Basic LED OFF Time Extender Circuit – B1P4

Let’s take a look at a new type of transistor along with a new component called a capacitor.

Capacitor symbols

This is one type of a capacitor and it has two terminals as shown in the symbol. To understand how it works we will take a look at the following simulation.

Capacitor - simulation

We have a capacitor along with two switches. One switch connects a power source to the capacitor with a current limiting resistor in series. The other switch connects a resistor to the capacitor terminals. We have plotted the current flowing through the capacitor and the capacitor voltage for analysis.

Capacitor charging - simulation

When the first switch is closed, there is a sharp rise in the capacitor voltage and this reaches a steady value with time. The capacitor is said to be charging. The current flowing through the capacitor follows an inverse trend compared to the voltage and it gradually reduces to 0.

Capacitor steady - simulation

If the switch is now opened or disconnected, then you will notice that the capacitor holds its charge and this will continue for a long time.

Capacitor discharging - simulation

By closing the second switch, we place a resistor across the capacitor and this causes the voltage across the capacitor to gradually drop to 0. The capacitor is said to be discharging.

Capacitor overview

In summary, capacitors are energy storage components that tend to hold on to their charge. They can also be used to filter out electrical noise. This type of capacitor is called an electrolytic capacitor and it has a positive and negative terminal. Just like an LED, the longer terminal is positive. Electrolytic capacitors generally have the negative terminal marked on the case which can also be used as a reference. Capacitance is measured in Farads and electrolytic capacitors generally have values in the uF range. They also have a voltage rating and applying a voltage greater than this will damage them.

Let’s quickly recap what we know about transistors. We know that transistors can be used as a switch or to amplify current and we know that we can do this by applying a positive voltage between the base-emitter terminals. This will result in current flowing between the collector-emitter terminals.

NPN transistor

This type of transistor is called an NPN transistor and you can remember this by assigning the P in the middle of NPN to the positive voltage that needs to be applied to the base. Also, the arrow in the symbol points outwards to help you remember that the base is positive with respect to the emitter.

PNP transistor

There is a transistor that is switched ON when the base is negative with respect to the emitter. The symbol for it is shown above and it is called a PNP transistor. Again, you can remember this by assigning the N in the middle of PNP to the negative base-emitter voltage that needs to be applied. Also, the arrow points inwards indicating that the base is negative with respect to the emitter. For a PNP transistor, the base needs to be lower than the emitter by at least 0.6-0.7V for the transistor to switch ON.

Potential difference

One thing to keep in mind is that potential difference or voltage is relative between two points. For a battery, it is the potential difference between the positive and negative terminals. When we talk about the base-emitter voltage, we are talking about the voltage at the base terminal with respect to that at the emitter.

Basic transistor circuit - schematic

Now that we have this information, we can take a look at the circuit diagram. The rightmost section should be familiar, we simply use an NPN transistor to control an LED. The LED will switch ON when the transistor is switched ON and this will only happen when the base is positive compared to the emitter terminal. Q2 is a PNP transistor and we know that the base has to be negative compared to the emitter for it to switch ON. The collector of Q2 is connected to the base of Q1 using a current limiting resistor and this means that the output of Q2 is the input for Q1.

To summarize, the LED will turn ON only when Q1 is ON. Q1 will turn ON, only when Q2 is ON. And Q2 will turn ON when it’s base terminal has a lower voltage compared to its emitter terminal.

If you look at Q2, the emitter is connected to the positive battery terminal which is at 3V. When the switch S1 is closed, resistors R3 and R4 form a voltage divider network with the common terminal being directly connected to the base of Q2. Since the base voltage is lower than the emitter, it will cause Q2 to switch ON, which will switch Q1 ON, and this will ultimately switch ON the LED.

LED OFF time extension simulation - closed

Let’s take a look at the simulation above to understand what’s the use of the capacitor in this circuit. When the switch is open, the capacitor voltage is 0, the transistors are switched OFF along with the LED. When the switch is closed, the capacitor starts to charge to the source voltage.

LED OFF time extension simulation - opened

When the switch is opened, the current which was previously flowing through the voltage divider circuit no longer has a return path to the battery and this current flows through the capacitor instead. The capacitor discharges over time causing its voltage to drop and eventually, the transistors and LED switch OFF.

Pull-up resistor

When the capacitor is fully discharged, it is as good as being non-existent as far as the circuit is concerned. Since the capacitor is no longer relevant and since the switch is disconnected, R3 is no longer relevant as well. R4 is connected between the positive supply voltage and the base and this causes the voltage at the base of Q2 to be the same as that at its emitter – which results in Q2 switching OFF. R4 is said to act as a pull-up resistor since it is pulling up the voltage at the base of Q2.

Basic transistor circuit - breadboard layout

Let’s build the circuit using the breadboard layout.

Basic transistor circuit - demo

Pressing the switch causes the LED to turn ON instantly, whereas releasing the switch causes it to slowly fade away. We can change this time by changing the capacitor value – a higher capacitor value would give us a higher OFF-time extension.

Take a moment to soak up all of this new information as we will be using it to build the future circuits. I’ll see you in the next one!