Measuring resistance or voltage with 1 digital I/O

May 23, 2011 10 Comments

Let’s say that you’re trying to measure some resistance, but running low on analog pins. Maybe you’re using a microcontroller (aka micro) that’s purely digital, such as the parallax propeller. Well my friends this post is for you! Let’s consider the fact that nearly all micro’s digital pins trigger at 1/2 of the voltage they micro is being driven at. If we slowly increase the voltage of a micro’s digital input, the micro will consider it HIGH at 2.5+ volts for a 5v micro.

If we use a simple charging RC circuit, we can think of the final voltage as 2.5 volts. What if we have the micro discharge the circuit, so we start at about 0v. After the circuit is discharged the micro will turn the pin to input and time how long the circuit takes to charge to 2.5v. Resistance is easily calculated knowing initial and final voltage, capacitance, and time elapsed.

Math:

At the top is the classical, time domain, voltage of a charging capacitor. Vcap is the capacitor voltage, Eemf is the electromotive force (charging voltage), C is the capacitor’s capacitance, t is the time in seconds, and R is the resistance being solved for.

Vcap will be 2.5v when micro triggers, C is .1uF, t is in micro seconds, Eemf is 5v. For arduino, which runs at 5v and digital I/O triggers at about 2.5v. Notice that the 10E-6 cancels out. This is good since Arduino doesn’t have alot of sigfigs to work with during calculations.

Make sure that the time that this circuit takes is reasonable. The time below is in Seconds. Everything else is same as before, R is approximately what resistance is being measured.

variables are same values as above.

Vcap was switched to Vc

Schematic:

As you can see the circuit is fairly simple. The 270Ohm resistor is to limit current the current as the micro discharges the capacitor. The R is the resistance we’re trying to measure.

This circuit is not meant for extremely high sampling rates. If you absolutely need a high sample, then I suggest that you get an ADC chip to do it all for you. Note that if the resistance is high, then you might want to decrease capacitance. If Resistance is very low, then increase capacitance. Only do this if you’re having problems though.

Sometimes the ADC chip is worth it, but if you don’t happen to have one lying around, or if you don’t want to mess around with spi or I2C, then this is a quick work around.

So in this screenshot I had channel 1 (yellow) connected to the capacitor. Channel 2 (blue) connected to a signal pin on the micro. The capacitor’s voltage suddenly drops down when the micro discharges it. When the blue line’s rising edge occurs, that signals the I/O measurement has begun and the pin is set to input. We can then see the blue line’s falling edge occurs when the capacitor’s voltage is at 2.5 volts. As soon as the 2.5 mark is it, the circuit is ready for another sampling. I just had the sampling set to some arbitrary length of time.

Here’s a video of it in action!

Original forum posts

http://arduino.cc/forum/index.php/topic,50642.msg361179.html#msg361179

http://adafruit.com/forums/viewtopic.php?f=25&t=19476

Code for the Arduino:

Any analog references are solely for debugging. Remove them when you are confident everything’s working.

Click to download resistor measurement program (pdf)

Measuring Voltage using this method: I do like the thought of using 1 i/o to measure nearly any range of voltage, but basically if you’re measuring up to 100 volts, then this method will only be able to measure 50-100 volts. This is because the voltage divider will make the 0-100v into 0-5v, and the pin won’t trigger below about 2.5v. This equation was calculated using Thevenin’s theory. It could also easily be solved using nodal analysis, but then we’d have some calculus to do! I feel as though this is impressive to do very accurate voltage calculations, but it’s a bit complicated and doesn’t work over the full range. An op amp could be added to give a wider voltage measurement range, but this is would require additional pins… and the op amp output won’t like charging capacitors!

Well the voltage calculation was off, but fitting the calculated versus real output results in a linear function with slope almost 1. Interestingly it’s off by a constant.

I decided not to investigate why, but it is probably a result of component 10% manufacturing tolerance.

If you decide to give this a shot, then use the top middle schematic with the equation on the very bottom “a” is 2.5 (on the mid left). I suggest you do a function fit of calculated versus real voltage output like I did. Code is extremely similar to the example above. Of course you could just do a function fit for the entire thing since the picture shows the form of the equation.

code for voltage measurement: voltage_measuring_rc

Filed under Guides Tagged with

Sharp proximity sensor Function Fitting

May 23, 2011 2 Comments

Ultrasonic distance sensors, such as the Ping))), are great, but they aren’t cheap and often have very low refresh rates around 50ms. The sharp sensor becomes an attractive alternative since it updates about 1-2 milliseconds, outputs a fairly consistent distance measurement, and it’s about 15$!

Long range Sharp Sensor

Long range Sharp Sensor (Image by sparkfun.com)

What an awesome looking little guy! The only problem with him is that it’s difficult to calculate an accurate distance from his analog output, but with a little bit of some curve fitting we’ll have the problem solved.

The graph that the data sheet provided gives a good bird’s-eye view of how to solve the problem. For this fit, It will be accurate about 15-150cm. I just simply plugged in a voltage meter and started taking measurements of the output voltage. I then used logger pro to plot and fit the points.

sharp Fitting (click to enlarge)

For this example, I was using the Arduino. One thing to keep in mind is that the arduino only has a 10bit ADC. The voltage of interest for the sharp sensor ranges from about .5-2.7 volts. In this range we only have about 450 points of measurement by the ADC. In order to improve this I set the analog reference to internal which maxes out at 1.1volts. Then used a voltage divider on the sharp sensor’s output so that 2.7+.1 scaled to 1.1 volts.

For the voltage divider, I just used a 20k potentiometer, so R1+R2 = 20k. Vin is 2.8, Vout is 1.1, so R2 needs to be 7857ohms (measure this with your voltmeter.). So the center tab of the pot will be Vout, And the sides will be R1 and R2. It doesn’t matter on your polarity, but make sure that the 7857ohms is what’s connected to ground. Once you’ve wired everything check Vout to ensure the 2.7v output is a little below 1.1v.

I will mention that when the sharp sensor updates, it will make a little ‘bloop’ in the analog voltage output. The signal to noise ratio tends to drop right before and after it updates. You can put a capacitor connected to the micro controller’s analog input pin and connect the other wire of the capacitor to ground. If you need to filter some of the readings, as I did, then I suggest don’t use the capacitor, filter, then do a running average. The method of read ->filter -> running average seemed to work best.

Below is example code for Arduino, and I simplified the equation a bit! I was using a 2.64v to 1.1 voltage divider, but you can just key in your values.

Note: All analogs are on 1.1v! Don’t go above this or you’ll probably fry your chip!

Code for Arduino: (ver ~ 0022)

Click to download Sharp Interpretation program (pdf)

Filed under Guides Tagged with ,

Newer posts →