As a result, the LED starts turning on.Whether autowiring is enabled. After some time, once the voltage on the base is high enough, the collector current starts slightly growing. It happens because the transistor starts opening even before the capacitor is charged to 80%. Note that the LED actually turns on much faster. If we the capacitor C1 is 470 mkF, and the resistor R3 is 200 KOhm, then the approximate maximum delay is going to be The rise time from 20% to 80% can be calculated as the following It is the time required to charge the capacitor, through the resistor, from an initial charge voltage of zero to approximately 63.2% of the value of an applied voltage. The charging time of the capacitor is related to the product of R3 and C1: The delay is related to the charging time of the capacitor C1. We can pick up a standard resistor of 3.3 KOhm. Now we can apply Ohm’s law and calculate R2: To make sure the transistor turns on fully, let’s add a factor of two for safety and use a base current of 0.7mA = 0.0007A.
Choosing a base resistor for a transistorįirst, let’s calculate the base current which keeps the transistor open We can pick up a standard resistor of 100 Ohm. Now we can apply Ohm’s law to calculate R1: The LED current is also the collector current. This current should be enough to turn the LED on. Let’s set the LED current to be 10mA = 0.01A which is less than the maximum allowed current for the LED. Let’s calculate the value of the resistor R1 which limits the current for the LED. This is an NPN transistor which has the following parameters according to its datasheetĬhoosing a current limiting resistor for an LED We use a standard LED which has the following parameters Let’s see how we can choose elements for the transistor delay circuit. The delay can be adjusted by changing the resistance of the variable resistor R3.Ĭhoosing components for the transistor delay circuit The current starts flowing through the transistor, and the LED turns on. After some time, the voltage applied to the base of transistor becomes high enough to open the transistor. It means that the more the capacitor is charged, the more voltage applies to the base of the transistor. While the capacitor is charging, the voltage across the capacitor grows. The RC filter implements a voltage divider. The more the resistance of R3 is, the slower the capacitor charges. The resistor R3 defines how fast the capacitor charges. If we hold the button S1, the capacitor begins charging. It means that no current comes to the base of the transistor, and the LED is off. The RC filter defines the delay.Īt first, the capacitor is not charged, and the switch S1 is off. The transistor is controlled by the RC filter which is built by the variable resistor R3 and capacitor C1. The transistor Q1 and the resistor R2 make a switch. The resistor R1 limits the current to prevent damaging the LED. The LED is connected to the collector of the transistor. Let’s see how we can choose elements for the circuit, and how the delay depends on parameters of the elements.īelow you can see the schema. The circuit uses an RC filter to turn an LED on with a little delay.
It only contains a transistor, a capacitor, several resistors, a switch and an LED. The transistor delay circuit may be helpful to learn some electronics basics.
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