Need to buy/make inline electronic flasher....

As above - need to flash two bike 12V 21W indicator bulbs (in parallel) in an environment with severe vibration - I have tried two makes of mechanical (bi-metallic strip) units which are prone to vibration upsetting the flash rate. So I want to buy/make an electronic flasher. The flasher unit is a two terminal device in series with the load - i.e., the passing current heats up the strip within. I am not sure there exists a two terminal electronic unit, although I assume adding an earth would not be unacceptable, but I don't really want to have to rewire the bike to accommodate e.g. a 555 timer circuit driving a FET or Power Transistor.

Anyone any suggestions ?

Apologies if a bit OT - it is DIY though !

Nick

Reply to
nick smith
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It's certainly not impossible. When the lights are off, the load side of the switch is practically earth, so it's possible to rig a charging circuit with an electrolytic and diode to give you supply lines of +12V and +1.2V approx assuming regular PN diode. Hint - drawing 1mA for 1 second from a 1mF capacitor will drop the capacitor voltage by 1V

You need minimum current draw though (unless you like *big* capacitors) so a low power varient of the 555 (7555 IIRC??) and a power MOSFET should work. Thing is you'll need a P-channel MOSFET unless there's a clever way of using an N-Channel which I can't think of. Overrate the MOSFET to allow for the inrush current of the load.

A really clever design could dispence with the 555 and just make an oscillator out of the MOSFET and a few auxillary components. I'm not offering a design because analogue was never my thing. Probably the only person who did electronics for a hobby and never made a radio (that worked) or an amplifier. Designed quite a few few nifty digital bits and bobs though which worked for years though :-)

Love to get into playing with PICs if I ever get the time.

HTH

Timbo

Reply to
Tim S

There used to be a two terminal plus ground IC available to do exactly this. I think Natsemi made it but can't remember the number. Have a search through their ap notes.

Reply to
G&M

There's a circuit for a flasher based on the 78L /79L series of voltage regulators. You'd need to beef up the output switching capacity, though.

You can download the data sheet from the RS site.

Reply to
Dave Plowman (News)

That should work with the 3A version of these regulators as well. Should be worth a try at least.

Reply to
G&M

I'd guess so, but it needs to switch two 21 watt bulbs.

Reply to
Dave Plowman (News)

Thanks for all the replies so far.......

I am worrying that the start up current of an incandescent bulb is roughly 10 times the running current - so 42 watts at 12 volts is about 3.5 amps and 10 times this is some 35 amps - or is this aspect not an issue ? I am tending towards a solution with a relay driven by a 555 timer or maybe just an RC network, but really wanted an electronic solution to get around the vibration problem.

Nick

Reply to
nick smith

Standard automotive relays are designed for high inrush currents and to have a shock resistance up to 20G.

Reply to
Tony Williams

Remember there will be some resistance in the wiring and switch contacts etc. However, Maplins HUF75337P3 power mosfet will handle 62 amps at 55 volts for 1.49 gbp...

Reply to
Dave Plowman (News)

Okay - how about soft-starting a 5 or 10 Amp regulator ? Haven't got one to hand here to try but should be possible to make it oscilate.

Reply to
G&M

Hi,

See:

Or switch the output to a low level instead of off that just keeps the bulb filaments hot.

cheers, Pete.

Reply to
Pete C

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