How to Use (and Choose) a Multimeter!

Checking your car battery life, debugging circuits, and finding that pesky short are all super useful functions that can be performed with just one awesome tool: the multimeter!

First of all, what the heck is a multimeter??   Excellent setup question! It’s a handheld device with bunch of different electrical meters — hence, multi-meter!

Measuring voltage, current, resistance, and continuity (aka electrical connection) are the most common uses of a multimeter.  Read on to learn what this means, how to do it yourself, and how to choose your very own multimeter!

Choosing a Multimeter!


There are a few key differences between multimeters, the main one being analog versus digital:
Analog multimeters show real-time changes in voltage and current, but can be difficult to read and log data.

Digital Multimeters are easier to read, but may take some time to stabilize.

There are also auto-ranging multimeters, that automatically detect the measurement range, and manual ranging multimeters where you have to choose a range yourself (or start with the highest setting and work down).

Other than those two main differences, you’ll want a multimeter that has separate ports for current and voltage measurements (this is a safety issue, both for the meter and for yourself).

Next comes the fun part: features! Multimeters all have voltage and current meters (otherwise they’d just be called voltmeters and ammeters!), and most can also measure resistance. There are a variety of other “extra” features depending on manufacturer and cost (e.g. continuity, capacitance, frequency, etc.).

Second-to-lastly, there are a ton of different types of probe leads, including alligator clips, IC hooks, and test probes. Can’t decide? Here’s a kit that has four different types!

Lastly, always check the multimeter maximum voltage and current ratings to be sure that it can handle what you want to use it for.

Using a Multimeter!

But first! A quick overview of voltage, current and resistance!

My favorite analogy for electricity is the “water flowing through a pipe” analogy. In this analogy, voltage is similar to the water pressure, current is like the water flow (except with current you have electrons instead of water molecules!), and resistance is akin to the size of the pipe. Check out this tutorial for an awesome and thorough overview of electricity.

Keeping these analogies in mind helps us to figure out how, and what, we are measuring.

Measuring Voltage:

A voltage measurement tells us the electrical potential, or pressure, across a particular component.

Voltage is basically the “oomph” in our circuit, s so we want to avoid drawing any power from the circuit when we take a voltage measurement. This means we need to measure voltage in parallel with a particular component using infinite (or really, really high) resistance to prevent any electrical current from flowing into the meter.

Using a multimeter to measure voltage across a component (or battery!):

1. The black multimeter probe goes into the COM port, and the red probe into the port marked with a “V”.

2. Switch the dial to the “voltage” setting (choose the highest setting if you have a manual ranging multimeter).

3. Place black probe on negative side of the component, and red probe on positive side (across, or in parallel with the component). If you get a negative reading, switch the leads (or just note the magnitude of the voltage reading).

Read the meter output and you’re done! Not too bad 🙂

Measuring Current:

Taking a current measurement tells us the amount of electricity flowing through a given component or part of a circuit.

To measure current, we need to measure all of the flow in our circuit without consuming any power from the circuit and reducing the current measurement. This means we measure current in series with a component and we want our meter to have zero resistance.

Using a multimeter to measure current through a component:

1. The black multimeter probe goes into the COM port, and the red probe into the port marked with an “I” or an “A” (or “Amp”).

2. Switch dial to the current setting (choose highest setting if you have a manual ranging multimeter).

3. Connect red probe to current source, and black probe to the input of the component, so that the current flows from the source, through the meter, to the component (in series with the component).

Read the meter output! If you’re not getting a reading, switch to a lower setting.

Measuring Resistance: 

Measuring resistance is pretty straightforward, but you do have to disconnect individual components from a circuit to get their actual resistance, otherwise the rest of the components in the circuit can interfere with your measurement.

Using the multimeter to measure resistance of a component:

1. Put the black probe in COM port, and red probe in the port marked with a “Ω” or “Ohm” — it should be the same port as the voltage port.

2.  Switch dial to setting marked with a “Ω” (may have to choose approximate range for manual ranging multimeter).

3. Place probes on either side of the component (orientation doesn’t matter).

Read the meter output and you have conquered resistance!

Bonus: Measure Continuity!

