# 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
2
0
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.

-David

Apr 7, 2009
1
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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 Senior Member

Apr 6, 2009
430
32
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.
http://www.play-hookey.com/digital/timer_555.html

Good luck, Mike

4. ### dbaptie Thread Starter New Member

May 7, 2008
2
0
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 Expert

Aug 7, 2008
5,019
567
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.

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