Design a circuit "simulate" opening/closing a valve for a water tank

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Joined: Thu Feb 01, 2018 1:45 am

Design a circuit "simulate" opening/closing a valve for a water tank

Post by lingyueqing » Thu Feb 01, 2018 2:24 am

I need a circuit that can "simulate" opening/closing a valve for a water tank. The circuit has only 2 inputs, 'm' (minimum) and 'M' (Maximum), and an output 'V' (valve). The valve is only open if V= High(1). The tank has a floating water meter that goes up and down with the tank water level.

When the tank is emptying, as soon as the water meter reaches a certain level, minimum(m), the entry 'm' is Low and the valve should open to fill the tank. As soon as water meter reaches minimum(m) again in the rising direction, 'm' will become High but the Valve should remain open (=1). When the tank is filling, as soon as the water meter goes above a certain level Maximum(M), the entry 'M' is High and the valve should be closed. If the tank starts emptying again, 'M' will become Low but the valve should remain closed.

m=Low(0) and M=High(1) will never happen simultaniously.

I need to make two versions of this circuit: one using only NAND gates, and another using a NXP Flip-Flop:

I tried using for simulation purposes. One of the answers has a working NAND gate implementation, but I just can't wrap my head around the circuit with the JK flip-flop.
NAND gate schematic

Posts: 4
Joined: Fri Mar 08, 2019 8:43 am

Re: Design a circuit "simulate" opening/closing a valve for a water tank

Post by bandtank » Fri Mar 08, 2019 9:56 am

I realize this post is very old, but I thought I'd answer it in case someone else has a similar question. It's a common situation for filling tanks.

You need to design a state machine to understand the logic before trying to implement a solution. It's very likely you will need to add more states and/or constraints after you dig into the problem. Start with this Karnaugh Map:

Code: Select all

m  M | V
0  0 | 1
1  0 | 1
0  1 | Bad
1  1 | 0
The Karnaugh Map shown above doesn't tell the whole story, however, because the transitions matter. The fill state is activated when m = 1 until M = 1. When m = 1, the fill state is triggered, which sets V = 1. When M = 1, the idle state is triggered, which sets V = 0.

The m = 0 and M = 1 condition is bad. You should set V = 0 in that case because it's safer; one of the inputs is bad and you don't know which one, so you should assume the tank is at the maximum instead of below the minimum to avoid overfilling. This would be a good time to trigger a notification that gets a human involved as well. You can't assume this won't happen; it will and you have to handle it.

I don't think the circuit you provided will actually work in this application because the Karnaugh Map doesn't support the intended results and it doesn't handle the error condition (m = 0, M = 1). Here's the first draft of the state machine that should govern your design and implementation:
Screenshot from 2019-03-08 09-21-43.png
Screenshot from 2019-03-08 09-21-43.png (25.27 KiB) Viewed 48 times
You're going to need state elements like a D flip-flop. I didn't draw the reset state to simplify the diagram, but that needs to be handled as well.

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