The continuity measurement checks if two points in a circuit are electrically connected, otherwise known as a conductance test. Before measuring continuity, be sure that the circuit power is OFF.

Using the multimeter to measure continuity: 

1. Place black probe in COM port, and red probe in voltage port.

2. Switch dial to setting marked with an audio symbol.

3. Place probes at points you want to check — if the meter makes a beep sound, it means the two points are connected.

Le fin!

Go forth and measure all the things!

Now that we know how to use a multimeter, get crackin’ on all those at home, DIY projects! To get you started, here are a few quick, practical, & fun projects:

1. Measure the resistance of your skin! Change the distance of the probe leads and see how resistance changes. Lick your fingers (or dip them in water) to see how moisture affects resistance!

2. Measure the voltage across AA, 9V, or other batteries around the house/workplace/school to locate dead, or dying, ones.

3. Make a lemon battery and measure the voltage and current output.

4. Use the continuity setting to check if different materials conduct electricity.

 

Looking for more info on multimeters?

Check out this in-depth guide by the folks at Tools Critic!

Interactive Survey Game!

A survey questionnaire come to life! Use (nearly) any object to gather helpful data through an interactive, engaging, and fun multiple-choice survey.

This project uses the Makey Makey microcontroller in combination with a Raspberry Pi computer to read in participants’ survey choices and save the results in a text file.

Planning & Design!

This general design is easily customized to fit a different theme. The only crucial design requirement is to use materials that conduct electricity for the survey pieces, or wrap non-conductive materials in aluminum foil.

Suggestions:
Prototype, prototype, prototype! Build different versions and test them on family, friends, co-workers, or (ideally) your target audience. Observe how folks interact with your survey, then use that to make it better! And always remember to keep it simple 🙂

Materials

Makey Makey Kit
– Computer: Raspberry Pi

– One (1) ground piece, five (5) survey response pieces, one (1) submit piece, and two (2) yes/no pieces*

22 Gauge (stranded) Wire — five (5) 10 – 16″ strips and three (3) 6″ pieces (ends stripped)

– Container:

— Wood Box (12.5″ x 12.5″)
— Plexliglass.(“12 x 12”)
— Three (3) 2″ x 2″ wood panels

* Specific materials used in this design are detailed with the corresponding procedure, although customization is encouraged!

Tools

Safety goggles, woo!
Multimeter
— Optional: Soldering iron, solder& desoldering wick
— Ruler (or calipers)
Drill w/ both drill and driver bits
Flat wood file (to prevent splinters!)
Hot glue gun
— Epoxy (permanent)
– Pliers

Reprogram the Makey Makey

To reprogram the Makey Makey, you’ll need to have the Arduino IDE with Makey Makey drivers installed. Here’s a thorough tutorial on how to do this.


1. Plug Makey Makey into computer and open the Arduino IDE.

2. Open (or copy) Makey Makey source code:
Here’s the GitHub page for the Makey Makey.
Here’s a direct link to download the full program. This is a .zip file, so be sure to extract all the files.

3. Reprogram the “click” key into an “enter” key.
For a thorough overview of how to do this, check out this tutorial.

4. Change the following keys:
These two keys are mapped in the survey program, but can be left as-is or you can choose to switch other keys (e.g. the arrow keys). Just be sure to change the mapping in the program.

A. Change the “g” into an “n”.
B. Change “space” key into “y”.

Build the Survey Response Pieces!

Specific materials used in this design:

– Two (2) wood blocks, two (2) golf balls, and one (1) jar lid.
– Aluminum foil
Unistrut 1/2″ Channel Nut with Spring
– Ten (10) 1/2″ washers
– Plexiglass [or wood] (12″ x 12″)

Procedure:

1. Wrap each of the survey response pieces at least 2 – 3 times with foil, hot gluing each layer.

2. For unistrut spring pieces, hot glue (or epoxy) the top of the spring to the bottom of each survey response piece — be sure that the metal of the spring is touching the foil of the survey piece.

3. Attach the survey pieces to plexiglass.

Determine location of survey response pieces and mark with tape. Drill a hole at each point.

Place a washer on either side of the hold and screw bolt into unistrut spring about 3 turns.

