exhaust fan timer - Timer Circuit for Exhaust Fans

Timer Circuit for Exhaust Fans

The following post describes a simple timer oscillator circuit which can be used for keeping an exhaust fan alternately switched ON/OFF as per a fixed predetermined period. The circuit was requested by Mr. Anshuman.

The Circuit Request

Hi Swagatam,

Here is a  suggestion for the circuits on your blog. The idea is to have a very simple Oscillating circuit will delays of 5-10 minutes to turn on and off exhaust fans which would otherwise go bad if left on by mistake.

Ideally this circuit should be small enough to fit behind the switch itself …I was thinking of a couple of RC delays with a transistors to do the oscillation and a simple relay to be operated which will operate the AC Fan itself.

Of course we will need a very very basic rectifier to make the DC to power the circuit…unless somehow this can all be done ALL in AC and I'm missing something.

Please drop me a reply if you are able to find the time to work this or let me know if you post it on the blog.


The Circuit Design

AS shown in the figure below, the proposed exhaust fan timer oscillator circuit may be understood as follows:

D1  along with C2, Z1 and C4 forms a standard transformerless power supply which provides the circuit with the required operating DC voltage.

The IC 4060 is a counter, divider chip which has a built in oscillator. Here it is configured as an oscillator whose timing is determined by the setting of P1 and the value of C1.

When power is switched ON, the circuit receives the required DC supply for initialization.

Current through C3 instantaneously resets the IC pin#12 so that the timing can begin from zero and not randomly.

Pin#3 which is specified to give the highest delay switching is wired as the trigger output for the connected triac load assembly.

Initially as the timer counts, this pin is held at logic zero.

As soon as the timing elapses, the above pin goes high triggering the triac and the connected load which is an exhaust fan here.

The situation persists until the ON time of the circuit elapses  reverting the output to zero and switching OFF the load.

The above cycle repeats, switching the load ON/OFF  at the predetermined time rate as long as the circuit remains powered..

The circuit can be made into a one-shot timer by inserting a 1N4148 diode across pin#3 and pin#11 of the IC (anode to pin#3, and cathode to pin#11)


exhaust fan timer 1 - Timer Circuit for Exhaust Fans

Parts list for the above exhaust fan timer/oscillator circuit

R1, R3 = 100K
R2, R4 = 1K
R5 = 1M
C1 = 1uF/25V
C3 = 0.1uF disc
C2 = 100uF/25V
C4 = 0.33uF/400V
Z1 = 15V 1watt zener
T1 = BT136

33 thoughts on “Timer Circuit for Exhaust Fans

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Based on my understanding of the circuit, I had the following questions:
    1. Ideally the switch should not be on the neutral line (which is how I understand the way the Triac has been setup here). It should be above the fan on the phase itself
    2. I assume the off delay is the same as the on time? What variation of this circuit would allow us to have asymmetrical on-off times (for instance if the device needs only 1 minute off for 5 minutes run time)


    • Phase or neutral points are not critical here, they may be put anyway round, the circuit will work both ways.

      With a 4060 IC you would get identical ON/OFF time, for variable duty cyce you would have to use a different circuit may be by using the IC555.

      any standard 555 astable design can be replaced with the 4060 stage for getting a discretely adjustable ON/OFF periods.

  3. Hi Mr. Swagatam,

    Thanks for your reply . can you tell me how the circuit determine that two hours elapsed and it has to be stopped?

    Amit Desai.

    • Hi Amit,

      as mentioned in the article, the values of P1 and C1 determines the output timing.

      P1 may be adjusted through trial and error to get the required timing.

  4. Hi Mr Swagatam,

    I've tried setting up a timer circuit using this 4060 ic and there seems to be some issue with it not getting the 12v and 24v.
    Could you help take a look at my circuit.
    How do i provide the circuit diagram?
    Appreciate your help and feedback.


  5. Hi dear, this is najeeb i've found your blog very usefull thnx.

    I've mobile charger who's output current is 7.8 volts and amperes are 400 but it's too slow to charge a mobile please suggest me circut to boost up amperes or voltage so that I can charge my mobile in no time.

    I've also tried to add capacitor of 10volt 1000uf to it's output but my nokia mobile doesn't support it. it indicates that charger is not supported please help.

