Solar Engine/Power PCB Kit – B1P25

Solar Power Kit

Let’s take a look at the solar power kit which is very interesting.

Solar Power Kit

A solar panel converts light into electricity and the output of a solar panel depends on the amount of sunlight.

Solar Power Kit

The solar panel provided is a 5V, 1.25W panel which means that the output voltage would be 5V when sufficient light shines on it. You could connect an LED or electronic device directly to the solar panel and it would work fine as long as it does not consume for than 1.25W and as long as there is constant light shining on the panel.

Solar Power Kit

The reality is that it is not easy to get a constant source of light from the sun and that is why we use some sort of a storage element. This is the theory behind such a setup. When the sun shines on the panel, it will generate a voltage and current that will charge the storage element. This charge will then supply the load with energy if there is drop-in sunlight which causes a drop in the output of the solar panel.

Solar Power Kit

The kit contains a supercapacitor which can be used as a storage element. It has a value of 1 Farad and with a maximum voltage rating of 5.5V. All the capacitors we have been using so far had a maximum value in the uF range, and 1F is 10^6 uF, that should give you an idea of how much storage a supercapacitor has.

Solar Power Kit

This is what the schematic looks like. We have a screw terminal for the solar panel and another one for the load or output. The supercapacitor is directly connected in parallel with the solar panel.

The supercapacitor will start charging from 0 and as the voltage rises it will cause an increase in the current flowing through R3, which will eventually switch ON T1 & T2. The number of diodes present determine the switch ON voltage. You can also replace these with LEDs for a higher switch ON voltage. When T2 switches ON, the negative output terminal will be connected to ground switching ON the load.

When T1 switches ON, it will lower the base voltage of T3 and this will switch it ON. This injects voltage into the junction between R4 & R5 causing T1 & T2 to latch ON. As the supercapacitor starts discharging, the voltage across the rails will decrease and this will also decrease the voltage at the base of T3 and it will eventually switch OFF. T3 was responsible for latching T1 & T2 and these two transistors will also switch OFF, ultimately causing the output to switch OFF.

This way, we have control over the turn-ON and turn-OFF voltages.

Solar Power Kit

Let’s assemble and test the circuit. Make sure you mount the supercapacitor with the correct polarity.

Solar Power Kit - Demo

Let’s connect an LED to the output and let’s wait for the supercapacitor to charge. The turn ON and turn OFF voltages are shown on screen.

Solar Power Kit - Demo

I’ve modified the circuit to power an Arduino microcontroller – a Piksey Pico in this case.

Solar Power Kit - Demo

Here’s what the updated circuit looks like. You can increase the run time of the circuit by adding more supercapacitors in parallel. This way, you can drive even heavier loads like motors to create a solar-powered car or wind chime.

That’s it for this video, we will quickly recap some of the main components in the next few videos.

Transistor Siren PCB Kit – B1P18

The Kickstarter edition of BBox 1 contains two addition PCB kits and we will look at the first one in this video.

Transistor Siren Kit

The first kit is a PCB siren, which is the same as project 18 so we will not be covering the theory again. You should have received a document which explains the working of the circuit as well.

Transistor Siren Kit

To assemble the kit, you will need a soldering iron and some solder with flux core at the very minimum. If you’ve never soldered before then please make sure you tin the tip of the iron.


You can do this by wrapping some solder around the tip when cold and then switching it ON while still applying solder as it melts. Also, make sure you heat both the component lead as well as the PCB pad for best results.

Transistor Siren Kit

Let’s quickly assemble the transistor siren circuit. Please ensure you mount the electrolytic capacitors with the right polarity.

Transistor Siren Kit

You need to connect the battery to the screw terminals shown here and the speaker to the other terminals. I’m using a 9V battery here. When you press the switch, the siren should rise and it should fall when you let go of the switch.

Let’s build the other solar power kit in the next post.

Universal Logic Gates (NOR, NAND) – B1P24

We’ve learnt about the basic logic gates in the previous video and we’re now going to use them to build two new gates – the NOR gate and the NAND gate.

