Introduction: "police" Light Flasher

Picture of "police" Light Flasher

How it works: the switching transistors (PNP) cant decide between each other and in doing so, it charges and discharges the caps and switches the leds on and off. resitors are for current limiting.

(originally called "two-transistor dual- led flasher.") This is a circuit that's used in RC cars, radio controlled things (if it fits), ETC.......,
it basically is a police light flasher. for some reason i had to re-do this circuit like 3 time or something, but it worked out GREAT!!!!!!!!!
This circuit will work off of a 3v or 6 v battery ONLY! 4.5 or 9.0 volts cannot be used because 9 volts changes the led "flippige", and 4.5 volts speeds it up at a really fast rate.

*NOTE*- this is not my circuit, i got it from Radioshack's electronics learning lab, i do not take any credit for this circuit.-*NOTE*

(but it was fun to build!!!!!)

Step 1: Parts List........

Picture of Parts List........

2) 9015 Switching PNP Transistors ($0.75)

2) 470 OHM Resistors ($1)

2) 100 KOHMS Resistors ($1)

2) 10 uF Capacitors ($1.19)

2) Leds (of any color) ($1.59)

This circuit won't work if either of the PNP transistors are backwards

*NOTE* to change the speed of the flashing, change the capacitor values and the 2 resistor values, not the led resistors though!

Step 2: Schematic-assembaly

Picture of Schematic-assembaly

Here is the schematic for the flasher.

I made this on smart draw 2009, sorry for the bad quality.

Step 3: Turn Out.

the turn out was great for me but if you have problems, email me at or comment.

(if anyone knows a circuit that allows you to time someone?, or a police siren! THANKS!!!!!!!!!!)


MJamesM (author)2009-05-30

The circuit above, quality aside, will not work as you claim - the transistor on the right is reversed L-to-R.

MROHM (author)MJamesM2011-12-08

The Emitter's of Both Transistors should be tied to +V---- You're Right!!! I verified this already!! One Capacitor goes from base of Q1 to collector of Q2 while another capacitor goes from base of Q2 to collector of Q1. And Q1= 1st Transistor while Q2= 2nd Transistor. Point well made MJamesM!!! We need these connections to allow for proper Circuit operation!! Awesome Instructable electronicfanatic1. Thanks MR.OHM.

coophdc (author)MJamesM2010-02-05


I built it the way you suggested and it worked perfectly. Thanks for pointing that out. 

Also, NPNs wont work... Just fyi.

MROHM (author)coophdc2011-12-08

Npn's Will work if you do the Following 1) Reverse Power Supply connections 2) Reverse Led "s 3) Reverse Polarity on the Capacitors, Also to provide proper Feedback. A capacitor is connected from base of 1st transistor to collector of 2nd Transistor and the 2nd Capacitor is connected from the base of 2nd Transistor to the collector of the 1st transistor,Without these 2 Feedback Loops properly connected the Led Flasher will cease to Blink. I wanted to clarify this because it isn't properly marked in the Schematic. I breadboarded this Circuit with PNP Transistors and NPN Transistors( with the Modifications already mentioned ) and They both work!! Still a Great Instructable!! Increase The values of the Capacitors causes the Flasher to Blink Slower(Capacitor takes longer to Charge and Discharge) Thanks from MR.OHM!! Awesome Instructable!!

Wesley666 (author)MJamesM2009-10-19

If its a bi-polar won't it still work?  I have had transistors that work the same either way.

i dont understand how it wouldnt work if i build it and it works! jokes on you

Here is a schematic for a siren that I built, it worked great the only disappointment was that it sounded like a foreign countries siren...( high-low) but the pot adjusts the pitch which can be piercing to the ears lol!


erveli (author)2010-01-01

Can this be done with only one led, capacitor, transistor and those two resistors. Because I want to have only one Led that blinks?

brandencota (author)2009-11-01


electronicfanatic1 (author)2009-05-29

no, im using it for radio controlled circuits

lemonie (author)2009-05-27

Hi, I'm building something not to dissimilar myself, but I'm taking photos as I go along. Was this something you built and then thought about making into an instructable? The photo opportunities may be lost, but you could explain how you actually put this together and detail changing the components & batteries? L

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