cooling fan circuit schematic

Smart Cooling Fan Circuit

A continuously running cooling fan is a big nuisance and is not very crucial in most instruments. The 12V smart fan control presented here allows the fan to run at rates varying from once a second to once in ten seconds approximately. In case of a battery powered instrument, this idea will save some power too!

Basically, the cooling fan circuit is an astable multivibrator built around NE555(IC1), in which the output level at pin 3 remains high for a long time and low for a short time. The low level output at pin 3 drives the cooling fan via T1 and T2. This timeout (delay) period can be adjusted using preset pot P1.

A simple temperature detector mechanism is deliberately added to the main circuit. Here, a negative temperature coefficient (NTC) type thermistor (TH1) functions as the heat monitor. Normally the resistance of TH1 is high and the astable is enabled through trimpot P2. When temperature increases beyond a limit set by preset pot P2, the astable is disabled by the grounding of its reset terminal (pin4) via TH1. At this time, output at pin 3 of IC1 remains in low level and the cooling fan sweeps continuously.

Temperature dependent cooling fan schematic

cooling fan circuit schematic


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  • Tomomi

    The sound card is designed to dzigtiie audio frequencies. It does not respond to very low frequencies or DC voltages. This prevents the sound card from being used for recording things like temperature or anything that needs a absolute reference. Now there are some tricks that you can do like converting your signal into an an audio frequency and then recording that tone. But then you have to use software to convert that back to numbers.

  • Mike

    Looks almost like what I need! I have very little electronics experience and want some timers for my boat, one for venting the bilges, maybe 5 minutes in every hour, one for the composting toilet maybe 5 minutes every quarter hour. Plus a few other fans. what would I need to modify in this circuit apart from removing TH1 in order to achieve a variable duty cycle long time delay astable?

  • Cabcapcan

    It doesn’t work in Multisim. Just DC current, no oscilation. Strange. Another fan controller on 555 works good.

    • Layc

      Hi Mark,Another great article.So you are still using the V9 clone? Looking at the speuliprs mentioned, it is difficult to work out who makes what and what is a clone and who is the originator.What went wrong with the MBE extruder? I assumed that would be OK ( as it seems most of the RepRap work that way) instead of the Bowden tube ( which has it’s own set of problems). I thought the Bowden system would reduce the carriage mass and hence faster printing, but I’m firmly stuck at 20-25mm.I’ve updated my extruder ( airtripper, of course) with a MK7 steel cog and it has much improved the situation; no more shredded PLA filament. Excellent!William

  • DEC

    hello sir , I am working with same thing but using 741 IC and a 5k NTC but i am getting output of mV . i checked the NTC its not changing values . can u suggest me something ?

  • Som

    My pin 3 of NE555 IC always remains high (12 V), any reasons ?

    • Estella

      Amazon Verified Purchase() This review is from: These are nice, ixnipeesnve and good quality. The blue is a very classic baby blue. I also ordered the coordinating skirt and sheet and several months later am still happy with all three.

  • subburaj p

    my b.e final year project s mainly based on solar and wind hybrid system so please give me a suggestions for the same……….plzzzzzzzz

  • fyp

    can you please list the function of each component in this circuit?

  • krunal

    Dear sir,

    my Fan current is 100mA , can i use the same circuit for my project ?

    • P. Marian

      T2 BC548 is designed for use as general purpose amplifiers and switches requiring collector currents up to 300 mA. If your fan requires 100 mA at 12V, this means 1.2W which is too much for this 625mW maximum power dissipation transistor. You can try and change it with other powerful transistor, maybe BD139.

  • electronics keralam

    BC 548 can handle 12 volt fan ?

    • Authro

      In prototype, a fan with maximum operating current of 50mA @ 12V was used.Anyway you can use another one like SL100B.

  • electronics

    This is a nice circuit thank you

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