upd

README.md

@@ -99,6 +99,134 @@ is the maximum recommended by the EPA. Water with a TDS level above 1,000 ppm
 is not considered safe to drink, and levels above 2,000 ppm may
 require a filtration system.
 
+
+### Resistance sensor with M5Stack AtomS3-Lite
+
+In order to measure the resistance you can measure voltage using ADC (analog) input esp32 pin on a voltage divider circuit.
+
+Voltage divider circuit consists of:
+
+- Resistor of a known resistance
+- Sensor's resistor for which resistance is to be measured
+- Known reference voltage (3.3v for esp32 case) applied to those two resistors connected in series
+
+Voltage is measured on ADC (analog) input pin of esp32 connected in between those two resistors.
+
+Ohm Law can be used to calculate unknown resistance from voltage drop (knowing reference voltage and reference resistor resistance)
+
+There are two kinds of configurations for this circuit:
+
+Either the variable resistor is close to GND (DOWNSTREAM) or it is closer to VCC (UPSTREAM).
+
+
+![image](img/VoltageDivider.png)
+
+
+$$
+\begin{flalign}
+\text{ Downstream: } \newline \newline
+&  \large {V _{ref} \over {R _{ref} + R _{sensor}}} = {V _{meas} \over {R _{sensor}}} \text{ } \Rightarrow \text{ }
+R _{sensor} = {R _{ref} {V _{meas} \over {V _{ref} - V _{meas}}}} &
+\end{flalign}
+$$
+
+
+
+$$
+\begin{flalign}
+\text{ Upstream: } \newline \newline
+&  \large {V _{ref} \over {R _{ref} + R _{sensor}}} = {V _{meas} \over {R _{ref}}} \text{ } \Rightarrow \text{ }
+R _{sensor} = {R _{ref} {{V _{ref} - V _{meas}} \over V _{meas}}} &
+\end{flalign}
+$$
+
+
+#### esp32 ADC
+
+esp32 ADC is not the best:
+
+- The ESP32 has two 12-bit ADCs
+- Noisy (can be corrected by statistical sampling which will reduce sampling rate for accurate measurements)
+- Non-linear (can be corrected by polynomial approximation)
+- Almost ignores ranges < 0.14v and > 2.6v
+- The ESP32 ADC can measure voltage levels between 0 V and 3.3 V. The measured voltage is assigned a value between 0 and 4095, with 0 V corresponding to 0 and 3.3 V corresponding to 4095
+- ADCs in the esp32 have an attenuator stage at their inputs; the suggested range for 11Db attenuation is 2600mV at best (even though the ADC's range is 0-3.3v)
+- However, the output can be made accurate to within 1% on input range of 0.14 to 2.6 volts
+
+
+#### For precise measurements use external ADC
+
+Examples:
+- ADS1115  https://docs.m5stack.com/en/unit/vmeter
+- INA219   https://www.adafruit.com/product/904
+
+#### Resistive boat sensors
+
+There are two types of commonly used resistance based sensors:
+- European 0-190 ohms range (often 10-180 ohms working range)
+- American Standard 240-33 ohms
+
+Examples:
+
+- Fuel level (10-180 Ohm or 0-190 Ohm range)
+- Oil pressure
+- Rudder position (10-180 Ohms)
+- Trim
+- Resistive Temperature Sensors
+
+Constraints for selecting a reference resistor size for these types of sensors:
+
+$$
+\begin{flalign}
+& V _{ref} = 3.3 \newline &
+8 \lt  R _{sensor} \lt 240  \newline &
+0.14 \lt  V _{meas} \lt 2.6 &
+\end{flalign}
+$$
+
+Appropriate sizes are (in Ohms):
+
+$$
+\begin{flalign}
+& \text{ Upstream: } \newline &
+15  \lt  R _{ref} \lt 33 &
+\end{flalign}
+$$
+
+$$
+\begin{flalign}
+& \text{ Downstream: } \newline &
+68  \lt  R _{ref} \lt 220 &
+\end{flalign}
+$$
+
+Pick up resistor with lower % tolerance.
+
+#### NMEA 0183 XDR Sentences for resistance
+
+NMEA 0183 is on USB serial with baud rate of 4800
+
+Example sentences:
+
+````
+$BBXDR,G,135.4,O,Ohms_esp32*6A
+$BBXDR,G,137.9,O,Ohms_esp32*65
+$BBXDR,G,137.6,O,Ohms_esp32*6A
+$BBXDR,G,137.6,O,Ohms_esp32*6A
+$BBXDR,G,138.1,O,Ohms_esp32*62
+$BBXDR,G,138.3,O,Ohms_esp32*60
+````
+
+#### Ready made voltage divider PCBs
+
+There are numerous ready made voltage
+divider PCBs. Look for
+
+- Grove Voltage Divider with adjustable gain
+- DFRobot DFR0051 Voltage Divider
+- Phidgets 1121 Voltage Divider
+
+
 ### INA219 Voltage and Current sensors (up to four on secondary i2c bus)
 
 Load and your sensor need to share common ground.