Although I understand DC circuits I never was taught anything about integrated electronics at school. So forgive my lack of knowledge in this area.

I am wanting to build a high water level alarm for my domestic sewage treatment plant that is independent of mains electrcity - in the same way a smoke alarm is.

As part of the project I thought I could make a simple LED flasher circuit using a 555 timer, powered by a 9v battery.
I wanted a short flash/blink with a long delay, say 5 seconds. However I didnt realise the limitations of the 555 in astable oscillator mode; in that the duty cycle can never be less than 50% - which would give a long 5 sec blink with a 5 second delay.

Can anyone suggest a way to obtain a short flash and 5 sec delay?

Ive searched the internet but most circuits using the 555 plus a few transistors are for things like multiple LED cascades or police car flashing lights. Jules

Hi Jules,

This does not answer your question directly - but on my small scale sewage treatment plant, i used one of these (purchased off eBay), with a 9V battery & simple float switch. High water level event in the system simply turns the strobe on (it has a short flash rougly once per second). Essentially all the elecronics are in one cheap, weather protected(ish) package. Been in service for just over a year now & has averted one "catastrophic incident" so far...

Click image to enlarge

Thanks
Thats a really good idea. I'm probably over-thinking the solution.

I had a blockage in my system earlier this year causing it to flood and the submersible outlet pump to become unreachable. It cost £200 for a suck-out and another £250 for a bloke to come out to tell me which holes linking two of the chambers were blocked.

I saved another £250 by unblocking it myself, whiilst it was empty.

All this inspired me to fit an early warning alarm.


The old 1930s septic tank ran without a problem for 90 years until tree roots grew through the bricks walls causing it to collapse. Rules now demand a sewage treatment plant. Jules

Years ago I built a unit for a light in our pheasant pen to deter nightly predaters.. This used a 555 timer to give a short 'on' and longer 'off' cycle. It was also used in a unit mounted on a quad bike to give feed dumps for deer at adjustable intervals. The 555 timer is well documented includeding details for adjustable duty cycle intervals. But, I've looked through all my archived circuits but can't find any info 11 GS Bali Blue
07 GS FL2. Usually muddy. Gone
Numerous LRs since 1963

I would agree with the suggestion above to just buy a ready made strobe unit.

The answer to the 555 question is that you connect the load between +12V and the 555 output, so that the load switches on when the output goes low. This allows you to achieve an output with long "Off" time and short "On" time.

thank you Jules