From: Keith Packard <keithp@keithp.com>
Date: Mon, 10 Feb 2025 22:42:41 +0000 (-0800)
Subject: bin: Add resistor divider calculator
X-Git-Url: https://git.gag.com/?a=commitdiff_plain;h=7155ca3d0d0e124c107e061b4524d9c4271da011;p=hw%2Faltusmetrum

bin: Add resistor divider calculator

Signed-off-by: Keith Packard <keithp@keithp.com>
---

diff --git a/bin/divider b/bin/divider
new file mode 100755
index 0000000..62844ba
--- /dev/null
+++ b/bin/divider
@@ -0,0 +1,88 @@
+#!/usr/bin/nickle
+#
+# Compute resistor dividers
+#
+#
+
+if (dim(argv) != 3) {
+	File::fprintf(stderr, "usage: %s <input voltage> <output voltage>\n", argv[0]);
+	exit(1);
+}
+
+real input = atof(argv[1]);
+real output = atof(argv[2]);
+
+real[...] resistors = {
+	10.0,	10.2,	10.5,	10.7,	11.0,	11.3,	11.5,	11.8,	12.1,	12.4,	12.7,	13.0,
+	13.3,	13.7,	14.0,	14.3,	14.7,	15.0,	15.4,	15.8,	16.2,	16.5,	16.9,	17.4,
+	17.8,	18.2,	18.7,	19.1,	19.6,	20.0,	20.5,	21.0,	21.5,	22.1,	22.6,	23.2,
+	23.7,	24.3,	24.9,	25.5,	26.1,	26.7,	27.4,	28.0,	28.7,	29.4,	30.1,	30.9,
+	31.6,	32.4,	33.2,	34.0,	34.8,	35.7,	36.5,	37.4,	38.3,	39.2,	40.2,	41.2,
+	42.2,	43.2,	44.2,	45.3,	46.4,	47.5,	48.7,	49.9,	51.1,	52.3,	53.6,	54.9,
+	56.2,	57.6,	59.0,	60.4,	61.9,	63.4,	64.9,	66.5,	68.1,	69.8,	71.5,	73.2,
+	75.0,	76.8,	78.7,	80.6,	82.5,	84.5,	86.6,	88.7,	90.9,	93.1,	95.3,	97.6,
+	100.0,
+};
+
+real divider(real input, real ra, real rb)
+{
+	return input * rb / (ra + rb);
+}
+
+real find_ra(real input, real output, real rb)
+{
+	real ra;
+	/* output = input * rb / (ra + rb)
+	 * output / (input * rb) = 1 / (ra + rb)
+	 * (input * rb) / output = ra + rb
+	 * (input * rb) / output - rb = ra
+	 */
+
+	return (input * rb) / output - rb;
+}
+
+real error(real input, real output, real ra, real rb)
+{
+	real actual = divider(input, ra, rb);
+
+	real e = abs(actual - output);
+	return e;
+}
+
+real closest_standard(real ohms)
+{
+	real exp = floor(log10(ohms));
+	real mul = pow(10,exp + 1);
+	real search = ohms * mul;
+
+	for (int i = 0; i < dim(resistors)-1; i++) {
+		if (resistors[i] <= search && search <= resistors[i+1])
+			if (abs(resistors[i] - search) < abs(resistors[i]+1 - search)) {
+				return resistors[i] / mul;
+			} else {
+				return resistors[i+1] / mul;
+			}
+	}
+	return 0;
+}
+
+void find_divider(real input, real output)
+{
+	real	best_a, best_b;
+	real	best_error = -1;
+
+	for (int i = 0; i < dim(resistors); i++) {
+		real rb = resistors[i];
+		real ra_ideal = find_ra(input, output, rb);
+		real ra_actual = closest_standard(ra_ideal);
+		real e = error(input, output, ra_actual, rb);
+		if (best_error < 0 || e < best_error) {
+			best_a = ra_actual;
+			best_b = rb;
+			best_error = e;
+		}
+	}
+	printf("input: %g ra: %g rb: %g output %g\n", input, best_a, best_b, divider(input, best_a, best_b));
+}
+
+find_divider(input, output);