Logic gates

These two are commonly referred to as universal logic gates as they can be used to build numerous digital circuits including digital memory.

NOR gate

The output of a NOR gate is the opposite of an OR gate. You can think of this as an OR gate, followed by a NOT gate.

NOR gate

Here’s what the transistor representation of a NOR gate looks like. We have the two inputs and an LED connected to the output. When both the inputs are 0, the transistors will be switched OFF. R2 will provide the necessary path for the current which causes the LED to switch ON. If any of the inputs is 1, then it will cause the corresponding transistor to switch ON which will force the LED to switch OFF.

NOR gate

NOR gate

Let’s build and verify the truth table.

NAND gate

Similarly, you can think of a NAND gate as an AND gate, followed by a NOT gate. The truth table for a NAND gate is given above and it is also the opposite of an AND gate.

NAND gate

Here’s what the transistor equivalent looks like. If both the inputs are 1, then both the transistors will switch ON, which will pull the output to ground, switching OFF the LED.

NAND gate

NAND gate

Let’s build and verify the truth table.

This post concludes all the projects for BBox 1. The Kickstarter edition contains two additional PCBs kits and we will look at these in the next posts, which will then be followed by a quick recap of the main components.

Basic Logic Gates (NOT, OR, AND) – B1P23

It’s time to learn about logic gates which make up most of the electronics that we see around us.

Logic gates

A logic gate is the building block of digital electronics and it generally has one output, with one or more inputs. We will be looking at 3 basic logic gates in this video.

NOT gate

The first is a NOT gate. This is the symbol for a NOT gate and it has a single input and single output. When working with logic gates, it is common to create something called a truth table that lists out all the possible input combinations along with the corresponding output value. Since we only have one input, it can either be 0 or 1. When the input is 0, the output will be a 1. When the input is 1, the output will be 0. In other words, the NOT gate is said to be an inverter as it inverts the input signal.

NOT gate

Here’s how you can build a NOT gate using a transistor. It is simply a transistor switch that we have used before. This is the input and we have connected an LED to the output. When the input is connected to the positive terminal, it is said to be logic 1. The transistor switches ON, which causes the LED to switch OFF. When the input is connected to the negative terminal or logic 0, the transistor is switched OFF, and the pull-up resistor causes the LED to switch ON.

NOT gate

Let’s use the breadboard layout for the NOT gate to build the circuit and verify its operation.

NOT gate

The output is 0 when the input is 1 and the output is a 1 when the input is 0.

OR gate

Let’s now talk about the OR gate. The OR gate has one output with a minimum of two inputs (A & B). Each of the inputs could be 1 or 0, which gives us 4 possible combinations as shown here. The output will be 1 when either or both the inputs are 1. The output will be 0 when both the inputs are 0.

OR gate

Here’s what the transistor equivalent looks like. These are the two inputs and we have connected an LED to the output. It’s easy to see that applying logic 1 to either of the inputs will cause the corresponding transistor to switch ON. This will cause current to flow through R6 which will create a voltage drop that will switch on the LED.

OR gate

Let’s build the circuit and verify its operation.

OR gate

The last logic gate for this video is called the AND gate.

AND gate

It has one output and at least two inputs. The output of the AND gate would be 1 only when both the inputs are 1. In all other cases, the output will be 0.

AND gate

Here’s what the transistor equivalent looks like. It’s easy to see that we need both the transistors to switch ON, in order for current to flow through R9 giving us an output of 1. In all other cases, there isn’t a path for current to flow and the LED will be switched OFF.

AND gate

Let’s build and verify the truth table.

AND gate

These are the basic logic gates that are used to create all sorts of digital circuits. In the next video, we will use these to build universal logic gates that are the building blocks for digital electronics.

Steady Hand Game (Buzzer) – B1P22

We now learn about buzzers and use them to build a simple game.

Buzzer - overview

There are different types of buzzers so we will not be getting into the details of their construction but in short, a buzzer produces sound when voltage is applied to it.

