This circuit allows for easy adjustment of a threshold you define for some sensor input. For example, you use a photocell to detect ambient light; if it gets dark enough, play a sound. You define the threshold in code as some number, but when you install the piece in a different space with different ambient light, the threshold needs to be changed. Rather than need to open up your piece, connect a computer, and reprogram, you can just turn a knob to “trim” the threshold value up or down.

You can test the above circuit using a potentiometer instead of the force sensor shown, remember it can be a simple voltage divider like the trimmer is, or you can use 2 legs and a fixed resistor.

For the fixed resistor, try between 1/10 and 1/2 of the highest value of your variable resistor. E.g. if you’re using a 5k variable resistor (or part of a potentiometer), which goes between 0 and 5000 ohms, try a 500-2500 ohm fixed resistor with it (a 1K prob easiest to find).

The optimal value for you math people is
SQRT (minimum * maximum)
https://ericjformanteaching.wordpress.com/2013/02/04/voltage-divider-circuit/

In practice the true minimum might be zero which messes up the equation, and the true maximum might be in the millions, with a very steep curve. So it’s important to test the real-world boundaries. It usually doesn’t matter too much if you are fairly close to optimal. The only way to really tell what’s going on is to graph the “response curve” – i.e. the resulting voltage output as you interact with the sensor – either with an oscilloscope or by sending arduino data to processing. We’ll do the latter in class soon.

```/*

The circuit:
- If potentiometer, center pin to analog 0, side pins to +5V and ground
- If variable resistor, one pin to +5V, other pin to both analog 0 AND
to a fixed resistor to ground
- trim pot to analog pin 1 (as voltage divider, see above)
*/

int threshold = 250;

void setup() {
Serial.begin(9600);
}

void loop() {

// convert trimmer # into a range which will be added to analogValue:
trimmer = map(trimmer,0,1023,-50,50);

// print the result:
Serial.print(analogValue);
Serial.print(" + ");
Serial.println(trimmer);

// if analogValue is over the "trimmed" threshold, do something:
if(analogValue > (threshold + trimmer)) {
Serial.println("HEY!!!!!!!!!!!!!!!!!!!");
}
}```