timer circuit

Thread Starter

me111

Joined May 11, 2012
10
hey im new at this forum
had to ask this one
i have to make a project for which the object is to start a motor for 2 sec and then switch it off automatically even if the power supply stays on. i used the 555 in monostable mode for the purpose but i cant get it to switching off part.
i need help ... PLEASE?
here's the circuit i have been using


 

sheldons

Joined Oct 26, 2011
613
heres a circuit for you to try- you need to change the timing components to suit for your application......and instead of the led substitute your motor and add a diode anode to collector ,cathode to supply on the op transistor.....you may also need to change the op transistor depending on the type of motor you are controlling....
 

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sheldons

Joined Oct 26, 2011
613
if pin 4 is left floating the 555 might not work properly.....pin 4 is the reset pin which if not used must go to supply +(join pin 4 to pin 8),if you ground pin 4 the 555 is reset....heres an output stage you can play with as well with your circuit...
 

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Thread Starter

me111

Joined May 11, 2012
10
im am working on first circuit that u gave with the LED.
i dnt know where am i going wrong but the LED dsnt light for a single second :(
im making the exact same circuit right now :|
 

Thread Starter

me111

Joined May 11, 2012
10
and oh by the way i did connect pin 4 to V+ in the last circuit
i attached an LED on the output with a 1K
the LED did light up but it ddnt switch off then :|
 

Thread Starter

me111

Joined May 11, 2012
10
the LED dsnt switch off :(
i replaced the 220k pot and 10k and 1k and the 47uF on pin 6 and 7 and replaced it with and short them together and still its the same..
where am i going wrong :(
 

tracecom

Joined Apr 16, 2010
3,940
Here is a schematic for a circuit that will work. For 2 seconds, change C1 to 100μf and R1 to 18kΩ. Or you can use your 1μF and 2megΩ for 2.2 seconds.

ETA: I notice that you don't have a .1μF filter cap from pin 8 to ground. You need that, especially if you are using anything other than a battery for power. What is your power source?
 

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Thread Starter

me111

Joined May 11, 2012
10
im using an 2 9V DC supply which are going into the 7812 to give out 12 v
so it should work if a connect it directly right?
 

wayneh

Joined Sep 9, 2010
16,102
Yes, although as the batteries age, or if you are powering more than just an LED, the voltage will likely drop below 12v. That's OK for the 555, I'm just saying...
 
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