4. Connect a wire to each of the unistrut spring pieces.

Wrap wire around base of bolt (between washer and plexiglass). Hand tighten the bolt to secure wire without squishing it

Build the Ground Piece!

Specific materials used in this design:
– Styrofoam ball
– Metal pipe
– Flange stand for pipe
– Aluminum foil
– Twelve (12) washers
– 4 wood screws
– Wood panel (2″ x 2″)

Procedure

1. Build a stand for the styrofoam ball — use conductive materials or wrap pieces in foil.

2. Wrap styrofoam ball in aluminum foil, leaving a “tail” of foil. Place ball on stand and push the foil tail against the inside of  Hot glue pieces together.

3. Cover the exposed end of the ground wire (24″) to the inside, or bottom, of base and adhere with tape or epoxy.

5. Add a layer of two (2) washers under base to avoid squishing the wire, then connect base to wood pane via screws or epoxy.

Build the Enter Key!

Specific materials used in this design:

– Clothespin
– Wood panel (2″ x 2″)
– One (1) wood screw + one (1) washer

The screw should be about 1/4″ longer than the wood thickness.

– Aluminum foil

Procedure:

1. Wrap one of the handles of the clothespin in foil.

2. Remove clothespin spring clamp, align other side of the clothespin on wood panel, and drill in a screw and washer.

Foil on the other side of the clothespin should make contact with the washer + screw when closed.

3. Reconnect spring clamp and other side (may need pliers). Epoxy bottom of clothespin to wood panel.

4. Use alligator clip or wrap wire around screw and secure with hot glue.

Make the Yes and No Keys! 

Specific materials used in this design:
– Two (2) plastic container lids
– Two (2) wood panels (2″ x 2″)
– Two (2) wood screws and washers

Each screw should be about 1/4″ longer than the wood thickness.

– Aluminum foil


Procedure

1. Cut circle out of container lids. Wrap in foil.

2. Align lids on wood panels and drill in a wood screw with washer on top — be sure the screw slightly pokes through the back of the wood panel.

3. Use alligator clip or wrap wire around screw and secure with hot glue. 

Connect Pieces to Makey Makey

1. Connect ground piece lead to Makey Makey ground pads.

2. Connect survey game pieces to the first five (5) Makey Makey back header pins on the left: “w”, “a”, “s”, “f”, and “d”.

3. Connect the no button to the last (6th) back header pin, “g”

4. Connect the yes button to the “space” pads.

5. Connect the submit piece to the “click” pads.



Load the Survey Program!

Using a Raspberry Pi computer means that all of the electronics can fit into the game box! Write up a program in Python to cycle through a series of survey questions and five possible choices that map to the survey response pieces.

Here’s my code:
GitHub page!
Python program only.

Final Touches & Case!

This case is designed to withstand high traffic, experimentation, and children — and to be easily (and cheaply) fixable and adjustable. Use this design or customize your own!

Materials:
12.5″ x 12.5″ wood box
1″ x 10 ” wood panel

Procedure:
1. Epoxy wood panel onto front of box.

2. Drill the submit, yes, and no keys into the wood panel.


Recommended to put the “submit” button on the far right (switched this after further testing and feedback).

 

3. Drill hole large enough to fit an HDMI port in the back panel of the box.

I used two 3/8″ bits and filed down the hole until the HDMI port fit.

4. Label the survey game pieces and the submit, yes, and no keys.

Test, & Install!

Connect the Raspberry Pi to a monitor, keyboard, and the Makey Makey. Test the program and double check all the keys. Once everything is up and running, remove the keyboard (and mouse if connected).

Load the python program, stand back, and let passersby have a blast participating in a survey!

Prototyping Magnetic Boots!

Walking across large, metal pipes in search of urban adventure, my inner voice joked, “Hey, magnet shoes would be handy right about now.” Well, no arguing with that! Off to build my very own magnetic shoes!

This tutorial gives an overview of my build process for a magnetic boot prototype in hopes of inspiring you to build and test your own whimsical ideas! ‘Cause seriously, making ideas come to life feels like a superpower.

 


Materials


— Sturdy Boots
These had to secure my feet (aka no slipping out) and withstand my body weight. I found a pair of sturdy (although rather large) snowboard boots at a local thrift store which work as a first prototype.

