Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.
Engineers design circuits to serve a purpose that will make our lives easier. Cell phones are very complex direct current (DC) circuits. Let's go through a few DC circuit arrangements designed to do something whether useful or not.
Let's say we need a circuit to make some noise and give off some light at the flip of a switch. Let's make a list of components we will need.
1. Electrical wires
2. SPST (single-pole, single-throw) switch
3. Buzzer
4. Lamp (source of light)
5. Cell (battery)
Diagram 1 shows how to connect these components.
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The SPST switch is the most basic of switches. It is either on (when the switch is closed) or off (when the switch is open). The switch engages the battery by completing the circuit. A circuit that has a break in, it will not work because there is no path for the current to flow through. Diagram 1 shows an open circuit, which means in order to see the light or hear the buzzer you have to close the switch. Let's take a journey from the battery through the circuit to analyze what is happening.
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When the switch closes, the battery starts pushing electrical current through the circuit from the positive side of the battery. It reaches the junction where the current splits with some of it going through the light bulb and some of it goes through the buzzer. The light lights up and the buzzer buzzes. The two currents continue on and reach the other junction and combine back to the original current, which flows towards the battery. The process continues until you open the switch or the battery wears out.
Let's pretend you are going to do a science project where you construct a circuit that detects a magnetic field. You need a setup that, when a magnetic field is detected, a buzzer goes off. You don't expect the buzzer to go off frequently, but you want to know that the circuit is functioning so you include a LED (light-emitting diode) to indicate that the circuit is operational. Let's make a list of the components in your circuit.
1. Electrical wires
2. Reed switch
3. LED
4. Buzzer
5. Cell (battery)
A reed switch has internal magnets that close the switch (completes the circuit) when in a magnetic field, and open the switch in the absence of a magnetic field. When the LED light shines, you know that the buzzer will make noise when in an electric field. Diagram 2 shows how to construct this circuit.
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The reed switch in diagram 2 is closed meaning it must be detecting a magnetic field. The LED stays lit as long as the battery is able to push current through the circuit. When current leaves the positive terminal of the battery one of two things can happen. If the reed switch is open (no magnetic field present) the current goes only through the LED and back to the battery. If the reed switch is closed because a magnetic field is present, the current splits right after leaving the positive terminal of the battery. Some of the current goes through the LED and some goes through the reed switch and buzzer before heading back to the battery. This circuit utilizes a series circuit when no magnetic field is present, and a parallel circuit when a magnetic field is present. The removal of either the buzzer or the LED does not affect the functioning of the other.
A friend of yours wants you to design a circuit that designed around the following criteria:
1. Run a motor that turns a plastic strip causing a loud clicking sound when significant light is coming through a window.
2. Have an adjustment knob to change the speed of the motor.
Why would anyone want a circuit that does this? It is summertime and your friend doesn't want to sleep his summer away so he wants to be woken up when the sun comes up. He wants to be able to adjust the noise the motor and plastic strip make through the use of a variable resistor. Variable resistors use a knob to change its resistance, which changes the amount of current flow. This will allow your friend to adjust the current through the motor before going to sleep. A photoresistor, called a LDR (light-dependent resistor), has a very large resistance when in the dark, but low resistance when light is present. When it detects light it will allow current to flow. We will add a switch to turn off the motor when your friend wakes up. Let's look a the schematic of this circuit in Diagram 3.
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Your friend will push the switch closing it, but current will only leave the positive terminal of the battery when the LDR senses light. When it senses light the motor turns causing the clicking sound. When your friend gets up he can push the button and turn off the motor. All of the resistors in this circuit are in series.
DC circuits can have a variety of resistors and switches to serve different purposes. Switches allow or prevent the battery from push current through the circuit. Reed switches are open in a magnetic field and are closed otherwise. LEDs let us know that a circuit has a functioning power source while LDRs act as a way to control the current through a circuit based on the amount of light present. Parallel circuits allow one resistor to function while another resistor is not functioning. Series circuits only have one current flowing so if one resistor malfunctions, the entire circuit doesn't function. Only your imagination can limit the different types of circuits you can make!
DC circuits can have a variety of resistors and switches to serve different purposes. Switches allow or prevent the battery from push current through the circuit. Reed switches are open in a magnetic field and are closed otherwise. LEDs let us know that a circuit has a functioning power source while LDRs act as a way to control the current through a circuit based on the amount of light present. Parallel circuits allow one resistor to function while another resistor is not functioning. Series circuits only have one current flowing so if one resistor malfunctions, the entire circuit doesn't function. Only your imagination can limit the different types of circuits you can make!
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