Design the circruit No.1

Built and test the circuit shows below:

1. Components

     In this circuit. There are 4 resistors, 2 transistors, 2 LEDs and one TTL

2. Calculation

    I chose BC547 npn transistor and find its datasheet show as below

    For collector circuit: 

    I have Vled=1.8V and Iled=20mA. When the transistor is saturated, Vce=0.6V.

    Then Vr=Vs-Vled-Vce=12V-1.8V-0.6V=9.6V

            R=Vr/I=9.6V/20mA=480R

    For base circuit:

    Ib=Ic/beta  I have Ic=20mA, if I want find maximum Ib, then Beta should be minimum, Ibmin=110

    Then Ibmax=20mA/110=0.18mA. But if Ib=0.18mA Ic=20mA, the transistor is cut off. show as below

     So i chose i chose Ib=5mA and Vbe= 1.1V from data sheet when transistor is saturated.

     Then R=V/I=(5V-1.1V0/5mA=780R.

3.Design the layout

     2 LED collector circuit connected in parallel share the 12V supply. Each LED controlled by and 5V supply connected to transistor base. When Turn on the 5V supplier the transistor switch from cut off to saturated and allow the current flow through collector to emitter, and LED glows

4. Built the circuit

    I can not find 780R and 480R resistors, so i take 1000R and 560R instead and for safety.

    I built the circuit and works fine show as below

     

5. Measuring and Explain

    I measured the voltage cross the components.

    Vs=12V. Thats the voltage supply

    Vinput=5V. Thats the voltage from the TTL

    For LED1 circuit:

    Vr14=10.01V Vled1=2.10V Vr13=4.28V Vbe=0.82V Vbc=0.77V Vce=0.04V

    The collector circuit Vce+Vr14+Vled1=12.114V=Vs.

    The base circuit Vbe+Vr13=5.1V=Vinput.

    For LED2 circuit

    Vr15=9.79V Vled2=2.13V Vr16=4.53V Vbe=0.80V Vbc=0.60V vce=0.19V

    The collector circuit Vce+Vr15+Vled2=12.11V=Vs

    The base circuit Vbe+Vr16=5.33V=Vinput