This e-book contains 100 transistor circuits. The second part of this e-book will contain a further 100 circuits. Most of them can be made with components from your “junk box” and hopefully you l-C High-Pass Filter Reduces Power Supply’s Hum And Ripple put them together in less than an hour. The idea of this book is to get you into the fun of putting things together and there’s nothing more rewarding than seeing something work.
It’s amazing what you can do with a few transistors and some additional components. And this is the place to start. Most of the circuits are “stand-alone” and produce a result with as little as 5 parts. We have even provided a simple way to produce your own speaker transformer by winding turns on a piece of ferrite rod. Many components can be obtained from transistor radios, toys and other pieces of discarded equipment you will find all over the place. To save space we have not provided lengthy explanations of how the circuits work. Transistor data is at the bottom of this page and a transistor tester circuit is also provided.
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There are lots of categories and I am sure many of the circuits will be new to you, because some of them have been designed recently by me. Basically there are two types of transistor: PNP and NPN. We have labelled the NPN transistor as BC547. This means you can use ANY NPN transistor, such as 2N2222, BC108, 2N3704, BC337 and hundreds of others. Some circuits use TUN for Transistor Universal NPN and this is the same as our reasoning – the transistor-type is just to let you know it is not critical. BC557 can be replaced by: 2N3906, BC327 and many others.
Don’t worry too much about the transistor-type. Just make sure it is NPN, it this is the type needed. If it is an unknown transistor-type, you need to identify the leads then put it in the circuit. The choice is up to you but the idea is to keep the cost to a minimum – so don’t buy anything expensive. This way they can be re-used again and again. No matter what you do, I know you will be keen to hear some of the “noisy” circuits in operation.
If you are starting in electronics, see the World’s Simplest Circuit. It shows how a transistor works and three transistors in the 8 Million Gain project will detect microscopic levels of static electricity! You can look through the Index but the names of the projects don’t give you a full description of what they do. You need to look at the circuits. And I am sure you will.
Talking Electronics supplies a kit of parts that can be used to build the majority of the circuits in this book. 11 x 15 hole, 6 x 40 hole, surface-mount 6 x 40 hole board or others. There are more components than you think. The 8 little components are switches and the LDR and flashing LED is hiding. In many cases, a resistor or capacitor not in the kit, can be created by putting two resistors or capacitors in series or parallel or the next higher or lower value can be used.
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Don’t think transistor technology is obsolete. Many complex circuits have one or more transistors to act as buffers, amplifiers or to connect one block to another. It is absolutely essential to understand this area of electronics if you want to carry out design-work or build a simple circuit to carry out a task. The first thing you will want to know is: HOW DOES A TRANSISTOR WORK? Diagram “A” shows an NPN transistor with the legs covering the symbol showing the name for each lead. The transistor is a “general purpose” type and and is the smallest and cheapest type you can get. The number on the transistor will change according to the country where the circuit was designed but the types we refer to are all the SAME.
Diagram “B” shows two different “general purpose” transistors and the different pinouts. You need to refer to data sheets or test the transistor to find the correct pinout. Diagram “C” shows the equivalent of a transistor as a water valve. Diagram “D” shows the transistor connected to the power rails. The collector connects to a resistor called a LOAD and the emitter connects to the 0v rail or earth or “ground. Diagram “E” shows the transistor in SELF BIAS mode.
This is called a COMMON EMITTER stage and the resistance of the BASE BIAS RESISTOR is selected so the voltage on the collector is half-rail voltage. In this case it is 2. To keep the theory simple, here’s how you do it. Use 22k as the load resistance. Select the base bias resistor until the measured voltage on the collector 2.
The base bias will be about 2M2. The base bias resistor feeds a small current into the base and this makes the transistor turn on and create a current-flow though the collector-emitter leads. This causes the same current to flow through the load resistor and a voltage-drop is created across this resistor. This lowers the voltage on the collector. The lower voltage causes a lower current to flow into the base and the transistor stops turning on a slight amount.
The transistor very quickly settles down to allowing a certain current to flow through the collector-emitter and produce a voltage at the collector that is just sufficient to allow the right amount of current to enter the base. Diagram “F” shows the transistor being turned on via a finger. Press hard on the two wires and the LED will illuminate brighter. As you press harder, the resistance of your finger decreases. This allows more current to flow into the base and the transistor turns on harder. Diagram “G” shows a second transistor to “amplify the effect of your finger” and the LED illuminates about 100 times brighter. Diagram “H” shows the effect of putting a capacitor on the base lead.
The capacitor must be uncharged and when you apply pressure, the LED will flash brightly then go off. This is because the capacitor gets charged when you touch the wires. As soon as it is charged NO MORE CURRENT flows though it. The first transistor stops receiving current and the circuit does not keep the LED illuminated. To get the circuit to work again, the capacitor must be discharged. Diagram “I” shows the effect of putting a capacitor on the output.
It must be uncharged for this effect to work. We know from Diagram G that the circuit will stay on when the wires are touched but when a capacitor is placed in the output, it gets charged when the circuit turns ON and only allows the LED to flash. This is a simple explanation of how a transistor works. It amplifies the current going into the base about 100 times and the higher current flowing through the collector-emitter leads will illuminate a LED. A capacitor allows current to flow through it until it gets charged. It must be discharged to see the effect again. You can change the voltage of many circuits from 6v to 12v or 3v to 6v without altering any of the values.
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Work out the current entering each load and see if it is less than the maximum allowable. Then, take a current reading on the lower voltage. Increase the voltage to the higher value and take another reading. If any LEDs are taking excessive current, double the value of the current-limiting resistor.
