Help Request Building a Monostable 555 Timer driving a mini DC motor

[UNUM]

Need help in figuring out a problem with a monostable 555 timer circuit. I have a basic
monostable 555 circuit and I'm trying to drive a mini DC motor. When I don't have the motor in the circuit and I measure the pulse output, everything operates as expected, giving an approximate 5 sec pulse at pin 3. When I put the motor in the circuit, it continuously runs and the output at pin 3 never shuts off. Any suggestions?

 

[DerStrom8]

Welcome to Electro-Tech-Online, UNUM!
How are you triggering the timer? I have had that same problem before, and it turned out that the electro-magnetic field emitted by the motor induced enough current in pin 2 to keep triggering it. Maybe this is your problem?
Hope this helps!
Der Strom

 

[sparkyIVXX]

Are you sinking or sourcing
sinking is having the chip supply the ground
sourcing is supplying the power

 

[Boncuk]

As already mentioned pin2 might receive continuous trigger pulses caused by the back EMF of your motor.

You should decouple the timer IC directly at its supply pins with a 100µF elctrolytic cap paralled with a 100nF and use a free wheeling diode on the motor to eliminate back EMF as much as possible.

Also triggering pin2 non-retriggerable (via a 10 to 100nF cap) and a pull-up resistor at the same pin should solve the problem.

Boncuk

 

[UNUM]

Unum

Thanks DerStrom8, sparkyIVXX, and Boncuk,

Right now I have a benchtop circuit and I'm triggering the timer with two wires. One coming directly from pin 2 (see schematic diagram attached).

I'm sourcing ... the timer is supplying the power to the DC motor.

I'll rework the circuit to decouple the timer IC as suggested and see if that works.

Thank you.
UNUM

 

[ronv]

You should move the 10K resistor to the other side of the switch so that pin 2 is tied to 6 volts until the switch is pressed. Right now it is "floating" just ready to pick up noise from the motor. You really don't need the 3.3k resistor to ground, just a jumper to ground would give more margin. I think the .01 cap is really on pin 5?

 

[Boncuk]

You didn't use decoupling caps as suggested and additionally you didn't use a free wheeling diode across the motor.

Use the circuit I attached.

Boncuk

 

[UNUM]

ronv, you're right ... that should have been pin 5
boncuk, you're right too ... my attachment was my original schematic without the suggested changes ... thank you for your schematic - i'll give it a go