Chapter 3 · Book pp. 41–56
Using a Digital Multimeter
การใช้งานดิจิทัลมัลติมิเตอร์เบื้องต้น
A digital multimeter (DMM) is the first tool every electronics learner should own. It is cheap now, it measures many things, and it shows the answer as a clear number. This chapter shows you how to use a real DMM, then the test instruments built into Tinkercad.
3.1 Getting to know it รู้จักกันก่อน
A DMM comes with two test leads (probes). The red lead goes into the positive socket and the black lead into the negative socket, marked COM.
| ✅ Advantages | ⚠️ Disadvantages |
|---|---|
| You read the value directly — no unit conversion and no scale to compare. | It runs on batteries. A flat or weak battery can give wrong readings. (A needle meter can measure voltage without a battery.) |
| Fewer human reading errors than with a needle meter. | If the signal changes quickly, the digits keep jumping and it is hard to tell which value is right. |
| An auto-ranging meter picks the range for you, so it is safer when you do not know the voltage — less chance of damaging the meter. | Trends are harder to see. A needle shows a value swinging; with digits you have to note the numbers and picture the change in your head. |
⚠️ Sharp probe tips
Some leads have plastic caps over the tips to stop you getting hurt. Take the caps off before you measure, and put them back when you have finished.
Fit the batteries before first use. The meter here takes two 1.5 V AAA alkaline cells: undo the screw on the back, lift the cover, and put the cells in the right way round. Some meters use a clip or a sliding cover instead.
3.2 Getting started เริ่มต้นใช้งาน
3.2.1 Measuring voltage
Turn the selector to the voltage setting (Fig 3-4). This meter has two voltage positions: normal volts (V) and millivolts (mV). Both measure DC and AC — press SELECT to switch between them.
3.2.2 Measuring resistance
- Turn the selector to Ω.
- Check the leads first: touch the two tips together. The reading should be close to 0 Ω.
- Measure the resistor. The value should fall within its tolerance. If it is far from the colour code, check the legs and look for burn marks — the part may be damaged or worn out.
3.2.3 Using the resistance mode to test parts
3.2.3.1 Thermistor (temperature-dependent resistor)
Fig 3-6 measures an NTC (Negative Temperature Coefficient) thermistor. Its resistance goes the opposite way to the temperature around it, which makes it an easy temperature sensor.
3.2.3.2 LDR (light-dependent resistor)
Measure an LDR in the light, then cover it with paper so no light reaches it. The resistance goes up. That makes an LDR a simple light switch.
3.2.3.3 Testing a diode
The resistance position also has two extra modes: diode test and capacitor test.
- Set the switch to Ω and press SELECT until the diode symbol appears.
- Red lead on the anode, black lead on the cathode. A small current flows through the diode (forward bias), and the meter shows the voltage drop — about 0.6 V is normal.
- Swap the leads (reverse bias). No current flows, so a good diode shows OL.
- If it reads as a short circuit both ways, the diode is faulty.
3.2.3.4 Measuring a capacitor
Set the switch to Ω and press SELECT until the unit shows nF. Then measure the capacitor and compare with the value printed on it.
3.3 Measuring frequency การวัดความถี่
To test this you need something that makes a frequency: an astable multivibrator built with the LM555 IC (more about the 555 in Chapter 5). An online calculator works out the frequency for you.
🛠️ Lab 3.1 · Calculate, build and measure a 555 frequency
- On a computer with internet, open a browser and go to the 555 astable calculator at ohmslawcalculator.com/555-astable-calculator (Fig 3-10).
- Enter the resistors and capacitor and calculate. With R1 = R2 = 10 kΩ and C = 0.047 µF you get 1.023 kHz.
- Build the real circuit (Fig 3-11). Turn the meter to Hz, put the red lead on the output (pin 3 of the LM555) and the black lead on ground. The meter shows about 1 kHz, close to the calculation.
- Now try a low frequency: change C from 0.047 µF to 10 µF. The calculator gives 4.81 Hz (Fig 3-12).
