![]() In this example, the two resistors are set so that the base of the transistor is at a sufficiently high voltage for current to flow into it and as a consequence, the transistor is on. In another case, there is current flowing into the base and so the transistor is “on” and the current can flow through it resulting in the light bulb being on. As a result, the transistor is “off” and no current can flow through the bulb. In the first transistor, the base is grounded and no current can flow into it. ![]() In this circuit, there are two transistors. Therefore, we can ignore the Q – Point and switch it between saturation and cut off areas. For switching purposes, we only need this device to operate either in fully-on or fully off region. In the saturation region, it remains fully ON. This diagram depicts the three operating regions of the transistor such as saturation region, active region and, cut off region. Many output devices will require a transistor switching circuit to operate a high current requirement load such as relays, solenoids, and motors. If we want to connect a load that requires a higher operating current demand more than 3mA, it will burn microcontroller. But the question that may come to your mind, Why do we need to interface transistor with a microcontroller? Because microcontroller pins can not provide output current more than 3mA and voltage more than 5V. In any application, we need to interface a transistor with a microcontroller. how to use it as a switch in microcontroller projects.How to use a transistor as a switch in electronics circuits.Many thanks for all the helpful replies.You can check these practical transistors: 2N2222, MPSA42, 2N3906 Using transistor as a switch So, removing the LCD shield and wiring everything directly into the Arduino with a separate push button, everything started working as expected. Eventually, after many attempts at getting the circuit to work, even the LED stopped working and that's when I realised that there must be a continuity problem with the header connections. This would explain the anomaly that only the LED lit up as expected, but the fan failed to work: presumably the weak current into the transistor's Base created sufficient gain to power the LED, but not the fan. It seems that the LCD shield I was using has some fault which only allowed limited current to flow from the corresponding Arduino pins. As the LCD shield occupies all the header pins on the Arduino, the LCD shield helpfully provides circuit connections for the unused Arduino pins. As mentioned in the question, I was using a button wired into an LCD shield. So the question is, why does the circuit work as expected for the LED, but not for the fan? Any help/suggestions will be greatly appreciated! I've tried different pins on the board, but always get the same result. But in my case, the current from the relevant digital pin to the transistor's base never goes higher than 1.4mA. My understanding is that an Arduino digital pin should easily supply this. Measuring the current from VIN (or the 5V pin) to base, I get 4mA, so my assumption is that 4mA is sufficient to saturate the base. ![]() If I take a wire directly from VIN or the 5V pin and touch it to base (through a resistor, of course), it does succeed in saturating the base, and the fan turns on. It does turn the LED on and off as expected, but the fan needs much more current than the LED. The problem that I have is that the Arduino's digital pin doesn't seem to be able to saturate the base, so not enough current passes through the transistor to power the fan. My expectation is that, when I press the button, the digital pin wired to the transistor's base will cause the transistor to close the circuit and turn the fan (and LED) on for five seconds. ![]() the relevant digital pin is switched from LOW to HIGH for a determined time in response to a button press recorded on A0.įor simplicity, I'm showing a separate button component in the diagram, but I'm actually using a button that's wired into an LCD shield. I'm using a 2N2222 transistor as a low-side switch to turn a 5V computer fan on and off with an Arduino digital pin, which responds programmatically to a button press - i.e. ![]()
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