AboutDave Nyce Expertise I have been an electronics engineer for 25 years. I can answer questions on analog and digital circuits and my specialty is sensors.
Experience I am the inventor on 23 US patents, and also some foreign ones. Developed sensors for over 25 years. Licensed private pilot (airplane and rotorcraft), have HAM radio license. I'm not an expert in computer networking.
I've prototyped another part of the circuit I'm designing. The two transistors in question each have a gain of about 100, but I'm getting a very very small current of only 2.6mA flowing into the LED. The LED is very dim. I can't figure out what is happening. Are the transistors working properly? Any help welcomed! I need a current flow of 80mA into the LED (4 LEDs will actually be used in the final circuit).
Answer With your circuit, you should get about 4.3 volts to the LED circuit. If it is a red LED, it has a voltage drop of about 1.7 volts. So, there is 4.3-1.7 = 2.6 volts across the 390 ohm resistor = 6.7 mA. To increase the current, decrease the resistance. If you will have 4 LEDs, they should each have their own resistor. With 4 LEDs and 4 resistors of 130 ohms, each red LED will have 20 mA, for a total of 80 mA. (Make sure that you are not using an LED that already has an internal resistor.)
