Arduino Programming: About time

Time is probably the most commonly controlled process variable. Timers are all over the place in industrial control. Odds are, if you need some type of timer, no matter how strange, you can find it off the shelf.

Now, many of these timers used a chip usually referred to as the “555.” The LM555 originally made by (I think; someone will correct me) National Semiconductor was a very versatile device, but it was at the heart of many time-delay relays, short timing circuits, etc.

So once upon a time, if you wanted to build a basic timer, odds are you would wire up a 555 into a circuit. To build a handful, or just one, you’d use a perf board,

perfboardmaybe you might use wire wrap or even dead-bug construction (my favorite!)


It would be time consuming,but maybe you had no choice because the timer had some weird requirement that no off the shelf timer had, or needed to fit into an oddly shaped space.

What does this have to do with Arduinos? Well, you can program any timing sequence into an Arduino. Say you want the heater on a commercial ironing board to come on for five seconds when the operator lowers it, a 555 does it easily. If you want the heater to come on for five seconds and when the board is raised again, a fan to blow for 10 seconds to cool the clothing, the 555 can still be used. Maybe you need two of them. But now, the Arduino becomes an easier solution. Whether you need one time sequence, or dozens, a single Arduino can be programmed to do it. When you factor in the labor of wiring a circuit board with the 555, the low off the shelf price of the Arduino makes it even more attractive.

This is the wonder of the time we live in: an off the shelf microprocessor board is now inexpensive enough to be used for logic replacement.


Arduino Programming: stuff we forget

This post will probably grow over time as I add suggestions from you guys. I see a lot of posts online from Arduino enthusiasts who want to build one thing or another. It’s easy to think of which Arduino version you need and the sensors, actuators, etc. The problem is that we forget all the small, incidental things and those costs can add up. When you’re making up your Bill of Materials (BOM), it helps to remember all this “extra” stuff you might not think of right off the bat so you get a good idea of what it will all cost.

So, what do makers/builders often forget?

  • Power. You need a power supply if you’re not planning on keeping your project connected to a USB port. And sometimes even if you are, you’ll need extra power to drive that motor, or multiple supply voltages because you can’t/won’t use a regulator
  • Enclosure. Is it going to be outside? Probably need a NEMA4-rated box. Even if it’s kept inside, a pretty enclosure is a great way to finish up your project and make it look professional.
  • Wire. Yep, something as basic as wire can really add up, especially if you need multiple gauges or ratings
  • Connectors. Round is better. Drilling holes is easy even with cheap tools. Making square, trapezoidal or just plain weird shaped holes is not. Well, not unless you have a machine shop at your disposal. If you do, maybe we can help each other!
  • Tools. Can you drill all the hole sizes you need?

Android Bluetooth serial port communication

Tablets and smartphones are everywhere and prices are dropping fast.  A tablet provides a great user interface: it’s inexpensive, has a high resolution color touchscreen and it’s an ideal method to control an embedded system.

The most straightforward way to do this is with an embedded system exposing a web  interface over Wi-Fi. In this case, the tablet only needs a browser to connect. However, smaller embedded systems may not have this luxury. Here we will look at using an Android tablet to connect to a small embedded system using Bluetooth.

We won’t get into the details of the embedded system but for clarity’s sake, let’s say it’s a small Arduino measuring room temperature and connected to a Bluetooth transmitter. It sends a reading automatically once per second.

There is a lot of information online about using Bluetooth with the Android. The problem is that it is fragmented and few sites have all the information in one place. So I figured I’d compile a set of the major points you need to know to get the Bluetooth Serial Port Protocol (SPP) working on an Android app.

First, your app needs the BLUETOOTH and BLUETOOTH_ADMIN permissions. This goes in yourAndroidManifest.xml file.

<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />

Now that we have given the app permission to access the Bluetooth API, let’s look at the classes that are relevant. We’ll need the BluetoothAdapter, BluetoothDevice and BluetoothSocket classes to connect to the external system.

First, we need to find and pair with the device. The following code snippet builds a collection of Bluetooth devices that were discovered by your Android device.

