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Design Project: Telegraph System
4 questions By Tony R. Kuphaldt
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Question 1 of 4
What voltage will you choose to use for each of the station batteries?
Reveal answerI recommend 9 volts, because small 9-volt batteries are easily available and provide more than enough voltage to energize light-emitting diodes.
Notes:I strongly recommend using batteries to power the telegraph stations instead of AC-to-DC power supplies, as batteries are easier and safer for beginning students to work with. I also highly recommend students choose the same voltage for each station (if using the example circuit), although it will function with different voltage batteries at each station. Of course, whatever voltage is chosen as the system standard will impact other factors in the design, most notably the dropping resistor sizes.
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Question 2 of 4
What resistor values will you choose for the “dropping” resistors in series with the light-emitting diodes? Remember that these resistor values will depend on the voltage and current ratings of the diodes, as well as the chosen power supply voltages.
Reveal answerThis may be determined by using Ohm’s Law, once the component ratings and operating voltage are known.
Notes:This is a very practical application of Ohm’s Law, in a context simple enough for beginning students to understand.
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Question 3 of 4
Explain how it would be possible to have more than two telegraph stations working on your system. Where would you connect the third, fourth, and fifth stations? Would the station designs have to be changed in order to accommodate more than two stations? Will your working voltage have to change?
Reveal answerIt is very easy to add stations if the example design is used. If other designs are chosen, it may be more difficult.
Notes:This question forces students to really think about how their system works, and what considerations arise when system expansion is suggested.