Buzzer - symbol

This is the symbol for a buzzer and applying 3V to the buzzer contained in BBox 1 will produce a sound. The sticker is meant to protect the buzzer from damage during automated assembly but you don’t have to remove it as they are generally loud either way.

Steady hand game - schematic

Here’s what the circuit looks like and you can see that the buzzer is simply placed between the positive terminal and transistor. When the transistor is switched ON, the negative terminal of the buzzer will be connected to the ground, generating a sound.

The rest of the circuit is very simple. We have two probe points and when they come in contact with each other, the transistor switches ON, which then switches ON the LED and buzzer. Technically, you could simply use a switch to control a buzzer but we used a transistor here as we would like to turn this into a game and it’s always better to prevent exposing battery terminals directly from a safety point of view.

Steady hand game - layout

Let’s use the breadboard layout to build and test the circuit.

buzzer circuit - demo

You can use this circuit to form a simple game by using two pieces of conductive metal wire.

Steady hand game - wires

One forms a loop while the other forms a unique shape or pattern, like this. They are both connected to the individual probe points and the buzzer sounds when they come in contact.

Steady hand game - demo

The goal is to navigate the pattern without touching the two wires. You can vary the diameter of the loop and change the pattern to increase the difficulty.

Now that you’ve learnt about buzzers, you can add them to the previous alarm circuits to generate a sound when they are triggered.

We will learn about logic gates in the next post.

DC Motors & Speed Control – B1P21


Let’s learn about motors and let’s build a simple speed control circuit.


A motor is a device that converts current to rotary motion. BBox 1 contains a 3V DC motor, which means that applying 3V DC to it will cause it to spin at the rated speed. For DC motors, you can vary the applied voltage to vary the speed – within limits depend on the motor.

Motor parts

This is what the symbol for a motor looks like and as can be seen, it has two terminals. The motor itself is made up of different components as shown above.

Motor magnet

We have the permanent magnets which create a strong magnetic field at the centre.

Motor armature

We then have the part that rotates which is called the armature. It is made up of a number of coils or windings that are ultimately connected to the motor terminals.

Motor brush

The third component is called the commutator which consists of brushes that are connected to the motor terminals.

When DC voltage is applied to the armature, it produces a magnetic field, and this causes the armature to align itself depending on the field of the permanent magnets. In order to cause the armature to rotate, we need to vary this field constantly which would create a varying magnetic field giving rise to rotation.

Motor armature slots

This is the reason why we have these slots on the armature, which come in contact with the commutator brushes. The slots and coil arrangements are designed such that they cause the magnetic field to vary which causes the armature to rotate continuously.

Moto speed control - schematic

This is what the circuit looks like. We have the motor and we have also placed a diode across its terminals. We’ve learnt about this diode in the project about relays, and it is necessary to add this to allow a path for current to flow when the motor is switched OFF. This diode is also called a freewheeling diode.

The transistor we will be using here is slightly different and has a different pin configuration. This transistor can handle higher current compared to the BC547 that we have been using previously. The rest of the circuit operation is pretty straight forward. A trimpot is used to vary the base voltage/current which changes the operating point of the transistor and this controls the amount of current that can flow through the motor, changing its speed.

Moto speed control - layout

Let’s use the breadboard layout to build the circuit. The motor does not have any wires attached to it so you please wrap jumper wires across its terminals. Instead of using a trimpot, you can also use something called Pulse-Width-Modulation or PWM to control the motor speed. We will look at this in BBox 2.

Moto speed control - demo

Let’s move on to the next project.

Heat Detection Using A Thermistor – B1P20

It’s time to learn about a new component called a thermistor.


A thermistor is a type of variable resistor, whose resistance depends upon the temperature. This is the symbol for a thermistor and there are commonly two types of thermistors.

Thermistor PTC vs NTC

If the resistance increases as the temperature increases, then it is said to have a PTC or positive temperature coefficient. On the other hand, if the resistance decreases as the temperature increases, then it is said to have an NTC or negative temperature coefficient. BBox 1 contains an NTC thermistor.

