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# Challenging homework problem

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#### E-design

Solve this problem

From another forum: (think it was someone's homework problem)

Original problem: You have 2 offices connected by 3 wires. You are allowed only 1 DC source total and 1 bell or buzzer in each office. Design a wiring diagram so that either office can call each other by ringing the bell or buzzer in the other office. You are only allowed to use mechanical switches.

While this one is very easily solved, someone suggested raising the bar by proposing the following:
Given: Still 2 offices, 1 DC source(2 terminal battery of any voltage -not allowed to split the battery into cells), bell or buzzer (any voltage) in each office,
BUT using only 2 low resistance wires between the offices
. (No extra ground return -that would count as a wire).

You may not use any active amplifying components like transistors,IC's or AC, RF, encoding, infrared beams, laser, tunnel-diode, super conductor or any other fancy component.

You may use LED's, diodes, relays etc. Under no circumstance are you allowed to short out the DC supply or insert a fixed resistance for current limit (for instance if both buttons in the 2 offices happens to be pressed simultaneously in your design would short the DC supply). Both buzzers/bells may not sound at the same time

No valid solution was given at the time of this posting (many readers thought it to be impossible). I think it can be solved however. (you need to think outside the box for this one)!

Let's see what our members can come up with. To make it more interesting, instead of posting your design, just mention the total amount of components you needed extra (besides the given battery, 2 bells and 2 wires) to complete the task. We may look and judge the winning designs later on.

If you think really hard, you can do it with 5 extra parts. If you think smart, you will need only 3 (I know of two different ways with 3 extra components, there may be more ways)

Re: Solve this problem

i can solve this problem with only 2 extra components , when you want i send the drawing. my email is vitor-ferreira@netcabo.pt

Re: Solve this problem

You can PM me with your circuit to evaluate. With only 2 components you may be the winner as long as the 2 components are not 2 telephones.

Re: Solve this problem

I count 2 components extra per side for a total of 4 extra components. Also there will be a dead short circuit for a short time (time it takes for relays to switch) with both buttons pressed. It is debatable if the relay K2 will even switch under this condition to clear the short.

Re: Solve this problem

I think it is possible to do it with 2 multicontact switches and one diode.

Added after 1 hours 48 minutes:

hope it is free of errors

Re: Solve this problem

Looks valid to me. My idea's is different and maybe much simpler.

Re: Solve this problem

This is my solution.

regards,
Davood Amerion.

Re: Solve this problem

I based my design on a component made famous by Bill Hewlett and David Packard in their Model 200A audio oscillators, for those that are old enough to remember this product. The component was of course the lamp. In his (William Redington Hewlett) 1939 Stanford thesis "A New Type Resistance-Capacity Oscillator" he produced graphs that showed the cold to hot resistance of the filament to vary with a factor up to 10×

So with this inserted in series with the battery, the lamp can be chosen according to wattage to offer low enough cold resistance for normal operation of the buzzers and to prevent high short circuit currents with both buttons pressed.

The lamp my be replaced by a PTC resistor if the time constant is fast enough

Solve this problem

dear E-design
i think
1- your circuit continusly consume power.
2- you use resistor (lamp) to limit current. (as opposed you mentioned)
3- selection battery voltage and buzzer [trigger]voltage and lamp spec. tightly depend to each other.
4- when battery goes to decharged. your circuit may fail to do anything.

Am i right?

Re: Solve this problem

I remember that lamp , and even made audio generator with it(transistor based). )

Your approach is based on fact that buzzers wont work if voltage is less than 12 V . BTW i got a chineese manufactored 12 V buzzers working pretty well from 5V TTL supply. Sure there should be buzzers stopping to work if supply is less than nominal voltage .

So switch approach is more technological to produce - no need for component matching .))

Davood Amerion said:
dear E-design
i think
1- your circuit continusly consume power.
2- you use resistor (lamp) to limit current. (as opposed you mentioned)
3- selection battery voltage and buzzer [trigger]voltage and lamp spec. tightly depend to each other.
4- when battery goes to decharged. your circuit may fail to do anything.

