1 clear red LED
1 green LED
2 x 560 Ohms carbon resistors
2 x 1 K Ohms carbon resistors
2 x NPN BJT (C547) transistors
By knowing How much current these LED can handle therefore we Know how much current we can have going thru before we damage the LEDs and that is 20 mA and we can find this out by looking at the data sheet of this LED on the internet .
So to calculate the resistance at R14 and R15 ...
R = V / I
R = [12 - 1.8(LED Vd)- 0.2(Vce)] / 0.02
R = 500 ohms
because there is no resistor at 500 ohms we use the next one above it to make sure we don't damage our circuit and the next resistor is 560 Ohms resistor .
By using the data sheet of this transistor we can see to have this transistor in saturation we need to have 0.005 A or 5 mA at the base so now that we know the Ib we can go ahead and calculate the resistance of R13 and R16 ...
R = V / I
R = [ 5 - 0.7(diodeVd)] / 0.005
R = 860 Ohms
Because there is no resistor as 860 Ohms resistor we go above and the next one is the 1000 Ohms resistor
Now here is the way I set my board up ....
This is a simplified injector driver circuit using very basic components and design.
By using LED we simulate the operation of grouped injectors in vehicle .
Operation:
Vs of 12 volts(DC) from our battery to supply our LEDs ,'' Injectors''
5 V signal supply from ECU(PWM) for switching the on and off
LEDs to show the circuit is working
Design:
After the caculation of the resistor I used Lochmaster 3.0 to design my board which makes it real easy to do and the only thing on my mind was to keep it simple so I wont have problems with wires and components touching each other so as you can see I have spread everything out .
How it works:
Well we are basicly using the BJT as a switch to turn the LEDs on and off . There is a constant supply of 12 V to the LED circuits but there is no earth so the ciruit is not complete as we the ECU puts out 5 V signal it turns on the BJT by putting 5 mA at the base therefor putting the BJT into saturation in other words fully on so then the circuit is complete and now our LED have a connection to earth so the electrons can flow thru the circuit thus turning on the LED
Beta:
Beta = Ic / Ib
Beta = 0.02 / 0.005
Beta = 4
Checking:
I checked my board visually and Voltage drop to make sure everything was correct
Vd of R14 and R 15 are around 10V( 10.08 @ R14, 9.98@ R15)
Vd of LEDs are 1.86V
Vd of R13 is 4.4 V and R16 is 4.45 V
Vbe 1 is 0.65 and Vbe 2 is 0.72
Vce 1 is 0.04 V and Vce 2 is also 0.04V
I didnt have any problems with my board and these are some pictures showing it working:
Due to the brighness of my LED the next pix is quiet dark.