- Swap the capacitor in the real circuit and measure again. The meter reads close to the calculated value (Fig 3-13).
3.4 Measuring current การวัดกระแสไฟฟ้า
Fig 3-14 measures the current through an LED with a 220 Ω resistor in series. The reading is 11 mA.
⚠️ Move the red lead — and move it back
Before measuring current, move the red lead to the current socket (A) and connect the meter in series. When you finish, move it straight back to VΩHz. If you measure voltage with the lead still in the A socket, the fuse inside blows at once and the meter stops working until it is repaired.
Simulate the same circuit in Tinkercad and you get the same kind of result.
3.5 Test and measurement tools in Tinkercad เครื่องมือวัดในโปรแกรม
Tinkercad has 4 main instruments. Choose Components > All and scroll down to the Instruments heading:
- Multimeter
- Power Supply
- Function Generator
- Oscilloscope
Click the one you want and drag it onto the workspace. To resize it, roll the mouse wheel: roll up to make it bigger, roll down to make it smaller.
3.5.1 Digital multimeter
Choose the mode first: Amperage (current), Voltage or Resistance. Then wire it up to suit that mode.
| Mode | How to connect | Example (Fig 3-19) |
|---|---|---|
| 1 · Amperage | In series in the circuit | LED + 220 Ω on 3 × AA (4.5 V): 11.3 mA |
| 2 · Voltage | Across the points to measure | Across the 3 × AA supply: about 4.48 V |
| 3 · Resistance | Across the part’s legs, with no power | A 1 kΩ resistor: 1.00 kΩ |
3.5.2 Digital power supply
A DC power supply adjustable from 0 to 30 V, giving up to 5 A. It shows the voltage and current as numbers, has separate knobs for each, an on/off switch, and output terminals for the DC supply.
3.5.3 Function generator
- Three output waveforms: sine, square and triangle.
- Frequency from 1 Hz to 1 MHz, shown in Hz to 2 decimal places.
- Amplitude from 0 to 10 Vp-p, shown in volts.
- Offset from −5 to 5 V, shown in volts.
- One output: red for the signal, black for ground, plus an on/off switch.
Measuring instruments matter a great deal in learning electronics. Checking values in a circuit helps you understand how it really works — and leads on to designing your own projects.
Which setting, which socket?
| To measure | Selector | Red lead | Connect |
|---|---|---|---|
| Voltage (DC / AC) | V (SELECT for DC/AC) or mV | VΩHz | Across the part |
| Resistance | Ω | VΩHz | Across the part, power off |
| Diode | Ω, SELECT to the diode symbol | VΩHz | Red to anode: ≈0.6 V; reversed: OL |
| Capacitance | Ω, SELECT to nF | VΩHz | Across the capacitor |
| Frequency | Hz | VΩHz | Red to the signal, black to ground |
| Current (mA) | mA | VΩHz (mA) | In series |
| Current (to 10 A) | A | 10A socket | In series — move it back after! |
🛠️ Activity · Measure it in Tinkercad
Repeat the chapter’s measurements with Tinkercad’s instruments.
- Current (Fig 3-15): 3 × AA cells → 220 Ω → LED, with a multimeter in Amperage mode in series. Record the current.
- Voltage (Fig 3-19): add a second multimeter in Voltage mode across the battery pack. Record it.
- Resistance (Fig 3-19): a third multimeter in Resistance mode across a 1 kΩ resistor that is not powered. Then change the resistor to 330 Ω and 4.7 kΩ.
- Change the resistor: replace 220 Ω with 470 Ω in step 1. Predict, then measure: does the current go up or down?
- Power supply (Fig 3-20): replace the batteries with the power supply set to 4.5 V and compare the readings.
- Function generator (Fig 3-21): place one, set 1 kHz square wave, 5 V amplitude and 2.5 V offset, to match Fig 3-21.
Check yourself
Chapter 3 of the book has worked measurements but no questions, so these are my own, based on its pages.