ISP Clip for ATtiny44/84 (or others!)

I got my start in microcontrollers with the Arduino, as many of us outside of electrical engineering did. (And still do!) The ease of programming that little board makes it a great gateway... and like any gateway, sometimes you go through.

Most of my projects now don't use Arduinos directly. For simple microcontroller applications, it's much more cost-effective to use a bare microcontroller selected for the memory size and pin-count you need to do the job. For me, that usually means either the 8-pin ATtiny45/85 or the 14-pin ATtiny44/84. Both can now be programmed directly using the Arduino IDE with either a commercial programmer or with an Arduino, as described on the excellent MIT High-Low Tech site.

One frustration with these smaller microprocessors, though, is that I either had to take them out of the circuit and wire up a breadboard as shown at MIT's site, or build a 6-pin ISP connector into my project. I built a 6-pin ISP-to-breadboard adapter assembly, which often helped during the prototyping process, but it was not ideal. What I really wanted was something that would clip directly to the chip and let me reprogram it without removing it from the breadboard or circuit.

It turns out the solution had been sitting in my toolbox for the last 15 years, I just didn't know it! Years ago, someone gave me a set of "DIP clips": they're a spring-loaded clip made by 3M that grabs a DIP chip from the top and provides easy contact points to each pin. They're still sold here and elsewhere. I glued a 6-pin ISP header on the side of one of these with wires from the ISP header to the appropriate pins on the clip, and voila! A ISP clip.

This one is for the ATtiny84 (or 44): I just plug my ISP programmer to the clip and put the clip on the ATtiny84: I can then test and debug the microcontroller while it's in the circuit —even if it's soldered into the circuit— without having to build an ISP header in the circuit. There's another 8-pin clip that does the same for me with the ATtiny85 (or 45).

New idea? No... Pretty? No... But it works nicely.

Random training interval timer

In addition to various things I'm supposed to do, I race triathlons. One of the training tools we use is the interval workout, in which we alternate high- and low-intensity time intervals during the course of a run or bike workout. There's been a bit of discussion during the Monday-Night run recently about whether it might be advantageous to train with randomized intervals instead of with set intervals.

A typical running interval workout might be a 20min warmup, 5 sets of 1min hard 3min recovery, then 20min cooldown. The idea is to replace the main set with 20 minutes of random intervals, during which the hard and recovery intervals vary randomly in length (within reasonable parameters, of course.)

Typical sports-watches have an interval feature that helps time regular intervals, but they don't do random intervals. Here's my solution: use an ATtiny45 to generate the random intervals, and indicate the intervals via a flashing LED.

And here's the code.

Programming of the ATtiny45 was done via the Arduino IDE: the fuses on the ATtiny45 are set to use the 1MHz internal oscillator. The circuit itself is simply power and ground to the chip (using a 1225 lithium battery) and an LED with current-limiting resistor on PB0 of the microcontroller.

Quantitative Two-Dimensional Temperature Measurements

Lab Experiments involving the Heat Equation are usually one-dimensional exercises involving a copper pipe and a half-dozen thermocouples. But with DS18B20 "One-Wire" thermometer chips and an Arduino, it's possible to measure hundreds of temperature values simultaneously. Here's what my students Daniel Lund and Lawrence Lechuga and I came up with for 2-D temperature measurements.

We began by laying out a 10x10 grid of sensor locations on a 30cm-square plate of 5mm-thick aluminum. We laid Kapton tape in strips under where the sensor leads would be located, to prevent the leads from shorting against the aluminum plate; then we glued each sensor to the plate using thermal epoxy.

Next, we temporarily attached each sensor to an Arduino microcontroller running FindAddress.ino. This program first determines how many devices are on One-Wire bus; then for each device, it sends the address of that device to the Arduino serial port. A terminal emulator on the attached computer displays those addresses. By running this program with the sensors attached individually, we could then determine the hard-coded addresses of each sensor. One hundred tests later, we had a complete list of sensor addresses, ordered by their physical position on the grid.