    BluetoothAdapter bta = BluetoothAdapter.getDefaultAdapter();
    Set deviceList = bta.getBondedDevices();

Now that we have a list of external devices, we can iterate over the collection and extract the name and address of each device like this.

    for (BluetoothDevice i : deviceList)
        String name = i.getName();
        String address = i.getAddress();

You can select which one you need from the list and connect to it. Selecting the device is something you can decide how to handle yourself. Connecting to the device is done using its Bluetooth address. Once you have a selected device, we connect to it by requesting the device through its address. The address is a unique 48-bit ID assigned to each Bluetooth device.
Once we have the device, then we’ll open a socket using the standard UUID for the SPP serial port protocol to indicate that we want to connect to the device as if it were a standard serial port.

    BluetoothAdapter bta = BluetoothAdapter.getDefaultAdapter();
    BluetoothDevice device = bta.getRemoteDevice(address);
    BluetoothSocket socket = null;
    if (device != null)
        UUID serialID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");
            socket = device.createRfcommSocketToServiceRecord(serialID);
        catch ( e)

Next, we’ll handle sending and receiving data from the socket.

Trimmable setpoint voltage divider

Let’s say you’re using a comparator like the venerable old LM339. You provide a setpoint “reference” and an input. If the input is below the setpoint, the output is clamped to ground. If the output is above the setpoint (plus any offset voltage of the comparator, of course), the open-collector output floats. Normally we tie the output to +5V if we want a TTL level output as we are often using the comparator to send a signal to a digital or microcontroller circuit. With me so far? OK.

The classic voltage divider is a good choice for a setpoint if it only needs to stay constant. But what if you need a variable setpoint? Simple, use a trimmer, trimpot, variable potentiometer, whatever you wanna call it. Now you can change the voltage of the setpoint. But wait a minute. If we have a regulated 12VDC and use a trimpot, even an expensive multiturn, it can still be difficult to set that voltage to within a millivolt. After all, a 10-turn pot with 12V at the input is still 1.2 volt per revolution, so with 360 degrees per revolution, you’d need fingers precise to almost 1/3 of a degree to set it to within a millivolt. Possible, but difficult.

Like most things, there are multiple ways to skin this particular cat (I can only write this while Lefty and Poncho are not in the room…).


In the olden, golden days, engineers with beards and slide-rules used verniers that geared the output down, so one turn of the knob might only be 1/10th turn of the potentiometer on the output.

This makes it easier to adjust, but those things are $$$. Gotta be a cheaper solution. Sure, use a voltage divider. Remember the divider can take an input voltage and give you a smaller output, but if we make the divider variable, we can make it so we only vary it by a small amount. So instead of trying to adjust a 0-5 volt range with a potentiometer, we can design the divider so we only have to adjust a 0 – 0.1V range with the same pot. Much easier!


Say Vin is 12V, R1 is 10k and R2 is 820Ω. Vout is then 0.91V. If we insert a 1k potentiometer between R1 & R2,that means that the output can now vary about that point.


So, how does this work? Let’s assume the pot is all the way in one direction, the voltage divider is then 1,820÷11,820 x 12 = 1.85V

with the pot all the way in the other direction, the output is 820÷11,820 x 12V = 0.83V

Nice! So now our trimpot only has to control a span of about 1V instead of a span of 12V. With high-resolution A/D converters and digital inputs, these basic techniques aren’t  used a lot these days, but they are still useful to have in your toolbox.

Now go design something!

Sensors for counting objects

In order to count anything, we need to detect it first. This usually means some kind of sensor. The sensor used will typically provide a signal that our counter can read. Most such sensors actually function as a type of switch because their output terminals are closing a circuit on the counter electronics that causes a count to increment.

The simplest sensor used to count objects is an actual physical switch. Microswitches are switches with very sensitive contacts: a light touch is all it takes to register the presence of an object. Often microswitches are made with levers to reduce the force needed or to have a greater reach.



One common application for this type of switch is in coin counters for arcade games. The coin falls through a slot,  tripping the lever as it rolls past the switch. The main advantage of microswitches is their low cost and reliability. A disadvantage of this type of counting sensor is that physical contact with the switch is required and the force required to trip the sensor can affect the object you’re counting.

