diff --git a/platformio.ini b/platformio.ini
index b73b326..657f13e 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -369,3 +369,16 @@ build_flags = ${in-debug.build_flags} ${out-usbradio.build_flags}
 extends     = micro, in-debug, out-switchusb
 src_filter  = ${in-debug.src_filter}  ${out-switchusb.src_filter}
 build_flags = ${in-debug.build_flags} ${out-switchusb.build_flags}
+
+# OhmMeter utility
+
+[ohm]
+src_filter = -<*> +<OhmMeter.cpp>
+
+[env:esp32-ohm]
+extends     = esp32, ohm
+src_filter  = ${ohm.src_filter}
+
+[env:micro-ohm]
+extends     = micro, ohm
+src_filter  = ${ohm.src_filter}
diff --git a/readme.md b/readme.md
index 19ed06b..95fbf85 100644
--- a/readme.md
+++ b/readme.md
@@ -33,7 +33,7 @@ Wiring
 |  6            |  6                     |  5         | -        | -      | -      | -        | -        | P1-9        |
 |  7            |  7                     | 18         | CE       | -      | -      | -        | -        | P2-7*       |
 |  8            |  8                     | 19         | CSN      | -      | -      | -        | -        | -           |
-|  9            |  9                     | 21         | -        | -      | -      | -        | -        | -           |
+|  9            |  9 > 1k Ω              | 35 > 1k Ω  | -        | 330 Ω  | 100 Ω  | 220 Ω    | 680 Ω    | 470 Ω       |
 | 10            | 10                     |  3         | -        | -      | -      | -        | -        | -           |
 | 11            |  0                     |  1         | -        | -      | -      | -        | -        | P1-2        |
 | 12            | -                      | 22         | -        | -      | -      | -        | -        | -           |
@@ -46,12 +46,13 @@ Wiring
 | 19            | 19                     | 25         | -        | DATA2  | -      | -        | -        | P2-2        |
 | 20            | 20                     | 33         | -        | DATA3  | -      | -        | -        | P2-3        |
 | 21            | 21                     | 32         | -        | DATA4  | -      | -        | -        | P2-4        |
-| 22            | -                      | 35         | -        | -      | -      | -        | -        | -           |
+| 22            | -                      | -          | -        | -      | -      | -        | -        | -           |
 | 23            | -                      | 3.3V VCC   | 3.3V VCC | -      | -      | 3.3V VCC | 3.3V VCC | -           |
 | 24            | 5V VCC OUT             | 5V VCC     | 5V VCC   | 5V VCC | 5V VCC | -        | 5V VCC   | PX-5 5V VCC |
 | 25            | GND                    | GND        | GND      | GND    | -      | GND      | GND      | PX-8 GND    |
 
 * 2nd player Genesis is incompatible with Radio because it uses the same pins, 1 player Genesis is compatible
+Ω This is optional and only used for dongle detection. For DB-25 pin 9: On the micro, run to pin 9 to 5V VCC with a 1k resistor. On ESP32, run to pin 35 to 3.3V VCC with a 1k resistor.  On each dongle, run a resistor of the given value between DB-25 pin 9 and GND.
 
 Credits / Links
 ---------------
diff --git a/src/OhmMeter.cpp b/src/OhmMeter.cpp
new file mode 100644
index 0000000..69f5a33
--- /dev/null
+++ b/src/OhmMeter.cpp
@@ -0,0 +1,64 @@
+
+/*
+ https://www.circuitbasics.com/arduino-ohm-meter/
+
+micro values:
+
+82: 81.31
+100: 98.71-99.89
+
+*/
+
+#if defined(ARDUINO_ARCH_ESP32)
+
+int analogPin = 35;
+int Vin = 3.3;
+auto steps = 4095.0;
+
+#else
+
+int analogPin = 9;
+int Vin = 5;
+auto steps = 1024.0;
+
+#endif	// ARDUINO_ARCH_ESP32
+
+// value of the known resistor
+float R1 = 1000;
+
+#include "Arduino.h"
+
+int raw = 0;
+float Vout = 0;
+float R2 = 0;
+float buffer = 0;
+
+void setup() {
+	Serial.begin(115200);
+}
+
+void loop() {
+	raw = analogRead(analogPin);
+	if (raw) {
+		buffer = raw * Vin;
+		Vout = (buffer) / steps;
+		buffer = (Vin / Vout) - 1;
+		Serial.print("raw: ");
+		Serial.print(raw);
+		Serial.print(" Vout: ");
+		Serial.print(Vout);
+
+		// this is R2 when it's connected to + and R1 is connected to ground
+		R2 = R1 * buffer;
+		Serial.print(" R2: ");
+		Serial.print(R2);
+
+		// this is R2 when it's connected to ground, and R1 is connected to +
+		R2 = R1 / buffer;
+		Serial.print(" R2: ");
+		Serial.println(R2);
+	} else {
+		Serial.println("disconnected");
+	}
+	delay(1000);
+}