Two Transistor LED Flasher Circuit
Intro: Two Transistor LED Flasher Circuit
How to make a circuit to make two LEDs flash back and forth. This is also called a free running multivibrator.
STEP 1: The Parts
For this project you'll need the following:
-PNP transistor x2 Q1, Q2
-4.7-22 uf capacitor x2 C1, C2
-470 ohm resistor x2 R1, R2
-100k ohm resistor x2 R3, R4
-5mm LED (any color) x2 LED1, LED2
-Solid core wire, 22 gauge
-Prototyping Breadboard
-4 AA holder x1
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-PNP transistor x2 Q1, Q2
-4.7-22 uf capacitor x2 C1, C2
-470 ohm resistor x2 R1, R2
-100k ohm resistor x2 R3, R4
-5mm LED (any color) x2 LED1, LED2
-Solid core wire, 22 gauge
-Prototyping Breadboard
-4 AA holder x1
-
STEP 2: Insert the Transistors
The transistors leads were to close together, so using needle nose pliers I bent them to fit in the breadboard. Insert the transistors two rows away from each other. Insert it so the rounded side is facing outwards and the side with the letters is facing towards the breadboard.
STEP 3: Circuit Diagram
STEP 4: Assemble the Circuit
STEP 5: Troubleshooting
If the circuit fails to operate:
-Check if the transistor is in backwards
-Make sure that they are both PNP transistors
-Make sure that the LEDs and capacitors are not in backwards
-Be sure that none of the bare leads are coming in contact with one another
-Check if the transistor is in backwards
-Make sure that they are both PNP transistors
-Make sure that the LEDs and capacitors are not in backwards
-Be sure that none of the bare leads are coming in contact with one another
27 Comments
Cannonball 7 years ago
In your schematic the caps are backwards. A charged cap in series with the power supply is the only way to reverse the bias on these transistors. When the capacitor discharges the transistor will turn on and place the charged capacitor in series with the power supply turning off the other transistor.
GianniM 9 years ago
It works because two transistors are not identical, due to imperfections and manufacturing issues, thus one of them starts conducting current befoer the other, and the cycle begins.
botdemy 7 years ago
That explains why I cannot get this to work in cicuits.io. :) In circuits.io, transistors are perfectly identical so they both conduct at the same time.
AtulV8 7 years ago
TylerH66 7 years ago
Are these really suppose to be PNP transistors? All the other tutorials I've seen use NPN (http://wild-bohemian.com/electronics/flasher.html)
hassaank98 8 years ago
you rocked dude
SHOE0007 8 years ago
I did this and with the proper functioning transistors I got it to work. Then I replaced the diodes with a speaker and replaced the values for C1 to 4.7 uf and C2 to 10 uf.
This made a simple buzzer. I am thinking of adding a 555 timer to it to adjust the buzzer.
90alx 9 years ago
Can you explain me please why the LED's are flashing one each at a time and not both of them in the same time ?I don't understand why the capacitor are not charging and discharging in the same time.
marcell.veszpremi 9 years ago
both of my leds are on, what should I do?
tqnata_1 12 years ago
-4.7-22 uf capacitor-V=?
-470 ohm resistor -W=?they works
-100k ohm resistor -W=?they works
PLS HELP
sghosh21 10 years ago
check the images
Adambowker98 11 years ago
squirrelchaser 10 years ago
lukeD 12 years ago
mania123456 13 years ago
Please reply soon and I really appreciate your efforts and creating and uploading such helpful ideas.
metsfan1938 12 years ago
mania123456 13 years ago
robot1398 13 years ago
mcmahanly 13 years ago
mcmahanly 13 years ago