#3 - What You Need to Know: Using a Digital Multimeter, Reading Schematics, and Using Ohm's Law

More Essential Tools and Skills

The DMM

The Digital Multimeter (or DMM for short) is a powerful tool for troubleshooting circuits and overall verification of components. It has many uses, such as measuring voltage [In both DC (solid line symbol) and AC (wavy line symbol)], measuring resistance in ohms (O.L. means that there is an open/no connection/no conductance, 0 for short/connectivity/conductance), measuring frequency, checking is a diode is functioning properly, measuring current (though we prefer the use of a VOM - volts ohm meter - since those have a built in circuit breaker; if you use the DMM to measure current, be sure to observe correct polarity), and countless other uses. In other words, it is an invaluable tool.

Schematics and OHM'S LAW

Knowing how to read schematics is an important skill to understand the blueprints for building your circuit. There are various symbols and parts to know, such as resistors, potentiometers, LEDs, switches, relays, transistors, and much more. Below, you can see some built circuits using resistors (cylinder shaped brown component with 4 bands of colors on them), LEDs (the red lights - LED stands for Light-Emitting-Diode), and a potentiometer (in blue with a knob). 

*To read a resistor, use a color chart to learn the values. Then look at the 4 bands: the first two are digits, the third is a multiplier, and the fourth is a tolerance band. So, for example, a Red-Red-Brown-Gold resistor would be 22 (red is 2, and uses the first two digits) times 10 (ten to the power of 1 since brown is 1), which gives us 220. The Gold represents a range of values (in percents) that the manufactured resistor should land between (it won't always be exactly 220).

Next, is Ohm's Law. This is a VERY important formula!

V = I * R

Ohm's law states that the voltage (V, in volts) across a load is the product of the current (I, in amps) flowing through it and the resistance (R, in ohms) of the load. For a constant voltage, raising the resistance lowers the current (current-limiting), and lowering the resistance raises the current. For a constant current, raising the resistance raises the voltage across it. For a fixed resistance, raising the current through a load raises the voltage drop across it. Thus, resistance and current are inversely proportional, while voltage and current are directly proportional (same for voltage and resistance). Remember this! It will help you determine many things, such as which resistor will provide the brightest light (see above pictures). The lowest resistance of the 3 resistors will provide the highest current, and thus the brightest light.

Use this knowledge well! For it is key to your success in electronics!