In this case, knowing that the transistor current gain () is approximately 300, the minimum base current for the transistor is saturated is: Ib = Ic / Ic = (12-2-0.2) / 2K7 = 3.6mA  Ib = 3.6mA / 300 = 12 A If by convention we call the V-input voltage at which the LED begins to look like and V + to the input voltage at which the LED will shine with maximum brightness, we can formulate two equations in two unknowns to determine the value of the voltage divider R1 and R2: Vbe = (V-x R2) / (R1 + R2) + Ib (Vbe / R2) = (V +-Vbe) / R1  Solving we obtain R1 and R2: R1 = (V +-V-) / Ib R2 = [Vbe x (V + - V-)] / [Ib x (V-- Vbe)] Looking at equation 1, as tensions have taken input (V + and V-) differing 1 V, R1 take equal value for all input blocks. This value will be: R1 = 1 / 12 A = 83,333 "82K
The R2 values taken will depend on the input voltages. These values are shown below: R2 Values: 2 V 44.5 K 47 K 4 V 17.5K "18 K 6 V 10.9 k" 10K 8 V 7.92 K "8K2 All-film resistors are carbon ¼ W To complete the scheme, it is necessary to unite the four inputs of each stage to make an entrance, and to make the circuit input impedance is low, we place a current amplifier, consisting of a transistor in common collector.
The entry also is equipped with a diode which rectifies the signal, which is a 1N4148 diode high speed switching, and a small value capacitor (47 nF), which is charged to peak voltage, so that no tensions or negative influence or excessively rapid variations of the input voltage. We must say that this diode increases the input voltage for the lighting of each LED at 0.7 V. Figure 3 shows the basic circuit of current amplifier.
'Lighting circuits' input circuit Figure 3. Commissioning. The once assembled circuit needs no adjustment. The 24 V voltage transformer is connected to the bridge input rectifier and the input signal Ve frequency input signal must not exceed 10 Hz, since at higher frequencies is not seen the movement of the LEDs bar.
Measures the output voltage of the power supply: 12.3 V. Input voltage at which each LED starts to brighten: 2 - 2.3 V 4 - 4.2 V 6 - 6.3 V 8 - 7.4 V per LED Max Current: 3.5 mA. Maximum current consumed by the circuit: 19 mA. Input voltage for all transistors saturated: 10.2 V. Contents: Introduction. Design specifications. Development of the calculations. -Power supply. Meter. Commissioning. Measures. Wiring diagram. Components used. Components used.
TYPE UNIT SPECIFICATIONS Bridge Rectifier electrolytic capacitor 2200uF / 35V ceramic capacitor 100nF / 250V Resistance 220 Ohm / 12V zener diode 2Watt / 2Watt LED 2 green, 1 yellow, 1 red resistance 2K7 Ohm / 0.25Watt. BC547B Transistors 47K Ohm Resistance / 18K Ohm Resistance 0.25Watt / blue led led 0.25Watt bacteria. 10K Ohm Resistance / 0.25Watt Ohm Resistance 8K2 / 82K Ohm Resistance 0.25Watt / 0.25Watt Capacitor 47nF / 250V Diode 1N4148 Transformer 24/12V included in the pack