— Rare earth (neodymium) magnets
Small, thin-ish (< 1/4″ thick) magnets with a 10 – 15 lbf rating (see previous step).

— One screw per magnet (or per magnet hole)
Use screws with a length shorter than the sole of the shoe (so they don’t poke your lil’ feetsies.. or add some sort of rubber sole inside).

— Suggestion: One washer per magnet
Supposedly, the washer helps increase the magnetic field of the exposed surface. I haven’t calculated this or done any serious research, so at this point it’s just a design suggestion.


Tools



Drill

— Ruler

— Pen/pencil.

CNC Router and a 3/4″ drill bit

 


Build Process!



1. Level bottom of the boot with a CNC router (or other available method).

Clamp the boots to the CNC table with the bottom facing up — a piece of wood was helpful to keep the boots straight.

Set the zero point of the CNC to be the lowest point on the sole of the shoe, then use a large bit (ours was 3/4″) and level the sole of the shoe to the zero point.




2. Mark boot with tape for location of magnets.



3. For each magnet, drill in screw, magnet, and washer into the bottom of shoe.


Testing!


To test the boot, I stuck it on a roof beam and pulled downwards. I added more magnets and repeated this until I couldn’t pull the boot off by hand, then (slowly) tried to hang from it.

Lessons learned during testing:
1. I ended up using waaay more magnets than I thought, so it is probably worthwhile to calculate how the individual magnet fields are adding together.

2. Magnets need to be level to maximize the total magnetic field strength.

3. There is a limit to how close you can place each magnet depending on the shape and size of its magnetic field. Smaller, round magnets are easier to work with than large, rectangular magnets.

4. Don’t place magnets close to parking passes (or other electronic devices). Also keep them far, far away from large containers of screws.


Results & Next Steps!


At this point, my magnetic shoes are more magnetic “gloves” (lol thanks @jayludden :D). But! I can successfully hang from one boot, so the concept works!

The lessons learned from testing will help improve this prototype design. Currently awaiting more magnets for the second boot (used most of them for the first one), trying different magnet orientations, and searching for a spot to test them upside down.

Stay tuned, will have them up and running, er, well, hanging, soon!

Many thanks to: Tinker Tank at Pacific Science Center for being my build and test center, and to Richard Albritton for the CNC help!

Hazardous Gas Monitor

Build a portable gas monitor to check for dangerous levels of hazardous gases in your home, community, or on the go and prevent your friends from lighting a cigarette during  a gasoline fight.*

This tutorial shows you how to build a web-connected “canary” monitor for three hazardous gases: Liquid Propane Gas (“LPG”), Methane (aka natural gas), and Carbon Monoxide (“CO”) . Using the Particle Photon microcontroller, the sensor readings are converted into parts-per-million (“PPM”) and uploaded to the data.sparkfun.com web service.

*Please note that this is solely a movie reference — gasoline fights should probably be avoided in real life.


Helpful Background Info!


1. How to set up the Particle Photon.

2. Pushing data to the data.sparkfun.com web server.

3. New to relays? Check out this a handy reference.

4. Here’s a helpful overview on the N-Channel MOSFET.

5. For powering the Photon, here’s a thorough guide on the Photon Battery Shield.

6. Highly recommended to peruse the datasheets for the three gas sensors.


Choosing a Battery!


The gas sensors used in this project require a fair amount of current, about 0.17 A each at 5V. To make the system portable, we’ll need a high capacity battery. One easy, and affordable, option is to use four (rechargeable) AA batteries in series. These batteries will last about 4 hours.

Another option is to use a lithium ion battery (“LIB”). LIBs have a higher capacity than AAs, but typically run at a lower voltage. If you go with this option, you may need to include a correction factor when you calculate the sensor value or boost the battery voltage with a transistor or other component.

The photo above shows a table with the approximate lifetime of a few different battery options.

If all of this sounds confusing, here’s a more thorough tutorial.


Materials!


Here’s a Wish List that includes all the necessary components for this project!