Another common sensor type used as input to counters or object detectors is a photoelectric switch. This optical sensor detects the interruption of a beam of light, often invisible infrared light. For example, to count boxes on a conveyor belt, an emitter, typically an infrared LED shines a focused beam of light across the belt. When the beam is reflected by an object passing by on the belt, the detector sees the returned light and closes a circuit and this sends a pulse to the counter module, updating the count of items going by.


Optical sensors have the advantage of not requiring contact with the switch, but may not work well in dirty or dusty environments where the optical signal may be blocked. Also, this type of sensor used for counting reflective items can be “fooled” by multiple reflections, causing an inaccurate count. In this case, a through-beam sensor, where the item must pass between the LED emitter and its detector, is often more reliable.

Magnetic sensors, as their name claims, detect magnetic fields. They are very useful when a non-contact sensor is needed in a dirty environment where light may be blocked.


Now that we’ve got sensors to detect the items, our PRT232 counter module is the ideal interface to do the actual counting. We can make modifications to the basic counter, such as a display, or special RS232 signal outputs,

Arduino Programming: Cycle timer

Sometimes you want an operation to repeat periodically. Say you are building a parts washer that circulates cleaning fluid around the dirty parts. The cleaning cycle might run for an hour and in that time you want the circulation pump to run for 10 seconds, stop for 5 seconds for particles to settle, then run for 10 seconds and repeat for an hour.

We need a timer. The type of timer that does this is called a Cycle Timer because it repeats a specific timing cycle and it’s pretty easy to build a cycle timer with an Arduino and a little bit of software programming. We’ll need an Arduino (any kind, from any manufacturer will work), a power supply, the power driver circuit, and the “load” which in this case is our pump.

Let’s get started.

// Which pin to use to control the load const int OUTPUT_PIN = 1; 
// Total number of cycles 
const int NUMBER_OF_CYCLES = 10; 
// On time per cycle in milliseconds 
const int CYCLE_TIME_ON = 500; 
// Off time per cycle in milliseconds 
const int CYCLE_TIME_OFF = 200; 

void setup() 
 digitalWrite(OUTPUT_PIN, LOW);

// Run the timer 
void loop() 
 int cycles = NUMBER_OF_CYCLES;
 while(cycles-- > 0)
    // Turned timed output on
   digitalWrite(OUTPUT_PIN, HIGH);
   // Turn timed output off
   digitalWrite(OUTPUT_PIN, LOW);
 // Hold forever

Measuring water flow

A flow meter is the sensor that is used to measure water flow, or the flow of a low-viscosity fluid. There are many different types of flowmeters, but perhaps the most common are turbine or paddlewheel types. In these types of flow meters, a blade spins from the force of the moving fluid. The rotation is detected by a sensor that generates pulses that can be counted by a reader interface.

Older flow meters such as fuel pulsers used a rotating magnet that pulled a tiny reed switch causing the switch contacts to close. Every contact closure results in a pulse at the input of the reader, leading to these flowmeters being called pulsers. There are still thousands of these devices in use.

More modern pulsers and flow meters of all types like the one shown below generate their pulses electronically, often using Hall-Effect sensors that, again, respond to a moving magnet that is spun by a turbine or a paddlewheel.


Now that we have a flowmeter device that can give a pulse output rate that is proportional to the rate that the water or other liquid is flowing at, we need to measure it. The reading device is basically a counter that is calibrated to the pulse rate.

For example, a fuel flowmeter may output 10 pulses per gallon of fuel dispensed, or a flowmeter used for water provides 100 pulses per liter. The reader must understand this calibration so it can display the correct value.

In many cases, the need is to read the flow and record or process the data on a desktop computer. Serial interfaces, RS232 or RS485 and USB are common here. By using a serial port flow meter interface, getting the data into the PC for software processing is a simple task since all modern programming platforms provide some form of serial data communication. Once the data can be received by your software, then you may record it, create graphs, log flow over time, etc.

Cedar Lake Instruments’s PRT232 flow meter interface is a serial port counter flowmeter reader that can measure and record flow. It reads pulser type flow meters, and can switch solenoid valves or pumps to control fluid flow.

Arduino Programming: Turn water on with Arduino and solenoid valve

Arduinos are popular small microcontroller boards that have many applications. However, they’re not designed to switch loads above a few milliamps: say a couple LEDs or so. While power-driver shields do provide this capability, they also can consume more resources than you may be able to give up.