If any transistor is getting hot, increase the value of the load resistor. In most cases, when the voltage is doubled, the current will will crease to double the original. This means the circuit will consume 4 times the original energy. This is just a broad suggestion to answer the hundreds of emails I get on this topic. 4 watt resistors unless specified on the diagram. When working on any project that connects to the “mains,” it is important to take all precautions to prevent electrocution. This project provides 240v AC but the current it limited to 60mA if a 15 watt transformer is used.
Although the output can produce a nasty shock and the voltage will kill you, the circuit provides isolation from the mains and if a short-circuit occurs, it will not blow a fuse, but the transformers will get very hot as start to buzz. You can use any two identical transformers and the wattage of either transformer will determine the maximum output wattage. If you don’t use identical transformers, the output voltage will be higher or lower than the “mains” voltage and the wattage will be determined by the smaller transformer. This arrangement is not perfectly safe, but is the best you can get when working on projects such as switch-mode power supplies, capacitor-fed down-lights etc. This simple circuit tests the capacity of a rechargeable cell.
Set the hands to 12 O’Clock and the clock will let you know how long the cell lasted until the voltage reached about 0. Now fit another cell and see how long it lasts. You cannot work out the exact capacity of a cell but you can compare one cell with another. The initial current is about 250mA for a 1.
This circuit tests the capacity of a rechargeable cell. You cannot get 6amp-hr capacity into 30gms! But what is the capacity of the cell? Our cell had less than 0.
The resistors and diodes are simply easy-to-get components that make up a load to discharge the cell at about 600mA. This is a load of about 2. 4watts and the resistors and diodes add up to a capability to dissipate 2. The voltage of the cell drops from 4v to 2v and the current drops too. You can measure the voltage across the resistors and work out the current-flow. Don’t fit an ammeter as the voltage -drops across it will reduce the current considerably. The buzzer starts to buzz when the voltage drops to 2v.
You will need to adjust the value of the biasing resistors to reproduce this value if you are using a different transistor as the detection voltage can change by as much as 200mV with different makes of BC547. Our cell lasted less than 2 hours and was obviously FAKE. Some cells have a much smaller cell inside and that’s why they weight only 28grams ! A Li-Ion cell must be charged on a charger that cuts off when the voltage reaches about 4. If you charge a cell from a variable power supply and do not monitor the terminal voltage of the cell, it will rise to over 5. This circuit indicates when a fuse is “blown. This circuit indicates when the soil is dry and the plant needs watering.
The circuit does not have a current-limiting resistor because the base resistor is very high and the current through the transistor is only 2mA. Don’t change the supply voltage or the 220k as these two values are correct for this circuit. This will clear-up a lot of mysteries of the solar panel. Many solar panels produce 16v – 18v when lightly loaded, while other 12v solar panels will not charge a 12v battery. Some panels say “nominal voltage,” some do not give any value other than 6v or 12v, and some specify the wrong voltage. You can’t work with vague specifications.
You need to know accurate details to charge a battery from a solar panel. There are 3 things you have to know before buying a panel or connecting a panel to a battery. The voltage of the panel when delivering the rated current. The Unloaded Voltage is the voltage produced by the panel when it is lightly loaded. This voltage is very important because a 12v battery will produce a “floating voltage” of about 15v when it is fully charged and it will gradually rise to this voltage during the charging period.
This means the panel must be able to deliver more than 15v so it will charge a 12v battery. Sometimes there is a diode and a charging circuit between the panel and battery and these devices will drop a small voltage, so the panel must produce a voltage high enough to allow for them. The Unloaded Voltage can sometimes be determined by counting the number of cells on the panel as each cell will produce 0. If you cannot see the individual cells, use a multimeter to read the voltage under good illumination and watch the voltage rise.
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You can place a 100 ohm resistor across the panel to take readings. The RATED VOLTAGE is the guaranteed voltage the panel will deliver when full current is flowing. This can also be called the Nominal Voltage, however don’t take anything for certain. This may occur for only a very small portion of the day. You can clearly see the 11 cells of this panel and it produces 6. It will barely produce 6v when loaded and this is NOT ENOUGH to charge a 6v battery. This panel claims to be 18v, but it clearly only produces 14.
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This is not suitable for charging a 12v battery. When you add a protection diode, the output voltage will be 13. A flat battery being charged will reach 13. 8v very quickly and it will not be charged any further. That’s why the output voltage of a panel is so important. The panel needs to produce 17v to 18v so it will have a small “overhead” voltage when the battery reaches 14. 4v and it will still be able to supply energy into the battery to complete the charging process.
The Rated Current is the maximum current the panel will produce when receiving full sunlight. The current of a panel can be worked out by knowing the wattage and dividing by the unloaded voltage. A 20 watt 18v panel will deliver about 1 amp. A solar panel can be used to directly charge a battery without any other components. The voltage of the panel does not matter and the voltage of the battery does not matter.
The output voltage of the panel will simply adapt to the voltage of the battery. Even though there is a voltage mismatch, there is NO “lost” or wasted energy. An 18v panel “drives into” a 12v battery with the maximum current it can produce when the intensity of the sun is a maximum. But here’s the important point: To prevent overcharging the battery, the wattage of the panel is important. To prevent overcharging a battery, the charging current should not be more than one-tenth its amp-hr capacity. This is called its 14-hour rate. But this rating is a CONSTANT RATING and since a solar panel produces an output for about 8 hours per day, you can increase the charging current to 330mA for 8 hours.
This will deliver the energy to fully charge the cells. 2AHr battery, the charging current will be 100mA for 12 hours or 330mA for 4 hours and a regulator circuit will be needed to prevent overcharging. 5AHr battery, the charging current will be 375mA for 12 hours and a larger panel will be needed. If the diode is Schottky, the voltage-drop is 0. Some panels include this diode – called a BYPASS DIODE.