    • Hi Swagatam, I am an average hobbyist. I desire to use your circuit to control a larger load say 2KW rated. and I also want mains isolation from the circuit. I propose to power it up using a 12v regulated power supply from a step down transformer and drive a relay that switches the load. Though I may sound like a nerd, I just need to confirm with you if it is ok to feed pin 16 with 12v +ve and use the output from pin 3 to fire the relay circuit using a power transistor and other passive components. Also would be grateful if you can provide a formula for the RC combination to calculate the latching and unlatching time so that I can choose the appropriate values to get a 3hrs on and 3 hrs off time please. Thanks for your time and response.

  6. i find this really an interesting DIY for my exhaust fan..really appreciate if you could share the exact detail of the component of the above circuit.

    1. wattage of R1,R2,R3,R4,R5?
    .2. type of capacitor for C1, C2, C3, C4?
    3. type and value of P1?

    thanks in advance

    • thanks!
      all the resistors are 1/4watt 5% CFR
      C1,C3,C4 are all metalized polyster type, should NOT be electrolytic.

      C2 is electroytic.

      P1 could be a 1M or a 500k pot

  7. hi again,

    im confused , when i first saw this diagram/ckt..there is a D1 even in your design explaination you mention the D1..but right now even how many times refresh this page i cant see the D1 in the diagram/ckt. pls help do clarify. as of this moment im assembling it

  8. i follow the diagram but my build wont work..may I ask

    1. Theoretically base on the diagram above, what voltage i can measure across?
    a. C4
    b. C2 or Z1
    c. C3
    d. pin#16 and pin#8

    thanks and regards..really appreciate your answers

    • C4 voltage is not important.
      across C2/Z1 it must be equal to the zener voltage.
      C3 is not important
      pin16/8 must also show the zener voltage.

      you ca replace the C4 power supply by a 12v ac/dc adapter, it would give better results and isolation from lethal mains.

  9. Hai, In the above circuit, The components R5,C2,C4,Z1 converts the 220v AC to 12v DC and gives 12v DC to the circuit. Correct me if I am wrong.
    If it is correct means, can I implement that same part of components to other circuits which require 12v DC??
    For example, I made a motion activated circuit which consists of one 555 IC and one PIR sensor and I am powering it by a 12v DC adapter. So can I implement the above same connection (parts) to my PIR circuit to avoid the 12v adapter??

    • Hai, yes it can be done, however C4 will need to be increased if you are using a relay at the output…also preferably C2 should be increased to 1000uF for a better filtration.

      with the above circuit you should be extremely careful not to touch the circuit while it's powered since it's not isolated from mains and could produce a lethal electric shock.

    • Thanx. I made a schematic as u said. Check whether it is correct.
      and what will be the ampere at the output??
      I made R1=1M/ quarter watt,
      C2=1000uf/ what volt, polar or non polar?,
      Am using a 12v relay at the output, so what should be the C2 value/volt and polar or non polar?,
      So the output will 12v DC or AC and how much amp at output?
      I am going to connect this to circuit to a motion sensor circuit which consists of one PIR, one IC555, and one 12v relay.

    • It looks OK to me.

      C2 = 1000uF/25V
      C1 = 0.68uF/400V, if relay does not operate then try 1uF/400V

      add a 1N4007 diode right across the zener diode to reduce stress on the zener, polarity same as the zener

      amp will be 0.07 or 70mA with a 1uF/400V cap

    • Some more doubts I have..
      1. Suppose if I need 500mA or 1A at output means what should be the C1 (connected parallel to zener z1) value??
      2. C2 (1000uf/25v) which is connected parallel to Z1 (zener 15v 1w) is polar and C1(1uf/400v) which connected parallel to R1 is non-polar or polar??
      3. At final, The output will be in 12v DC or AC?

    • anything above 100mA is not recommended through a capacitive power supply…however you can try the following design for acquiring the same, if it works, it would be great.

      1uF/400V will provide 70mA…so adding the value will produce proportionately more current.

      1000uF will be always a polar capacitor.

      1uF/400V is strictly a non-polar

      the output will be a DC. although with some ripples.

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