We developed a high current driver to make it easy to control a solenoid valves with Arduino. It will also control pumps and motors. With an adapter cable, it can easily connect to your Arduino, BeagleBone, Raspberry Pi or other digital controller without soldering or crimping any connections. Doesn’t get any easier than that.


The power driver board was born out of a need for controlling a 1 amp solenoid valve using an Arduino.  The solenoid valve was being used to control the water flow to fill a tank automatically. Now there’s a simple way to use your Arduino or compatible to switch up to 3A at 24VDC. Two output connections (the white wires shown above) connect directly the load (your solenoid, relay, motor, etc) and the power (red, black) go to the power supply (5 -24 volts). The orange lead is used to switch on and off. This is a low-voltage (5V) control that can connect directly to a microcontroller, or development board. An onboard LED indicates when the load is switched on.

Here’s some sample code that implements a timer with an Arduino. When the pushbutton is pressed, it turns on water flow for 3 seconds

// This sketch demonstrates a simple timer
// A load (motor, solenoid, relay, solenoid valve is on Pin 1
// A pushbutton to trigger the timer start is on pin 2
// When the pushbutton is held down for more than 0.1 second 
// then released, the timer starts
// and times out after 3 seconds
// Timer is retriggerable: if pushbutton pressed 
// during the timeout period, timer restarts
// Constant definitions
#define LOOP_INTERVAL 10
#define TRIGGER_PIN 0
#define OUTPUT_PIN 1

void setup()

void loop()
  static int count = 0;
  static int timer = TIMER_INACTIVE;
  // Process loop periodically
  // Check trigger input
  if (digitalRead(TRIGGER_PIN) == LOW)
    // Must hold down pushbutton for the entire interval and 
    // then release to trigger
    // push button released. Check if we should start timing
    if (count >= TRIGGER_INTERVALS)
      // Turn output ON (timeout is retriggerable)
      digitalWrite(OUTPUT_PIN, HIGH);
      timer = TIMEOUT;
    count = 0;
  // If timer active, count down
  if (timer != TIMER_INACTIVE)
    timer -= LOOP_INTERVAL;
    if (timer == 0)
      // Turn output OFF
      digitalWrite(OUTPUT_PIN, LOW);
      timer = TIMER_INACTIVE;

Let’s find out what new applications you can come up with.

Power Driver ($11.95 shipping included)

Reading temperature on BeagleBone with AD592

A very common task is to measure temperature at various points. In fact, temperature is the most commonly measured and controlled variable in Process Control. The BeagleBone single-board computer is becoming more popular because of its low cost and sophisticated capabilities. It’s one of the easiest ways to serve up a web page that allows the operator to monitor data and control devices.

Temperature can be measured with a variety of sensors. Here we are concerned with the range of liquids from around 0C to 80C. A straightforward way to measure temperatures in this range is with a semiconductor analog-output sensor. They are easy to use, accurate, and low cost.

I happened to have a number of AD592 temperature to current sensors on hand. These have been traditionally used for process control because their current output makes them very noise resistant when long cable are used. We convert the current (1 microvolt/Kelvin) to a voltage by using a resistor. In this case, we use a 3.3kohm resistor to produce a voltage of around 1V at room temperature. The 3.3k resistor gives a resolution of .003V/Kelvin which with the 273.15K offset gives 0.003V/degree Celsius.

Using the sensor connected to 5V, AGND and the analog input on P9, pin 36, we use this code snippet to read the sensor:

// Read an AD592 temp sensor and return degF
function readTemp()
   var volts = 1.8 * trap.analogRead("P9_36");
   // Convert to Kelvin. AD592 outputs 1uV/K
   var k = volts / 3300 / 0.000001;
   // Convert to Fahrenheit
   var f= (k - 273.15) * 9 / 5 + 32;
   // Log the measured values
   console.log("V,k,F: " + volts,k,f);
   return f;

For improved accuracy we can measure the actual resistance of the 3.3k resistor and use that measured value in the program.

This concept is easily extended to remote reading. Since the BeagleBone is a powerful little Linux computer, it can be used to serve web pages. We have a simple node.js webserver and sensor data reader program available at this download link