# Help with circuit will pay for help

#### makel

Feb 17, 2016
9
Need help with designing a truth table and safety circuit if anyone can help me I will gladly pay you for your time

Here is the problem
A powered guillotine uses a motor to position the cutter. The cutter must not operate while the motor is running. There is a guard which must be closed and locked before the cutter can operate but there is a maintenance facility which will allow the cutter to operate with the motor stopped but the guard open. A traffic light system must be incorporated to show when the Guillotine is unsafe (red), safe (green) and when in maintenance mode (amber). The guillotine can only operate once when in maintenance mode.

Draw the Karno map and design the safety circuit using two input NOR gates (use as many as you require).

If each gate has a MTTF 1526173, calculate the MTBF for your circuit (in hours).

How could you redesign the circuit for an improved MTBF?

How could you achieve a MTBF of 1 × 109 hours?

What will the reliability be after 5 years?

(Show the calculations).

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,454
This is a task you've beeen assigned to solve. We will not provide the complete solution, rather guide you along to find the solution yourself.
Show us your effort so far. We need an anchor to start from.

#### makel

Feb 17, 2016
9
This is my truth table I made a circuit using AND gates but I don't know how to make it with 2 NOR gates

A B C Safety Expression

0 0 0 Green

0 0 1 Green

0 1 0 Amber B

0 1 1 Red BC

1 0 0 Red A

1 1 0 Red A

1 0 1 Red A

1 1 1 Red ABC

A is the motor
B is the Guard
C is maintenance mode off
If A is true (1) motor is running
If B is true (1) Guard is open

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,454
I made a circuit using AND gates but I don't know how to make it with 2 NOR gates
A AND B = NOT(A) NOR NOT(B)
NOT(A) = A NOR A

You will need 3 NOR gates to substitute 1 AND gate. However, using a karnaugh mao you may be able to optimize the circuit using less gates (which will also help improve MTBF of the whole circuit).

#### makel

Feb 17, 2016
9
What would the inputs be with 2 additional gates? Is there a way to include the cutter only operating once in maintenance mode?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,454
Sure there is. Its like "operate cutter" = "maintenance mode" AND "start cutter signal".

There is also the issue of the following requirement:
The guillotine can only operate once when in maintenance mode.
How do you think this can be done? Hint: You'll need soemting to "remember" whether the guillotine has been operated or not: This "something" has a dedicated name (which one?) and can be constructed from NOR gates, too.

#### makel

Feb 17, 2016
9
Ok ive been using LogiSim to make my circuits is there any better software you could recommend for circuits/truth tables?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,454
Sorry I can't.
For this task paper and pencil should suffice. You'l have to learn the basics: any tool is only as good as the person using it.

#### makel

Feb 17, 2016
9
I don't know a great deal about electronics, i'm doing engineering and electronics is one module however the lecturer broke his hip a few weeks in so I have received almost no tutoring that's why im asking for help here. I will try that circuit but how will I know it works without simulation?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,454
I don't know a great deal about electronics
This task is not so much about electronics as it is about logic and boolean algebra. You could solve the same task using e.g. pneumatic or gydraulic components (valves etc.).

I will try that circuit but how will I know it works without simulation?
Using the simulation to check your theoretical results is another thing, and a good one. I was referring to the use of the simulator to "design" the circuit, which is not good engineering.

#### makel

Feb 17, 2016
9
So I guess my circuit should have 3 inputs and the output would be yes/no whether the cutter will work? How would I represent the cutter operating once in maintenance mode in my truth table though? Sorry for all the questions im working a 40 hour a week job and doing this course on nights, I have never done electronics past basic school level and with the lecturer off I am struggling with this module with little time to teach myself.

#### AnalogKid

Jun 10, 2015
2,842
For one-time-only cutter operation you will need a latch. The input to the latch is the cighal to the cutter, and the output from the latch is a 4th logic variable that indicates that the cutter has been operated once.

ak

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