Complete newbie here with very little electronics background needing some help. I want to use Sparkfun’s force-pressure sensor to determine when a person moves their weight from both feet to 1 foot or the other. I’m picturing a person swaying back and forth with force-pressure sensors under each foot. At the beginning, before the person starts to sway, their weight will be more or less evenly distributed on both feet. I’d like to have the sensors under the feet be connected to a vibrating motor so the person knows when weight has been shifted to one foot or the other. I think this is a relatively simple circuit involving a battery, sensor and vibrating motor. What I’m unsure of is how to make it so the initial weight of the person on the sensor doesn’t activate the vibrating motor. I’d like the vibrating motor to only be on when additional weight is added to one foot or the other. I hope this is enough information so someone here could provide some help. Thanks in advance!
Dave
I see it as an initialization problem. The ‘alarm’ must be properly initialized before it can go off. I’d expect near zero readings from both pads prior to a person stepping on. Then (unless he hops) it’ll be 1 pad giving a “high” reading followed by the other. Then both pads will give semi-high readings. I’d use this condition (both pads >> non-zero) to enable the alarm. Thereafter when 1 pad is > semi-high and the other is < semi-low, the alarm goes off. Should both pads ever read near zero again (for longer than some preset time period), the alarm is again disabled until the above condition is met.
The circuit is a simple microcontroller (MCU), a power supply, your 2 pads and motor and a motor driver of some sort. The MCU will need 2 analog inputs to read the 2 pads and just one digital output to control the motor. A Trinket would be just sufficient. A ProMicro would allow for expansion (more I/O pins) and be easier to debug code on (serial monitor).
https://www.adafruit.com/products/1501
https://www.sparkfun.com/products/12640
Another possibility is use of some op-amp and comparator circuitry. An op-amp could amplify and rectify the difference between the pad voltages and feed that into a comparator circuit that also detects when pads have some weight, > zero, on them. That way when there’s enough of a difference between the pads but both pads still have some weight on them, the motor runs. It stops the motor from running when stepping on and off.
Thanks so much for taking the time and effort to put some thought into this. I’ll follow-up on the suggestions/links you provided.
I’d say the MCU option is less wiring and so easier to do. But some people are programming-averse and the op-amp&comparator option isn’t too much harder. One advantage of the MCU is that it can be self-calibrating. That is, when a person has stepped on w/both feet, a measure of their total “weight” (inferred from pressures) can be done and the threshold(s) for motor on/off can then be set as a % of the total weight. The op-amp circuit can only alarm when a fixed “weight” (pressure) difference is sensed. That means a heavy person trips it’s alarm more easily, they have to keep a closer to even balance, %-wise, than a lightweight child would have to.