Once we knew the individual addresses and corresponding locations of the sensors, we permanently wired all the sensors to the Arduino. Power and ground are provided by the USB connection to the computer, and the data pins for each sensor are all connected via a single wire to one input pin of the Arduino. The data bus is also connected to power via a 1k pull-up resistor. (Note: the datasheet for the DS18B20 calls for a 4.7k pull-up resistor, but we found that with 100 sensors on a single bus a 1k pull-up resistor provided more reliable operation.) The Arduino itself is connected to the computer via a “FTDI Friend” USB-serial converter board.

The completed hardware (bottom side) can be seen below. The plastic stand-offs allow it to be placed flat on a table with the bare top side up.

The final step in construction is to program the Arduino to measure the temperatures and send those temperatures (in order of grid position) to the computer. With this many sensors we found that we were straining the capacity of the microcontroller: although the ATmega328 chip on the Arduino board has 32k of program space, it has only 2k of RAM. This RAM is used by the serial and One-Wire communications libraries as well as by our program, and when the RAM is full the Arduino behaves erratically. Our solution was to store the array of sensor addresses in the 1k array of EEPROM on the microcontroller. This required a second Arduino program (address_storage.ino) which was run once to store the sensor address array in EEPROM. Once that program did its job, we uploaded our final program, ThermoPlate.ino.

The ThermoPlate.ino program operates in two steps after the initialization procedures: first, it sends out a “broadcast” message on the One-Wire bus telling all sensors to record the current temperature. Next, it goes through all the sensor addresses stored in EEPROM. For each address it sends a temperature inquiry to that address, converts the sensor response to °C, and sends the temperature as text down the serial line to the computer. When the Arduino has gotten the temperature from each sensor it sends an EOL character to the serial port, then repeats the process. That’s the final output of the device: line after line of serial data, each line containing 100 temperatures in left-to-right, top-to-bottom order. With our 16-MHz Arduino, it takes just under 2 seconds to measure all points, with the limiting factor being the speed of the One-Wire bus itself.

The computer has to be able to listen to the apparatus and make sense of the incoming data. Since the incoming data is simply text, there is a wide range of possible tools to use for this purpose, ranging from LabVIEW to IDL to C++. For simplicity, we chose to write our program, TempMovie.py, in Python. This program captures each line of data coming in the serial port, splits the line into individual temperatures, rearranges those temperatures into a grid, and then draws a filled contour plot based on that grid data. One can set the temperature scale in the program to cover the expected range of values, and if desired the program will save the raw data for later analysis. The figure below shows a single frame of output of TempMovie.py when a small bag of ice was placed at the upper right and a hot soldering iron was placed near the center.

The image changes as the plate moves towards its new equilibrium state, and one can observe the heat flow in real time. For the sequence of images shown below, the plate was hit with a flame from a propane torch, which was removed after a few seconds. (Time in the video below is sped up by a factor of 4.) Time-dependent thermal data such as this can be compared to numeric solutions of the Heat Equation.

 

Note: These temperature sensors can detect changes as small as 0.1°C. If you look closely at all of these images, you can see that an area near the lower left corner of the plate is about 0.3°C warmer than the rest of the plate, due to the power dissipation of the microcontroller.

This technique of using multiple DS18B20 sensors controlled by a single Arduino can be used for one-dimensional experiments as well. Twenty-five sensors, evenly spaced along a meter of copper pipe, would make a wonderful replacement for the typical half-dozen thermocouples we’ve all used and hated as undergrads!

The sensors can be ordered from Newark.com or Jameco.com. For our thermal epoxy we used "Arctic Silver Alumina" from Newegg.com.

Off-topic, but... AT&T sucks.

For the past several years we've gotten our internet through AT&T DSL. Unless you want to support Comcast with your money (and nobody with an interest in net neutrality does) AT&T DSL has been the only option in this town.

I got tired of slow network. Calling AT&T did no good, they claimed things were fine. So I measured the actual speed. Repeatedly!

It just happens that I have access to a pair of very nice servers at my work; I wrote a script on my home machine that downloaded a small file from one of them and recorded the download speed every 15 minutes. An identical script on the second server served as control: should the problem be the server rather than the DSL line, the second server would show it.

