Light Dependent NPN BJT switch circuit example: NPN 2N3904 Bipolar Junction Transistor BJT indicator LED Nightlight circuit schematic diagram by Electronzap They have the same basic limitations, other than passing current in opposite directions. The NPN 2N3904 and PNP 2N3906 are complimentary pair bipolar junction transistors. Always consult the datasheet for that part number to learn the pin layout, and all other important electrical properties. Keep in mind that different transistor part numbers have a different pin layout and limitations. Not many pictorials are available to follow. Trying to do so just by looking at a schematic diagram only. It is good to find simple example circuits like these, and build them on a breadboard to study them. PNP bipolar junction transistor BJT switch pictorial and schematic diagram by electronzap But, these circuit are an easy introduction to controlling a transistor with 2 voltages (0V= off versus 5V = on), at low currents (through a 10,000 ohm resistor). In a practical circuit, you would not normally use a mechanical switch to control a BJT switch as mechanical switches do just fine switching by themselves. I added a couple example circuits that involve switching an LED on and off with an NPN, and a PNP BJT. Always design circuits as if the minimum gain can be expected. There is a range of gain that it could be based on circuit conditions and individual component variations. At least 0.067mA Base current if the Collector needs to pass 20mA. 300 Gain BJT needs to have the Base pass at least 0.34% as much current as the load is expected to need.At least 0.2mA Base current if the Collector needs to pass 20mA. 100 Gain BJTs need to have the Base pass at least 1% as much current as the load is expected to need.In switch circuits, the load is what limits current. The BJT Collector just needs to conduct better than the load in order to be considered fully on. NPN bipolar junction transistor BJT switch circuit pictorial and schematic diagram by electronzap The same amount of current flows through the load as the collector in a switch circuit.PNP BJT current flows in the opposite direction of NPN.Therefor no current will flow through it either. The Load of a switch circuit is on the collector side of the transistor. Without Base current, there will also be no current flow from Collector to Emitter if using NPN BJTs. The base to Emitter drops about 0.6V from the resistor used to set the Base to Emitter current.And also, the BJT needs to not conduct at all while off. BJT switch circuits, need the transistor to conduct freely compared to the load while on.Exact amount depends on the part number of the transistor. A small amount of Base current allows many times that amount of Collector current.There’s no Collector current without Base current.Switch circuits using BJTs are normally off. BJT Switch: Low side NPN BJT switch versus high side PNP BJT switch using bipolar junction transistors schematic diagrams by electronzap Includes 2N3904 NPN BJTs and 2N3906 PNP BJTs. Nice assortments of semiconductors that I use in my videos. Schematic symbols for NPN PNP BJTs N channel P channel JFETs and common MOSFET variations diagram by electronzap electronzapdotcom Whereas, the FET symbols i mostly use are just 1 of many ways that you will see them if you look at a lot of schematic diagrams. The BJT symbols shown below are used by almost everyone, but often they are not circled. NPN PNP Bipolar Junction Transistor BJT gain or beta current flow illustrated by electronzap It helps to remember these things when looking at a schematic with BJTs. A small amount of Base (to Emitter) current allows many times that amount of current to flow from Collector to Emitter. To support this site, check out the following links:īipolar Junction Transistors ( BJTs) have a variable amount of current that is allowed to flow from Collector to Emitter.Light Dependent PNP BJT switch circuit example:.Light Dependent NPN BJT switch circuit example:.

Bipolar transistor