Need help - simple timer for small motor

Discussion in 'The Projects Forum' started by dbaptie, Apr 7, 2009.

  1. dbaptie

    Thread Starter New Member

    May 7, 2008
    Hey all,

    I am currently working on a project where I need to have a motor that turns on when a switch is pressed, and then turn off again automatically after a certain amount of time. It doesn't seem like it should be too difficult a task, however I'm actually a mech student and have only limited electrical knowledge.

    The motor will need to be on for around anywhere up to 20 seconds, and the motor will need purely a voltage to power it. I have looked into 555 timers, and though I'm not 100% sure of their functionality, it seems that they are mostly useful for making something turn on after a certain amount of time, whereas I'm trying to make something turn off after a certain amount of time. Whilst I don't have an extensive knowledge of electrical stuff, I can read a schematic circuit diagram and hopefully understand the terminology.

    Thanks in advance,

  2. Alhadrad

    New Member

    Apr 7, 2009
    by using a 555 with a period of 40 seconds and a duty cycle of 50% to trigger a npn bjt transistor will do the job.
  3. mbohuntr

    Active Member

    Apr 6, 2009
    Here is a link to a 555 application, use the output to bias a NPN transistor for (T=40sec.) Just plug in the capacitor and resistor values. Just like Alhadrad said. Size the transister to your voltage and current needs.

    Good luck, Mike
  4. dbaptie

    Thread Starter New Member

    May 7, 2008
    Ok cool thanks. I've been having a look at the monostable circuit in that link, which i assume is the one you were referring too, and it seems to be the sort of thing I'm looking for. From what I can see though, as long as i set RC to the right value, the output should rise to +Vcc when the pulse is given, and it will remain that way until the capacitor is charged. For what reason would you suggest to put in a npn transistor at the output?
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    For a "small" motor, under 200 mA, 6-12V this might do. Skip the jumper near R to 555, forgot to block it out. Diode any Si @ motor rating. The 1k really not needed, just makes me nervous dumping large caps into P7. R is 1 Megohme pot. 10k , anything, just dont like putting B+ into P7.