Thermistor - resistance vs temperature

The datasheet gives you the relationship between temperature and resistance and this is usually in the form of a table or a chart.

Thermistor - resistance vs temperature

This means that you can determine the temperature if you know the resistance. The thermistor contained in BBox 1 has a resistance of 10K at 25 degrees Celsius.

Heat detector circuit - schematic

Here’s what the circuit looks like. We have an NPN transistor that controls an LED. The thermistor and trimpot form a voltage divider circuit whose output is fed to the base of the transistor. We can adjust the value of R3 and use the voltage divider formula to set the threshold.

Heat detector circuit - layout

Let’s build the circuit and adjust R3 such that the LED is OFF. I’m using a candle lighter to simulate heat and the LED will start to glow as I hold the flame closer to the thermistor. The LED will continue to glow until the surface temperature of the thermistor drops below the set threshold.

Heat detector circuit - demo

This is how easy it is to build circuits that react to temperature. Let’s move on to the next project.

Bi-Stable LED Circuit – B1P19

In project 6, we built a circuit that alternatively toggled two LEDs and we called this an astable multivibrator circuit as it was a free running circuit and didn’t have a stable state. We’re now going to build a circuit that can toggle two LEDs with the help of switches. This is called a bi-stable multivibrator circuit as it has two stable states – one state being when LED 1 is ON, the other when LED 2 is ON.

Bi-Stable Multivibrator - schematic

This is what the circuit diagram looks like and again, we’ve worked with all the components so let’s move to the simulation.

Bi-Stable Multivibrator - simulation

We have two LEDs that are being controlled by two NPN transistors. This means that the base has to be positive compared to the emitter for them to switch ON. The circuit is symmetric in nature, but because no two transistors are exactly alike, one will switch ON before the other. In this case, Q1 has switched ON, causing LED 1 to switch ON as well.

If we press switch S1, then it will force the base of Q1 to be at ground potential and this will switch Q1 OFF. Current can flow through R1, R5, R7 and this will switch ON Q2, turning ON LED 2. If we press S1 again, there will not be any change to the circuit as Q1 is already OFF and Q2 does not depend on S1. This is the first circuit state.

Now, if we press S2, it will force Q2 to switch OFF and current can flow through R2, R6, R8 causing Q1 to switch ON. This is the second circuit state. Pressing S2 again will not cause any change in the state.

This circuit forms the basis of one of the main components used in digital electronics, which is called a flip-flop or Set-Reset circuit. If we consider LED 1, pressing S1 will cause it to switch OFF or RESET itself, while pressing S2 will cause it to switch ON or SET itself. The converse relationship exists for LED 2.

Bi-Stable Multivibrator - layout

Let’s use the breadboard layout to build and test it. Flip-flops are used to create digital memory elements and this makes a bistable circuit very useful.

Bi-Stable Multivibrator - demo

Transistor Siren Circuit – B1P18

Link to the PCB version of this kit.

We will now learn how to build a two transistor siren that is manually operated with a switch. First, let’s try to understand the basics behind a speaker.


A speaker converts an electrical signal to sound by using a magnet. We already know that when a varying current flow through a coil, it generates a magnetic field. A speaker uses this principle to generate sound.


A speaker has two terminals to which two wires are connected and wound to form a coil – as seen in the centre of the speaker. One end of this coil is connected to a very light material which is called a diaphragm. The other end is suspended into a magnetic field that is created by a permanent magnet. When a varying electric current flows through the coil, it generates its own magnetic field which interacts with the magnetic field produced by the permanent magnet and this causes the diaphragm to move according to the current – producing sound waves. The speaker coil contained in the kit has a low resistance value of about 8 Ohms.

Transistor Siren - Schematic

This is what the circuit looks like and it is made up of two main sections – an RC charging and an oscillator section. Let’s simulate the oscillator section.

