoff, lamp off; (b) saturated, lamp on.

if the switch is open as in (figure above (a), the base wire of the transistor will be left “floating” (not connected to anything) and there will be no current through it. In this state, the transistor is said to be cutoff. If the switch is closed as in (figure above (b), however, electrons will be able to flow from the emitter through to the base of the transistor, through the switch and up to the left side of the lamp, back to the positive side of the battery. This base current will enable a much larger flow of electrons from the emitter through to the collector, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated.

Suggested corrections:

transistor: (a) cutoff, lamp off; (b) saturated, lamp on.

if the switch is open as in (figure above) (a), the base wire of the transistor will be left “floating” (not connected to anything) and there will be no current through it. In this state, the transistor is said to be cutoff. If the switch is closed as in (figure above) (b), however, electrons will be able to flow from the emitter through to the base of the transistor, through the switch and up to the left side of the lamp, back to the positive side of the battery. This base current will enable a much larger flow of electrons from the emitter through to the collector, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated.

OK, moving forward...

Two points can be made here, actually. First is the fact that when used in this manner, the switch contacts need only handle what little base current is necessary to turn the transistor on; the transistor itself handles most of the lamp's current. This may be an important advantage if the switch has a low current rating: a small switch may be used to control a relatively high-current load. More important, the current-controlling behavior of the transistor enables us to use something completely different to turn the lamp on or off. Consider Figure below, where a pair of solar cells provides 1 V to overcome the 0.7 VBE of the transistor to cause base current flow, which in turn controls the lamp.

Solar cell serves as light sensor.

Two points can be made here, actually. First is the fact that when used in this manner, the switch contacts need only handle what little base current is necessary to turn the transistor on; the transistor itself handles most of the lamp's current. This may be an important advantage if the switch has a low current rating: a small switch may be used to control a relatively high-current load. More importantly, the current-controlling behavior of the transistor enables us to use something completely different to turn the lamp on or off. Consider Figure below, where a pair of solar cells provides 1 V to overcome the 0.7 VBE of the transistor to cause base current flow, which in turn controls the lamp.

Solar cell serves as light sensor.

moving forward...

Amplified microphone signal is rectified to DC bias the base of the transistor providing a larger collector current.

Amplified microphone signal is rectified to DC to bias the base of the transistor providing a larger collector current.