This blog will keep to a simple theme of my tinkering with electronic and computer related toys, gadgets, projects and the like. I do hope from time to time there is something for someone to find when they are exploring these things for themselves. From an early age, I was always attracted to electronic gadgets an I continue to enjoy myself with my hobby. Unfortunately with a busy life, time becomes short an I can not indulge myself like in the past.
This experiment is an improvement of experiment #20. The circuit is completed with one wet finger between two resistors. In addition, to show the basic principle used in touch lamps and touch devices. The lamp does not stay lighted because there is no memory built into the circuit.
This experiment demonstrates that the body can be used to conduct electricity. By using one's body to complete the circuit the transistor is turned on and the LED lights. More over, the experiment show that dye fingers do not conduct as well as wet fingers.
In this experiment, the transistor configuration is called the Darlington configuration, current is amplified twice. All the current flowing through the emitter of the first transistor (left) will flow to the base of the second transistor (right). This means that the current flowing into the base of the left transistor will be amplified twice (once by each transistor), or twice the amount of base current is needed to control the larger current in the circuit turning on the LED. To turn on both transistors the capacitor voltage must exceed 1.4V before the LED will light. And, since the input current to the base is so small, it will take much longer to discharge the capacitor. In the video, I show the voltage at the capacitor. First, when the switch in on, charging. Second, when the switch is off, discharging. In addition, midway through the video, I show the voltage at the collector of the right transistor as voltage is pooling. Last, in the video I short the capacitor to discharge it much faster. The pictures below the video show the voltage at each transistors in the order of base, collector, and emitter. For some reason, at times, Blogger does not let me insert text for all my images.
This experiment combines transistor basic principles and what was learned in experiment #8 (capacitor charge/discharge). When the switch is first turned on the current flows through the 100k ohm resistor (controls charge time of capacitor) to charge up the capacitor, the transistor and LED will be off. When the capacitor rises to 0.7V the transistor turns on first and than the LED will turn on. Current will increase as the capacitor voltage rises. When the switch is turned off the capacitor will discharge through the 470 ohm resistor and the transistor base (resistor controls discharge of capacitor), the LED will dim as this discharge current decreases. When the capacitor voltage drops below 0.7V the transistor will turn off.
This experiment uses a variable resistor to show that 0.7V is needed at the transistors base to allow current to flow in the circuit. As the voltage increase the transistor turns on until a larger collector current lights the LED.
In this experiment, the NPN transistor base is connected to the collector making the transistor function as a diode. Once 0.7V is applied to the base current flows with only slight resistance and no current gain. Exactly as a diode functions.
In this experiment, the right LED in the collector path is brighter than the left LED in the base path because the base current is amplified by the transistor. In other words, a smaller current is used to control a larger current. The term "amplified" is misleading and does not amplify an electrical current(Go to Figure 4 and read text below). Remember, in experiment #14 the transistor was used as a switch. Here, it is still functioning as a switch, and in this sense the transistor is used as an amplifier. This is called current gain. Current gain by a transistor can vary anywhere from 10 to 1000 depending on the type of transistor. But, battery voltage and circuit resistance will limit the current gain. The circuit resistances, not the transistor itself, are limiting the current and the transistor is said to be saturated.
