9v Battery powered timing circuit

Discussion in 'The Projects Forum' started by djc, Mar 20, 2012.

  1. djc

    Thread Starter New Member

    Mar 20, 2012
    I am working on a project with my son. We need to build a small circuit powered by a 9v battery, which is activated by a push button on/off switch. The objective is to run a small (.13 amp) fan for 2-3 minutes. The fan will need to turn off automatically.

    I have limited knowledge, so please forgive my amateurism. I understand that I may need a 555 chip and a potentiometer to have the time adjustment. I need to know the values for each resistor, capacitors and diodes necessary. I believe that I may also need a relay, I prefer a small 5v if possible due to the lack of space.

    Most importantly, I will need to have step by step wiring instructions, as I can't read a schematic. If I can view a strip boad diagram from both the top and bottom, I ca wire, solder and reconstruct from there. Thanks for helping a dad in need.
  2. wayneh


    Sep 9, 2010
    So, you need a one-shot timer that holds your fan on 2-3 minutes following a button press?

    Congratulations on choosing a 555 circuit - I think it should do the job. I say "think" only because 2-3 minutes is a pretty long time for a 555 circuit. But doable. You'll only need a pot if you want to fine-tune the time. Otherwise you can choose from several resistor-capacitor values and get close enough.

    The 555 can probably power your fan directly without a relay. Do you know what current the fan draws when operated directly off your 9v battery? If the current spec is quoted at 12v, it will be lower at the roughly 8v or less that your battery will output when loaded. If the fan current is too much for a 555, a MOSFET is a better choice than a mechanical relay since it will be cheaper and use less power. They're surprisingly easy to use once you learn them.

    On this site, if you search for 555 timer circuits, I'm sure you can find a one-shot timer. They're used for switch de-bouncing (although usually far less time than 3 min) and many other such things. Radio Shack used to sell a little booklet by Forrest Simms containing a variety of 555 circuits for hobbyists. It'd be a handy guide for you and show how to adjust the timing.
    djc likes this.
  3. Bernard


    Aug 7, 2008
    At the top of this page, find VOL VI - EXPERIMENTS- Ch 8- 555 TIMER- Monostable Multivibrator [ One Shot]; with a few changes should give you a fan driver. Eliminate R3 & C1, connect push-button SW between Gnd & " B"; Make Rt a 100k resistor in series with 1 meg. variable resistor; make Ct 200 micro farrads, neg side to ground. Change D2 to a 1N4002 or similar; replace R2 with fan; R1 & D1 optional; place another 1N4002 across fan, cathode to gnd. See also Vol VI- DC Ckts. - Ch 3 - Pot as voltage divider & also following chapter. Check Vol VI - Ch - 3 - Pot. as voltage divider, & following Ch.; also Vol III- CH 3
    Last edited: Mar 20, 2012
    djc likes this.