Transistor Siren - Simulation

If we apply a voltage at R2, it will cause C2 to start charging and as this voltage builds up, it will cause Q1 to switch ON. When Q1 switches ON, it will cause Q2 to switch ON as well. This will cause the voltage across C2 to reverse its polarity, switching Q1 OFF, which switches Q2 OFF and the cycle repeats when a positive voltage is applied across R2 again.

Transistor Siren - Simulation

The positive voltage applied will determine the rate of charging or oscillation. A higher voltage will increase the oscillation frequency. We’ve applied a constant voltage at R2 which gives us a steady oscillation frequency but a siren has a varying frequency which starts low and increases with time. In order to create this effect, we add the RC charging circuit that is made up of R1 and C1.

Transistor Siren - Simulation

When the switch is pressed, C1 starts charging from 0 and this causes an increase in the oscillator frequency giving us the rise effect for the siren. The voltage reaches its peak when the capacitor is fully charged and when the switch is released, it starts discharging giving us the fall effect for the siren.

Transistor Siren - Layout

Let’s use the breadboard layout to build and test the circuit. This is a very nice project for beginners and if you’ve received the Kickstarter edition of BBox 1, then you will have also received a PCB version of this that you can solder together for something more permanent.

Transistor Siren - Demo

Let’s move on to the next project.

LED Clap Switch Circuit – B1P17

We’re now going to learn how to activate an LED with sound. We will be using a microphone to detect sound, so let’s talk about that first.

Electret Condenser Mic

The microphone we will be using is called an electret condenser microphone. A condenser is another name for a capacitor so the microphone is a capacitor at a very basic level. We have the two capacitor plates as usual but the microphone is designed such that sound waves can change the distance between the capacitor plates, which changes the resulting capacitance.

Electret Condenser Mic

This is the symbol for a MIC and to use it in a circuit, we would need to connect a biasing resistor. In the absence of sound, the output would be a steady DC value. Sound waves will cause an AC signal to appear on top of this DC value, that nature of this AC signal will depend on the sound waves.

Clap switch - circuit

Here’s what the circuit for this project looks like and it’s best we analyse it using the simulation.

Clap switch - circuit simulation

We’ve replaced the microphone with a voltage supply and switch. Capacitor C1 blocks the DC component from the microphone output and only allows the AC components to pass through. Resistors R2 & R3 form a voltage divider which biases Q1 at about 1.16V, meaning it is switched ON. The base-emitter voltage drop for Q1 is 0.46V which means capacitor C2 is charged to about 0.7V. Resistors R5 & R6 form another voltage divider and the voltage across R6 is about 0.17V which is not enough to switch ON Q3.

If look at the top section, then we can see that C3 can charge through Q1, giving rise to the following polarity. This is the idle state of the circuit.

A clap generates a sudden change in the microphone output which is amplified by Q1. This output voltage increases the voltage across R6 and causes Q3 to switch ON. This switches ON the LED and also connects the positive end of C3 to ground which places a negative voltage at the base of Q2, switching it ON. When Q2, switches ON, it causes a high current to flow through R6, which increases the voltage drop across R6 and this forces Q3 to stay ON. Q2 & Q3 are thus latched ON. Capacitor C3 eventually discharges and then starts charging with the opposite polarity. As the voltage rises it will cause Q2 to switch OFF which will switch OFF Q3 as well. C3 will then discharge through Q1 and it will charge with the opposite polarity again, bringing us back to the idle state. Another clap will cause this cycle to repeat.

Clap switch - circuit simulation

Let’s run the simulation and observe these states. Here we have the circuit in the idle state. Toggling the switch will cause Q2 & A3 to switch ON and C3 to reverse its charge. The transistors will eventually switch OFF and C3 will reverse its charge again, bringing us back to the idle state.

Clap switch - circuit layout

Let’s use the breadboard layout to build and test it.

Clap switch - circuit demo

Since a clap generates a very abrupt sound wave, the latching section of this circuit is necessary so as to extend the ON time of the LED. You can vary the bias point of Q1 and Q2 by adjusting the voltage divider resistors to increase or decrease the sensitivity of this circuit.

Let’s move on to the next project.