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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 = 5000; 
// Off time per cycle in milliseconds 
const int CYCLE_TIME_OFF = 2000; 

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.

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

Control a small DC pump

In an earlier post I showed how to wire a small DC pump to a power supply and a switch to turn it on and off. Even with simple manual control, there are still things to be aware of. Let’s say that you are building a small aquaponic or hydroponic system and you need a pump to circulate nutrient fluid to the plant roots. This is a situation where the pump only needs simple on/off control and can be left to run continuously.

There is a danger that over time the liquid will evaporate and its level will drop too low. Many liquid pumps cannot safely be run dry. This is because they are designed to use the liquid flowing through to cool them to a normal operating temperature. Without a continuous fluid flow, the motor will overheat.

This means we have to be aware of the liquid level. A float switch level sensor can be used in the control circuit to make sure that the pump does not run when the liquid is below a certain level. Most liquid level switches can’t control the current needed to run a pump so we use the switch to control a relay that has this needed current capacity.

It’s also important to remember that some pumps also have a maximum on-time rating. The pump may be able to run only (e.g.,) one minute continuously before it needs to be turned off to cool for three minutes. This proportional on/off time is referred to as its duty cycle.

Why RS-232 serial?

Why do we use RS232 serial ports to communicate with our hardware? Well, first and foremost, it is probably the least-common-denominator communication method in the industrial and embedded physical computing arena. If a computer has just one port, odds are it’s a serial port. This is mainly because serial ports are so easy to implement.

Those computers that don’t have RS232 serial ports, will usually have USB (another serial bus) ports. Because of the decades-long history of RS232, USB to RS232 ports are easy to find. Nothing wrong with USB, but it has more complexity and is limited to fairly short distances.

So, in a nutshell, RS232 serial ports are easy to work with, ubiquitous and reliable.

Detecting when water flow stops

Flowmeters are used to measure liquid flow. However, if you only need a notification that flow is present or not and don’t need to know the quantity of liquid, there are simpler methods.

A flow switch is an on/off device that will indicate whether or not a liquid is moving. They can be either Normally Open (NO), meaning that the switch is not conducting when there is no flow, or Normally Closed (NC) meaning that the switch is conducting when there is no flow. Having both types available offers flexibility to a design. One of the nice things about a flow switch is that is is very simple to build a visual indicator of whether the liquid is flowing or not. By using a light (bulb or LED) or a buzzer and a flow switch you can build a simple low flow alarm.

They are available in various pressure ratings that set the threshold of flow/no flow.

Flow switches generally present less resistance to flow than a flow meter, so there is less pressure loss across the switch.

As with flowmeters, our PRT232 is an easy way to read a flow switch and get an indication of fluid flow into your PC via an RS232 serial connection.

More on LEDs

The post on how to (light an LED from an RS232 serial port) got more interest than I expected. There is an old design that I played around with a couple years ago for a high-power RS232 serial port LED light that I am thinking of resurrecting. It’s so bright, it hurts to look directly into it! A USB version might also be nice, but RS232 has a great advantage in distance and most of the people we talk with need something more rugged than USB. You can run serial lines to the LED controller for over 100 feet under good conditions. The only thing that really competes with that is wireless or Ethernet.

Note to self: Ethernet LED controller? Hmmm.