Hi, I'm trying to build a circuit that will cause fans to be on when the power is on, and remain on for 10 minutes after the power turns off (I have separate battery for the fans). So far, I've got it where wire from the battery splits in two, one going straight to an OR gate and one doing the same but through a 555 timer.

I have looked up falling edge triggered 555 timer circuits, but none seem to work and all just trigger on the rising edge of the power input.
Would anyone be able to help? Thanks a lot

 

Hi, I'm trying to build a circuit that will cause fans to be on when the power is on, and remain on for 10 minutes after the power turns off (I have separate battery for the fans). So far, I've got it where wire from the battery splits in two, one going straight to an OR gate and one doing the same but through a 555 timer.

I have looked up falling edge triggered 555 timer circuits, but none seem to work and all just trigger on the rising edge of the power input.
Would anyone be able to help? Thanks a lot

Did you, by any chance, draw a schematic? Can we have a look if not confidential?

 

I have looked up falling edge triggered 555 timer circuits, but none seem to work and all just trigger on the rising edge of the power input.

It sounds like you are powering the 555 from the power source that turns on and off. Why not power it from the battery? Use the reset pin to trigger a monostable when the intermittent power goes off.

 

10 minutes is very long for a 555 timer. The leakage currents of large capacitors required for such a time makes the timing problematic.
Better to use a counter/timer circuit using a CD4060 counter such as shown here, which requires relatively small values for the timing resistor and capacitor.

 

FANS??? What sort of fans?? I have seen cars with electric engine cooling fans that have a scheme to run the fans after the engine is switched off. I have also seen computer fans that tend to be kept running after shut down. About a 1000:1 power ratio between those applications.
I have seen circuits with 555 timers have long delays of up to 15 minutes. But a 555pulser generating one pulse per second, clocking a binary divider counting to 1024 counts would be quite reasonable. And the binary counter can reset to zero when the power switches off.

 

And we still have no hint about the fans voltage or power or the accuracy required for the time. Nor any hint about how the fans get switched back on after the delayed switch off. And a 555 with a very low leakage capacitor should be good for a longer time. The cap would be a 100 volt polypropylene unit perhaps 2 MFD, or maybe lower capacitance units in parallel And the resistor 4.7 Megohms.

 

Your question is missing a lot of information.

Power source for the fans: Type / Voltage / AC / DC - ?
Number of fans?
Fan voltage
Current for one fan - or - Power rating for one fan?
Power source for the timer circuit? Is this the same as the battery supply for the fan?

AND - why is all of this secret?

ak

 

Your question is missing a lot of information.

Power source for the fans: Type / Voltage / AC / DC - ?
Number of fans?
Fan voltage
Current for one fan - or - Power rating for one fan?
Power source for the timer circuit?

You want the fans to come on when a power source goes off. OK, what is the power source that is going off? Voltage? AC? DC?

AND - why is all of this secret?

ak

No, not come on when power is switched off, but TO REMAIN ON. That was mentioned in post #1, but not emphasized.
If it is in a vehicle the very simple scheme will be a thermostat and a temperature switch. But if it is for an electrically heated convection oven it will be a bit more complex.
There are industrial quality delay-on-release timers available, check "Graingers". Not really cheap, but both reliable and adjustable.

 

No, not come on when power is switched off, but TO REMAIN ON.

Corrected.

 

Things like fan voltage and current aside for a moment what you want is a delay on break timing circuit. A Google of Delay on Break Relay or Delay on Break Timer should get you plenty of off the shelf modular solutions. The rest depends on your specific application. Heed the warnings as to a 555 as for long durations like you mentioned a 555 is not going to work out well. I would just buy an over the counter solution. If this is for a commercial application budget is nice to know?

Ron