# Capacitive Force Sensor Circuit Design Challenge

#### Evlov

Joined Jan 4, 2018
2
Hi!

I've been asked to build a circuit that utilizes a capacitive force sensor like this one:

https://www.singletact.com/micro-force-sensor/standard-sensors/15mm-45newton/

The problem statement is this:

If a force of 1.3 lbs or more is applied to the sensor continuously for 10 seconds, then a digital numeric display (7-segment display, for example) begins to count up from zero. (It's counting the time that has passed, so after one second it displays "1", after two seconds it displays "2", and so on.) If the force momentarily drops below 1.3 lbs before the 10-second mark is reached, then the counter resets to zero and will begin counting to 10 seconds again when the applied force is back to being 1.3 lbs or more.

The digital numeric display continues to count up indefinitely until the force applied to the sensor is 1.0 lbs or less for 10 seconds straight. Again, if the force momentarily spikes above 1.0 lbs before the 10-second mark is reached, then the counter resets to zero and will begin counting to 10 seconds again when the applied force is back to being 1.0 lbs or less. If the force applied to the sensor remains at 1.0 lbs or less for 10 seconds without interruption, then the digital numeric display shows "0" or is blank (either is fine).

The circuit is free to cycle between these two states, where State 1 is when the digital numeric display is counting up, and State 2 is when the digital numeric display is showing "0" or is blank.

I'm fairly confident that this can be solved without using an Arduino, and instead with using only fundamental electrical components and simple ICs. However, all ideas are welcome

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#### WBahn

Joined Mar 31, 2012
29,874
Sounds like some kind of assignment (which is consistent with the notion of it being a "challenge"). If so, then aren't YOU supposed to be the primary source of the design ideas?

The description is contradictory. It doesn't start to count until the load has been at least 1.3 lb for at least 10 seconds. If the load falls below 1.3 lb, then it resets. But shouldn't it then wait another ten seconds once the load goes above 1.3 lb before it starts counting again? If not, then there are two different "reset" states.

Then you say that it counts to 10 seconds, but right after that you say that it continues to count until the force applies if 1 lb or less for 10 seconds. After that, it gets really confusing.

What might really help is if you tried to put together a few example timing diagrams of what it is supposed to do under various situations and your best attempt at a state diagram.

#### Evlov

Joined Jan 4, 2018
2
Thanks for the quick response. I agree that the problem is a bit confusing. Please allow me to clarify as best as I can.

In each of the first 2 paragraphs of the problem statement, I'm discussing two separate counters. One counter is the delay counter, which is 10 seconds in length. This counter delays the start (or reset) of the other counter, which acts like a stopwatch (a timer that counts up), and which is represented in the digital numeric display.

Or if you like, you can break it down. If you ignore the 10 second delays, the problem statement becomes much simpler:

If a force of 1.3 lbs or more is applied to the sensor, then a digital numeric display (7-segment display, for example) begins to count up from zero. (It's counting the time that has passed, so after one second it displays "1", after two seconds it displays "2", and so on.)

The digital numeric display continues to count up indefinitely until the force applied to the sensor is 1.0 lbs or less. If the force applied to the sensor is 1.0 lbs or less, then the digital numeric display shows "0" or is blank (either is fine).

The circuit is free to cycle between these two states, where State 1 is when the digital numeric display is counting up, and State 2 is when the digital numeric display is showing "0" or is blank.

Now, instead of instantly changing between the States upon meeting the associated force requirement, we're adding a 10-second delay. However, the change in state will only be successful after the 10-second delay if the force applied to the sensor remains within the specified range. If it doesn't remain within the specified range, then a reset of the 10-second delay is triggered.

Does that help?