After sitting through more than a year of dreadful AT&T service, here are the results.

Notes:

• Up through the end of April of this year, my actual download speed was at most HALF the advertized "up to" speed. For most of last year, I got a quarter of the advertised speed on average.
• The jump in speed at the end corresponds to the date at which many of my neighbors gave up and got Comcast. Speeds after that point are still less than 2/3 the advertised "up to" speed.
• The "control" server (graph not shown here) showed a flat constant speed of nearly 100 Mbps. In other words, the limiting factor was the transfer, not the source.

Conclusions: AT&T sucks, and we need for them to have some serious competition in this area.

They just switched me to "U-Verse" (still the same "up to 1.5 Mbps" speed) so we'll see if that's any improvement. I'm not holding my breath, and the script is still running.

Oh, and here's the code, which was executed every 15 minutes by cron.

#!/bin/bash

DATESTRING=`date "+%Y-%m-%dT%H:%M:%S%t"`
DOWNLOADSPEED=`curl -s -o /tmp/whatever -w '%{speed_download}' http://REDACTED/randomblock`
echo "$DATESTRING    $DOWNLOADSPEED" >> /REDACTED/speedresults.txt

rm /tmp/whatever

Driving multiple Sparkfun 7-segment displays with an Arduino

I'm currently helping a couple of engineering students finish a senior project that didn't get finished last year. Long story... Anyway, it's a bicycle brake tester being built for Paul Components. The mechanical design is great, but they had trouble with the electronics.

Part of those electronics involved writing numbers to a pair of Sparkfun 7-segment displays. There's a lot of discussion on the Sparkfun board about these: apparently they're difficult. I had some issues making them work ---baud rate, for example, which should be set to 9600 in setup() not 2400 as stated in some references--- but it's relatively straightforward once those issues are straightened out. Here's my code, in hopes that it'll be helpful to anyone else trying to write to several 7-segment displays simultaneously.

The serial TX line (second pin given in the SoftwareSerial declaration) should be connected to the RX pin on the corresponding Sparkfun breakout board; the RX line in the SoftwareSerial declaration should be left unconnected. I've only tested it with two boards at a time, but it should work with as many as you have room for on the Arduino. It works fine for just one display, also!

Remote key-switch operation

One of my colleagues, Dr. Shane Mayor, has built a very nice LIDAR system for atmospheric research. It's located at a remote site, at the end of a dirt road, and everything about the system can be controlled remotely except the main pump laser power supply. That supply has a key-switch, which has to be manually turned to activate the system. He asked me to see what I could do to make it all remote-controlled.

Due to the cost of the laser, and Dr. Mayor's unwillingness to void the warranty, my suggestion that he just replace the key-switch with a computer-activated relay was met with some resistance. He absolutely did NOT want me to do anything inside that case! So instead, I built a servomotor key-turner.

The basic idea is to use one of the spare digital lines on his main National Instruments control board to signal the servo controller. When the line goes high, the servo turns the key to the 'on' position, and when the line goes low, the servo turns the key to the 'off' position. Any simple microcontroller should be sufficient for the task: rather than commit an entire Arduino board to the job I just used an Attiny85. Still overkill, but it's so cheap that I don't stock anything cheaper in my parts bin anymore!

Here's the circuit:

And here's the board, which I etched using Pulsar's "Fab-in-a-box" toner-transfer paper.

The software uses Ilya Brutman's Servo8Bit library, and I programmed the Attiny85 using MIT's High-Low Tech instructions. Here's my code. There's a 6-pin ISP header on the board (J1), which I use with an Arduino-as-ISP programmer.

I used the top of a scrapped project box to mount the servo directly above the key and hold the servo output shaft coaxial with the key. The actuator consists of a rubber stopper with a slot cut in it, mounted on a servo control arm. The slot in the stopper fits over the key so that the servo turns the key directly.

It took one re-programming cycle to make the microcontroller turn the servo the right direction (what kind of key turns left for on, anyway?) and then a very simple LabVIEW program to control the digital line to the servo, but that's it! Works great.