Microcontroller and Accessory Components

Particle Photon microcontroller

SparkFun Photon Battery Shield

– One 2000 mAh Polymer Lithium Ion Battery

Surface Mount DC Barrel Jack

Barrel jack to USB power supply cable

One (1) Lamp Switch

– Optional: Male-to-Female JST connector cable

Gas Sensor Circuit

One (1) Project Case

– One (1) 4 AA battery case

– Four (4) AA Rechargeable Batteries

One (1) Toggle Switch (SPST switch)

Piezo Buzzer

Three (3) Red LEDs

– Three (3) 10 kΩ resistors

One (1) PCB

22 Gauge stranded wire

– Optional: Electrical connectors (3-5)

LPG (MQ6) Gas Sensor

MQ6 LPG Gas Sensor

Gas Sensor Breakout Board

– One (1) 4.7 kΩ resistor

– One (1) 5V Voltage Regulator

Methane (MQ4) Gas Sensor

MQ4 Methane Gas Sensor

Gas Sensor Breakout Board

– One (1) 4.7 kΩ resistor

– One (1) 5V Voltage Regulator

Carbon Monoxide (MQ7) Gas Sensor

MQ7 CO Gas Sensor

Gas Sensor Breakout Board

– One (1) 4.7 kΩ resistor

– One (1) 5V Voltage Regulator

– One (1) 5V SPDT Relay

– One (1) N-Channel MOSFET

– One (1) 10 kΩ potentiometer

– One (1) 10 kΩ resistor


Tools!


– Soldering Iron

– Wire cutters/strippers

– Drill

– Screwdriver

– Epoxy (or hot glue)


Build it! Electronics


1. Solder gas sensor breakout boards to gas sensors. Orientation doesn’t matter, just be sure that the silkscreen (aka labels) are facing down so that you can read them (had to learn that one the hard way..). Solder wires to the gas sensor breakout board.

2. Solder three voltage regulators to the PCB board. For each regulator, connect positive battery output to the regulator input, and connect middle voltage regulator pin to ground.

3. Connect the LPG (MQ6) and Methane (MQ4) sensors.

For each sensor:

  1. Connect H1 and A1 to the output of one of the voltage regulators (recommended to use an electrical connector).
  2. Connect GND to ground.
  3. Connect B1 to Photon analog pin (LPG goes to A0, Methane to A1)
  4. Connect a 4.7 kΩ resistor from B1 to ground.

4. Connect the CO (MQ7) gas sensor.

*Aside: The MQ7 sensor requires cycling the heater voltage (H1) between 1.5V (for 90s) and 5V (for 60s). One way to do this is to use a relay triggered by the Photon (with the aid of a MOSFET and potentiometer) — when the relay is not powered, the voltage across H1 is 5V, and when the relay is powered the voltage across H1 is ~ 1.5V.

  1. Connect GND to ground.
  2. Connect B1 to Photon analog pin (A2). Connect 4.7 kΩ resistor from B1 to ground.
  3. Connect A1 to third voltage regulator output (5V source).
  4. Connect Photon 3.3V pin to positive relay input.
  5. Connect Photon Digital Pin D7 to left MOSFET pin, and a 10 kΩ resistor to ground.
  6. Connect middle MOSFET pin to relay ground pin. Connect right MOSFET pin to ground.
  7. Connect relay Normally Open (“NO”) pin to H1, and the Normally Closed (“NC”) pin to middle potentiometer pin.
  8. Connect right potentiometer pin to ground, and left pin to H1.
  9. Adjust potentiometer resistance until it changes the relay output to ~ 1.5V when the relay receives power.

5. Connect an LED and 10 kΩ resistor to each of the Photon digital pins D0, D1, and D2. Connect buzzer to Photon digital pin D4.


6. Connect toggle switch between battery pack and PCB board power. Recommended to include an electrical connector for the battery pack to make it easier to switch out batteries.


7. Connect lamp switch between LIB and Photon battery shield — recommended to use an extra JST cable for this to keep the LIB battery cable in tact (and make it easier to install the lamp switch).

8. Label wires!


Build a Case!


1. Drill hole for toggle switch on case lid.

2. Drill 3 holes in the case lid for the LED lights to shine through, and 3 holes for the gas sensors to have air contact. Adhere components on the inside of the lid.

