Schematic :
Calculations :
Vs = 12v
PWM = 5v
Vs = 12v - LED 1.8v - Vce 0.05v = 10.15v
Ic = 20mA so 10.15 ÷ 0.02 = 507.5 therefore
R14 = 560 Ω
R15 = 560 Ω
Ib = 5mA
Beta = Ic/Ib = 0.02/0.005 = 4
PWM = 5v - 0.7 = 4.3/0.005 = 860 therefore
R13 = 1k Ω
R16 = 1k Ω
Design :
Component List :
R1 470Ω 0.25w carbon film resistor
R2 470Ω 0.25w carbon film resistor
R3 1kΩ 0.25w carbon film resistor
R4 1kΩ 0.25w carbon film resistor
LED 1 : 20mA 5mm red
LED 2 : 20mA 5mm green
Transistor 1 : NPN 547
Transistor 2 : NPN 547
Circuit Explanation :
This injector circuit has two 20mA LED's in parallel that when switched on and off represent injectors working. This is done by using two NPN 547 transistors as switches. Each transistor has a 5v supply PWM connection with a 1kΩ resistor in series that limits the current to 5mA to the base of the transistor. This will fully saturated the transistor switching it on. The LED's have a 12v supply, each with a 470Ω resistor that limit the current to 20mA for the LED's and use up the voltage the LED's don't use. The transistors have a common emitter that goes to earth completing the circuit.
Test Procedure : Circuit working properly with 5 v constant supply.
First I did a volt drop test on the components in the LED 1 circuit.
R1 = 8.79v LED 1 = 3.2v Vce = 49mv .The LED voltage was slightly higher because I replaced the 560Ω resistors with 470Ω. Vbe was 0.82v and R4 = 4.7v.
2nd LED circuit : R2 = 9.77v, LED 2 = 2.3v, Vbe = 8.3v, Vce = 53mv and R3 = 4.7.
When I added up the components voltage drop they equaled supply voltage. Vce was 53mv and Vbe was 0.8v this showed the transistor was saturated.
Problems :
LED 1 switching on and off intermittently connected with a constant 5v supply. So I tested available voltage at Vs wire and got a intermittent reading between 12v and 0v. Then I checked soldering for the Vs wire and found a poor solder so I removed faulty solder with solder brade and resolderd and this fixed the 1st problem. LED 1 was now on but this now showed me a 2nd problem LED 2 was not on. So I tested the transistor Vce for LED 2 and my reading was 12.9v which showed an open circuit between collector and emitter. My solution for this was to replace the transistor which fixed the problem. Now both LED's were working but were quite dim so I replaced the 560Ω resistors with 470Ω ohm now the LED's are very bright because they have more current and voltage.
Reflection :
If I had the chance to do the circuit again I would take more time making sure my solders were good and try reduce the size of the circuit and generally improve the build quality.