Am i right?

Clause number 2 is not relevant as :
"short out the DC supply or insert a fixed resistance for current limit "

Clause number 4 is relevant for all of us )

Rest is about means to accomplish task that wont be done )

Re: Solve this problem

Davood Amerion said:
dear E-design
i think
1- your circuit continusly consume power. Yes
2- you use resistor (lamp) to limit current. (as opposed you mentioned) It is NOT a fixed value resistor.
3- selection battery voltage and buzzer [trigger]voltage and lamp spec. tightly depend to each other. Not necessary
4- when battery goes to decharged. your circuit may fail to do anything. It could be another DC source as nothing said the battery should last forever.

Am i right?

Like I said, this is my design. Could I make it work? YES

Having the lamp also has another advantage. You could select it so that it will have a very faint glow in the standby condition (idling current through buzzers). This way you could at least tell that you don't have a break in your lines if the buzzers don't want to work. It is almost the same principle as in a 4-20mA control loop where the 4mA is to indicate continuity of the loop.

Re: Solve this problem

Hi E-design
If you think really hard, you can do it with 5 extra parts. If you think smart, you will need only
3 (I know of two different ways with 3 extra components, there may be more ways)
In first post you said, you know 2 method for making this circuit can you give us 2nd circuit.

2- you use resistor (lamp) to limit current. (as opposed you mentioned) It is NOT a fixed value resistor.
3- selection battery voltage and buzzer [trigger]voltage and lamp spec. tightly depend to each other. Not necessary [/red]
4- when battery goes to decharged. your circuit may fail to do anything. It could be another DC source as nothing said the battery should last forever.

In cases:
2. why you dont alow to useing FIXED resistor!? and you used lamp!! what is the difference? is resistor more complicated than lamp? i think both them have same purpose.
3. i dont agree with you.
4. in 1st post, you said DC source is battery, I dont underestand your description.

Regards,
Davood Amerion.

Re: Solve this problem

Davood, some people will like your design and some mine. It is a question of choices. If you think a resistor and a lamp is/do the same thing, then good for you.

Battery can be recharged, replaced or be large enough to last for months, years - who cares? Battery was meant to indicate a single DC supply. Sorry if that confused you.

I have some buzzers rated 24V max that works happily from about 10V, so trigger tolerances should not be a problem

Also the (picky) argument can be made: What happens if the diode fails for some reason - go short circuit? It may be a dangerous short circuit condition (if the diode is what prevents the DC supply from shorting in some mode) and set something on fire unless you add another fuse component. If the lamp fails, it will be noticed (no faint glow) and worst, the circuit will stop working.

Regards E

Re: Solve this problem

Regarding Davood's solution, I don't see how Office 1 can call Office 2.

Re: Solve this problem

I have not looked at Davood's circuit in detail. Oh, I see what you mean - there's no way buzzer 2 can get connected to the (-)battery terminal with office 1 pressing the button. Maybe it is just a drawing error that Davood made when doing the CAD version.

Added after 1 hours 13 minutes:

Here is yet another design with 1 extra component, but maybe Davood will object to this

When you want to call the other office, just pull on the wire on your side!

Re: Solve this problem

Thats a really neat trick!! I thought about the solution given below.
Another interesting problem is. Suppose you are in a normal office and that office has a gate. Now you can extend 1 wire from the gate to the office. Now set up a system such that a person at the gate can call up or indicate 2 different people in the office independently. Well, for this you can use active devices if you want, but a simplest solution will be the best.

You were right, I have uploaded a corrected solution below. I think this is error free.

Re: Solve this problem

Won't you be ringing both buzzers by pressing the LH switch?

Re: Solve this problem

dear aryajur
you are right. when i convert my drawing (in protel) i mistake in switch connection (in office1)
I correct it and attached corrected one.

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