Monthly Archives: June 2015

Maker Module (MM02) – Arduino battery charge monitor


This design pattern uses a voltage divider, registering via an analog input pin, in order to enable an Arduino to monitor its own power supply voltage. This is particularly useful if you have a requirement to trigger shutoff when battery charge gets low (e.g. when using a LiPo battery), or else report battery voltage visually on screen in some application.


  • 2 Resistors (see selection notes, below)
  • Arduino
  • Battery
  • (optional) Something to report the battery voltage in real time, e.g USB/TTL serial convertor [like this] for serial monitoring, or else a display, or series of coloured LEDs, etc.


Let’s say you have an Arduino UNO, and wish to use 6 AA batteries to power it. At a nominal 1.25V apiece, this makes a total supply voltage of 9V. The Arduino will happily consume this via its onboard regulator, to create an onboard logic voltage ~5V.

Let’s assume that we’ll use the default (5V) analogue reference voltage on the Arduino. In this case, a voltage of 5V (or higher) on an analog pin will be read using analogRead() as integer value 1024. Equivalently, half this input voltage will report the value 512, and 0V will report the value 0.

In order to get sensible pin readings from our supply voltage, we need to map the input range (in this case 9V to 0V) into that required for logic input (in the case of our chosen reference voltage, 5V to 0V)…

Let’s select a suitable resistor pair to create this ratio. Using a voltage divider calculator, plug in the input (peak supply voltage at 9V), desired output (reference voltage at 5V), and 1Ω for R1. Note the value that the calculator returns for R2. The ratio between R1 and R2 is key to the correct voltage reduction: by inputting 1Ω for R1, R2 is reported (ignoring the unit) as the other half of the key ratio.

In our example case, the perfect ratio for R1:R2 is 1:1.25. Any resistor pair with the same ratio difference as that calculated here, will scale the maximum supply voltage perfectly to the input reference. Resistor values aren’t ranked in a full continuum however, so it can be tricky getting as close to the desired ratio as possible. Armed with a vague sense that ‘low’ resistor values are a source of current wastage, yet high resistances can create ‘noisy’ input, I found a close mapping (9V to 4.9V) where R1:R2 = 1:1.2, by choosing R1=1000Ω and R2=1200Ω.


Wire the circuit, including the divider as in the following diagram. The analogue pin on the Arduino takes its voltage reading in between R1 and R2:


int analogPin = 2; // any analogue pin

int r1 = 994; // actual readings of my resistors in ohms...
int r2 = 1196; // ...close, but never exactly as advertised!
int rp = r1+r2 ;

// a value that can be used to tune out any discrepancies -
// there's expected error in the Arduino reference voltage;
// '5V' may not be 5V on your board!
float calibIVRef = 1.0; 

float vRef ;
int val = 0 ;

void setup()
    vRef = 5.0;


    // wait a moment for analog reference setting to take hold

void loop()
    // the core calculation
    val = analogRead(analogPin);
    float vOut = (val/(float)1024);
    float calibratedVOut = vOut*(vRef*calibIVRef);
    float vIn = (calibratedVOut*rp)/(float)r2;

    // report back the 'real' voltage at the analogue pin

    delay(500); // don't spam samples

Closing remarks:

You can use an alternative reference voltage, so long as the resistance ratio maps the supply to an appropriate input voltage. In my smartwatch for example, I’m using 1.1V reference voltage on a 3.3V Arduino Pro Mini, mapping from a LiPo 4.2V peak supply to just a little over the reference voltage (1.125V), using R1=3270Ω and R2=1196Ω. As the input voltage drops below 1.1V, I start to see the ‘real’ battery voltage reported on the tiny screen, and I can make sure that I power down for a recharge as the battery approaches 3.7V.

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Maker Module (MM01) – BLE Microcontroller


I’m really excited about the potential with this building block – it’s core to any ‘smart’ home, and I can think of tons of wireless, phone controlled toys to make with this as a foundation!


  • Arduino Pro Mini (or any equivalent clone via eBay)
  • HM10 BLE module


I choose a 3v, 8mhz, ATMega328 Arduino Pro Mini (clone) – perfect for low energy applications, and has a very small size with just the right amount of space for the HM10 to sit onboard. Note that the HM10 has its VCC at 3.3V, so is unsuitable for any 5V Arduino without extra engineering.

The HM10 I purchased from eBay, shipping all the way from the far East; these take a while to arrive, but are very low cost.


Refer to the photo above – I flipped the Arduino over to expose its relatively bare back, and attached the HM10 with a double sided sticky foam pad. Leave a little room at the antenna end to maximise communication range.

Pin soldering was simple (if a little fiddly); refer to this pinout []. The only pins that are needed for basic wireless serial communication are VCC, GND, TX and RX. Couple the TX/RX with any digital pins on the Arduino – simply use the SoftwareSerial library to use it.


The following is all that’s needed to echo anything arriving via BLE into the Arduino serial monitor, and likewise push anything typed there back over BLE.

#include <SoftwareSerial.h>

SoftwareSerial mySerial(7, 6); // RX, TX

void setup() 
    // Open serial communications and wait for port to open:

    //  set the data rate for the SoftwareSerial port

void loop() // run over and over
   if (mySerial.available()) Serial.write(;
   if (Serial.available()) mySerial.write(;

Writing code for ‘any other thing’ to push data over to the board, is a whole other post… I’ll be sure to cover it when I write about my smartwatch. To give a short hint for now: I had great success with a PhoneGap app I built for iOS and Android, using a plugin called EvoThings BLE.

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Maker Modules

Well it’s been well over two years since my last post (that’s what having a child does to you, don’t you know). It seems that a whole mass hobbyist movement has happened in my absence – 3D printing, the Internet of things, etc. I have lots of catching up to do.

I’d like to start by sharing a series of posts about ‘functional modules’ that are pretty easy to procure parts for and assemble, which can form building blocks in numerous projects. I call these little recipes ‘MakerModules’.

So far they are:

MM01 – A tiny Arduino microcontroller married with a Bluetooth low energy module, for wireless anything.

MM02 – A battery charge monitor for Arduino.

MM03 – A battery coupled with a micro USB charger, creating a small, portable, easily rechargeable energy source.

These elements have been ‘discovered’ during development of my original ‘smartwatch’ project, which I’d also like to write about soon.