Instructables

555 Car Flasher unit circuit

Hi,
I'm wanting to put some led indicators in my car because they are so efficient but my current flasher unit isn't designed to run such a low load.  It is also relay based which probably isn't the most efficient way of doing things and with it being an old mini it sometimes rattles itself at whatever rate it feels like flashing at at the time so I wanted to use a transistor instead.
I've come up with the circuit attached and built it but when it is running all the indicator lights for the hazzards, the transistor gets hot!
Surely there is a better way of doing it?
Could someone have a look and see what could be changed? I don't know much about transistors but I know that I'd be better getting it to 'saturate' and act as a low resistance switch but I don't know how to achieve that.  It also has to run under two loads, indicators and hazzards, and has to flash at the same rate for each - between 1hz and 2hz.

http://www.dprg.org/tutorials/2005-11a/index.html
I think a variation of this circuit might be better (it makes use of the discharge pin rather than the output pin), with the load at the ground side of the transistor because it has to be an output pin on the flasher unit.

I've got some BFY51, BUY82, BD743B so it would be great to use one of them.

in short: what's the best solid state fixed rate 12v flasher driver using a 555 that can supply around 2 amps using as few parts as possible?

Thanks for your time, and sorry if I've done anything wrong!

Picture of 555 Car Flasher unit circuit
jim_20002 years ago
I second the N channel Mosfet, less voltage drop, less heat the IRF540 looks like a direct swap
Goodhart2 years ago
I think you are trying to sink too much current.
arichardson (author)  Goodhart2 years ago
But I thought if I got the transistor to saturate then its resistance drops to nearly zero and it won't warm up. The leds that I want to put in will draw less than an amp in total. Thanks for your help, I'll look into building a small heatsink, or use a higher rated transistor :)
Can anyone think of what else I could do to improve the circuit?
Cheers!
It ISN'T nearly zero ! The voltage across the transistor is probably still 1V or so, and you haven't said how much current your lamps draw.

Try an IRF540 mosfet instead.
arichardson (author)  steveastrouk2 years ago
If the led lamps draw less than 1amp, and the transistor drops 1v then there's just 1w to dissipate which I might be able to make a small heat sink for.

Would a irf540 be a straight swap into this circuit?
it is a mosfet after all....a metal oxide semiconductor field effect transistor.
You can get away with a small heatsink.