# Measure Resistance with an Ohmmeter

An ohmmeter is a device that measures resistance.  The basic unit of resistance is the ohm, and it’s abbreviated with the uppercase Greek letter omega (Ω).

This activity will guide you through:

• Building a simple micro:bit powered ohmmeter.
• Using the ohmmeter to measure the resistors you experimented with in the Build the LED Circuit activity.
• Practicing an important resistance measurement rule: Individual resistors need to be removed from the circuit to be measured.
• Calculating the equivalent resistance of:
• Two or more resistors connected end-to-end (in series).
• Two or more resistors connected side-by-side (in parallel).

A test circuit cannot have power applied to while measuring resistance. This second, important rule normally applies when using a repair manual, which might direct you to measure the resistance in a much larger circuit that’s part of some device.  The device’s power would have to be turned off for an ohmmeter to take a valid measurement.

## Circuits

In this activity you will have an ohmmeter circuit and an LED circuit. You will use the ohmmeter circuit to measure different resistors used in the LED circuit.

### LED Circuit

Let's once again re-use the setup from First Breadboard Circuit — Build the LED Circuit.

The LED circuit may still be on your board. If not, re-build it now.

### Ohmmeter Parts

You will also need the following additional parts, along with some jumper wires:

### Ohmmeter Circuit

This ohmmeter circuit can measure resistances in the 100 to 10,000 Ω range.

• Use the animation and the instructions below it to create the ohmmeter circuit.
• Optionally, view the full-size measure-resistance-meter-circuit.mp4 clip to play and pause it between steps.

• Plug one 3-pin header into row 29, columns g, h, i.
• Plug the other 3-pin header into the lowest sockets in the center bus strip’s red (+) column.
• Plug a 2000 Ω resistor’s leads into (j, 29) and one of the center bus strip’s blue (-) sockets.
• Use a jumper wire to connect (f, 29) to (e, 29).
• Bend the resistor and jumper wire you just connected to make them sit flat against the breadboard.
• Connect the ends of your alligator clip probes to these 3-pin headers:
• Black to the 3-pin header in (g,h,i, 29)
• Red to the alligator clip in the 3-pin header in the bus strip’s red (+) column.
• Connect the other ends of the alligator clip probes to these sockets on the right terminal strip:
• Black probe to (j, 21).
• Red probe to (a, 21).