3. Drill hole in the side of the case for barrel jack USB cord to connect to the Photon Battery Shield.

4. Drill two small holes on the side of the case for the lamp switch cable. Adhere lamp switch to side of case.

5. Label the LEDs with its corresponding gas sensor on the outside of the case.

6. Check electrical connections and, if everything is good to go, coat electrical connections in epoxy or hot glue.


Calculate Gas Sensor PPM!


Each of the gas sensors outputs an analog value from 0 to 4095. To convert this value into voltage, use the following equation:

Sensor Voltage = AnalogReading * 3.3V / 4095

Once you have the sensor voltage, you can convert that into a parts per million (“PPM”) reading using the sensitivity calibration curve on page 5 of the gas sensor datasheets. To do this, recreate the sensitivity curve by picking data points from the graph or using a graphical analysis software like Engauge Digitizer .

Plot PPM on the y-axis and V_RL on the x-axis, where V_RL is the sensor voltage. There is a lot of room for error with this method, but it will give us enough accuracy to identify dangerous levels of hazardous gases. Estimated error bars are around 20 PPM for the LPG and Methane sensors, and about 5 PPM for the CO sensor.

Next, find an approximate equation for the PPM vs. V_RL curve. I used an exponential fit (e.g. y = e^x) and got the following equations:

LPG sensor: PPM = 26.572*e^(1.2894*V_RL)

Methane sensor: PPM = 10.938*e(1.7742*V_RL)

CO sensor: PPM = 3.027*e^(1.0698*V_RL)


Program it!


First, set up a data stream on the [data.sparkfun.com service](http://data.sparkfun.com). Next, write a program to read in the analog value of each gas sensor, convert it to PPM, and check it against known safe thresholds. Based on OSHA safety standards, the thresholds for the three gases are as follows:

  • LPG: 1,000 PPM
  • Methane: 1,000 PPM
  • CO: 50 PPM

If you want to get up and running quickly, or are new to programming, feel free to use my code! Use it as-is or modify to suit your particular needs.

Here’s the GitHub page!

Here’s the raw program code.

Change the following in the code:

1. Copy and paste your data stream public key to the array called `publicKey[]`.

`const char publicKey[] = “INSERT_PUBLIC_KEY_HERE”;`

2.Copy and paste your data stream private key to the array called `privateKey[]`.

const char privateKey[] = “INSERT_PRIVATE_KEY_HERE”;

To monitor the Photon output, use the Particle driver downloaded as described in the [“Connecting Your Device” Photon tutorial](https://docs.particle.io/guide/getting-started/connect/photon/). Once this is installed, in the command prompt, type `particle serial monitor`. This is super helpful for debugging and checking that the Photon is posting data to the web.


Be a Citizen Scientist!


Now we get to test and employ our gas monitor! Turn the batteries for the gas sensors on using the toggle switch, wait about 3 – 5 minutes, then turn the Photon on with the lamp switch (the gas sensor heater coils take some time to heat up). Check that the Photon is connected to WiFi (on-board LED will slowly pulse light blue) and is uploading data to the server. Also check that the gas sensor readings increase when in proximity to hazardous gases — one easy, and safe, way is to hold a lighter and/or a match close to the sensors.

Once up and running, use the sensor to monitor for dangerous gas leaks around your home, school, workplace, neighborhood, etc. You can install the sensor in one location permanently, or use it to check gas levels in different locations (e.g. SoCal..).

Educator Extension!

This project is a perfect excuse for a hands-on chemistry lesson! Use the monitor to learn the fundamentals of various gases — what kinds of gases are in our environment, how are different gases produced, and what makes some of them hazardous or dangerous.

Study the local environment and use a lil’ math to record and plot LPG, Methane, and CO in specific locations over time to see how the levels change. Use the data to help determine what causes changes in the gas levels and where/when gas concentrations are the highest.

 


More to Explore!


Monitor hazardous gas concentrations around your neighborhood or city and use the results to identify problem areas and improve public safety.

Use Bluetooth, or your smartphone WiFi, to connect to the Photon and upload data to the web wherever you are!

Include other sensors, gaseous or otherwise , to create a more comprehensive environmental monitoring system.