Hello everyone,

I am looking for a simple and reliable circuit to alternate two fans (FAN A and FAN B) as follows:

• FAN A runs for about 30-70 seconds
• Then a pause of ~3-5 seconds (both fans OFF)
• FAN B runs for about 30-70 seconds
• Then again a pause of 3-5 seconds
• The cycle repeats continuously until power is turned off

Requirements:
- Power supply is 10 V (both fans are 10 V type)
- No relays, no counters, no microcontrollers – only analog solution with RC timing and MOSFET switches
- Each fan should be driven by a MOSFET (low-side switching)
- I would like to have that 3-5second "dead-time" between switching from one fan to the other
- Fan about 1-3 W
- Timing stability 10%
- Or do a digital solution with day timer

 

4 555s can do the job.

 

I'd do it with one oscillator, a counter, and some decoding logic.

Edit: oh, no counters allowed.

Edit 2: why not?

 

Yea, I use a circuit in some of my projects that use a 555 and a 4017 with the outputs of the 4017 tied back to the 555 thru diodes that control the timing, that would be perfect for this.

But yea...no counters.

 

sorry, 555 is not available

 

But TLC555 is OK

 

TLC555 will work just fine.

 

Your acceptable wide time tolerance should help. Charging a high value capacitor through a large value resistor with the output into a comparator with wide hysteresis, shorting the charged capacitor with a transistor via a lower resistor value might give you the long on/short off sequence. Somehow into a flip flop to select alternate fans. I’d need to sit down with pencil and paper to check this out, but maybe someone can progress this? Or tell me why it won’t work?

 

Your acceptable wide time tolerance should help. Charging a high value capacitor through a large value resistor with the output into a comparator with wide hysteresis, shorting the charged capacitor with a transistor via a lower resistor value might give you the long on/short off sequence. Somehow into a flip flop to select alternate fans. I’d need to sit down with pencil and paper to check this out, but maybe someone can progress this? Or tell me why it won’t work?

You are basically describing a 555, but a single F/F can't produce the dead time.

 

I was thinking, if the dead time of the first fan is gated to keep both fans off?

 

hi Tamm,
Can you acquire the items to build this circuit option.?

E
@Jerry-Hat-Trick added the asc file.

 

Not interested. I would use a microcontroller and have it working in a hour.

 

You're slow.

I learned very early in my career to estimate projects at about double what it would take. My management would then triple my estimates for the official schedule, and I had a lot of time to do whatever I wanted and still delver ahead of schedule.

 

hi Tamm,
Can you acquire the items to build this circuit option.?

E
@Jerry-Hat-Trick added the asc file.

View attachment 354725

Thank you ! Can you pl tell me how much power it consumes?

 

hi T,
This plot the Power, excluding of course the MOSFET Load.
E

 

Eric, I added a modification of the 555 timer to get the appx 70 seconds when first powered ON.
Otherwise the first timing cycle was over 110 seconds when tested on the breadboard.

 

Thank you ! Nice work !
If you use it to control two 2W PC fans for ventilation, I guess BC547B / BC548B transistors are enough and no amplifiers are needed?

 

Eric, I added a modification of the 555 timer to get the appx 70 seconds when first powered ON.
Otherwise the first timing cycle was over 110 seconds when tested on the breadboard.
View attachment 354737

But it constantly takes power

 

So do the fans when powered ON.
It's only an extra 1.35 ma.
How much current does a fan draw?

 

Eric, I added a modification of the 555 timer to get the appx 70 seconds when first powered ON.
Otherwise the first timing cycle was over 110 seconds when tested on the breadboard.

hi sg,
Thanks for the suggestion.
I did consider that, but as he stated The cycle repeats continuously until power is turned off
that the longer start up period would not be a problem.

I see now that he is concerned over the initialising circuit power usage, which is so small, when compared to the fan motors.

@Tammepuu What is the power source for the timing circuit??

E