Christmas-tree water-level alert

Update 12/11/11: A fix to the program--- changed to capSenseRaw() instead of capSense(). The problem with capSense() is that it auto-adjusts the output for touch sensing, rather than absolute capacitive measurement. capSense() reports whatever capacitance it observed the first time as zero from then on. This means that if the water level is high when the Arduino is turned on, high will be interpreted as dry... And it's not very convenient to make sure the sensor is outside the tree-stand whenever you first turn the lights on! Kinda misses the point of this whole exercise if you have to take the sensor out, dry it off, turn the lights on, put it back... But using capSenseRaw() takes care of the problem.

One other note: the sensor has to be very waterproof! The 4 coats of polyurathane were marginal: I've since added a thin coat of silicon RTV and it works much better. If water touches the electrode strips, it shorts out the capacitor and the sensing function returns -2, which leads to erroneous reports of low water level.

-- --- ----- ------- ----------- -------------

About a year ago I made a Christmas-tree water-level sensor that didn't hold up well. Here's a link, but don't make that one. It had a flashing LED, then an audible alarm, to let you know that the water was either low or gone. But why use an LED? As "Joe" suggested in his comment last year, the tree has a lot of lights on it already and why not flash them instead?

Good idea, Joe. Thanks!

The functional problem with last year's model is that the copper traces used for the water-level sensor corroded quickly, and the device was useless in less than a week. The fix I came up with this year is to use a capacitive measurement of water level instead of a conductive measurement. This method (and the necessary CapSense library) is described on the Arduino Playground wiki as an idea for a proximity sensor. Since water has a very high dielectric constant, the presence or absence of water significantly changes the capacitance of a pair of copper traces. Measuring the capacitance of those traces then gives you a measurement of water level while keeping the traces isolated from the water.

Here's my sensor. It's just a strip of strip-type protoboard. (It's 4 traces wide, but I'm only using 2 traces.) It's been covered with 4 coats of spray-on clear polyurethane, so it is (one hopes) completely waterproof. There are two neodymium magnets glued to the back so I can mount it to the inside of the Christmas-tree stand. I forgot to put something in the picture for scale, sorry: it's 9cm long.

With this improved sensor, I can now use the CapSense library to measure water level without worrying about corrosion of the copper traces. And I probably should have stopped there, but Joe's suggestion just seemed to good to pass up. So here's the schematic for the rest of it:

Everything inside the dashed lines is built into a small box. The box has a plug receptacle on the top, into which are plugged the tree's lights. Power to that receptacle is switched by a relay, controlled by an Arduino Pro Mini. (The Pro Mini and relay are powered by a 9V wall-wart transformer, also mounted inside the box.)

If the Arduino detects a "good" water level, it leaves the relay alone and the lights plugged into the receptacle stay on continuously. When the water level drops to "fair", the Arduino cuts the lights briefly once every 5 seconds. When the water level drops below that, the Arduino flashes the lights once a second. Here's the box: I wrapped it in shiny paper to help it "blend in" under the tree.

Here's the code. The values of "fair" and "good" will depend on the exact geometry of your sensor and sensor cable. Uncomment the serial lines in that code and see what values are coming back from the Arduino to determine what levels are appropriate for your setup.

A note for the Arduino-haters: Yes, this could be done with an ATtiny85. Here's why I decided to use a whole Arduino this time:

1. You need fairly precise timing to measure capacitance repeatably with the CapSense library. That means you need an external crystal rather than the microcontroller's internal oscillator.
2. The 12A relay I used needed 9V to activate. (My local Radio Shack didn't have a 5V relay that could switch more than 1A.) An ATtiny needs 5V at most, so I would need a voltage regulation circuit.
3. I needed to know the values coming out of the CapSense routines so as to be able to set the cutoffs for "good" and "fair" in the final program. That meant I needed a serial link back to the computer for the setup process.
4. Put all this together and hey, it's an Arduino. The Arduino is built already, and I have to write an exam this week also.

But yeah, it'd be cheaper and more elegant if I just used an ATtiny85. :-)

Update: Since I have the memory available, I tweaked the program a bit. Instead of a fast blink when the tree gets dry, it now blinks "water... water... water..." in Morse code.