# First post... question about small capacitor and DC motor

#### HPS USA

May 2, 2017
23
Hello! Thanks in advance for any help...

I am a novice with circuit design. I have the following project on my bench:

A small DC motor, 3.7 volt, 50,000 rpm. Commonly used for tiny drones.

I need to be able to run the motor for just several seconds each time the circuit is closed.

I assume this could be done with a capacitor?

Thank you!

#### Minder

Apr 24, 2015
3,335
You would be better off with a timer, the motor is going to decrease in rpm as the charge in the cap falls.
Also for several seconds may take quite a large cap.
A simple 555 timer would do it.
M.

#### HPS USA

May 2, 2017
23
You would be better off with a timer, the motor is going to decrease in rpm as the charge in the cap falls.
Also for several seconds may take quite a large cap.
M.

I am okay with the motor RPM decreasing after the initial burst... the object is to spin a small fan blade and create a burst of air.

That said, I am interested in the timer... please school me. ;-)

#### Audioguru

Sep 24, 2016
3,650
The voltage on a charged capacitor drops quickly as it is discharged. Then the motor would slow down then stop.
Maybe you want the motor to run at full speed then stop? Then you need a timer and electronic switch circuit.
A Cmos 555 can be the timer and it can drive a transistor used as the switch.

#### Audioguru

Sep 24, 2016
3,650
The minimum supply voltage for an ordinary 555 (LM555 or NE555) is 4.5V that you do not have. The minimum voltage for a Cmos 555 timer IC (LMC555, TLC555 and ICM7555) is 1.5V.
An ordinary 555 timer will drain the battery all the time but the Cmos one does not.

#### HPS USA

May 2, 2017
23
The minimum supply voltage for an ordinary 555 (LM555 or NE555) is 4.5V that you do not have. The minimum voltage for a Cmos 555 timer IC (LMC555, TLC555 and ICM7555) is 1.5V.
An ordinary 555 timer will drain the battery all the time but the Cmos one does not.

Ok... then it should be a CMOS, because this device needs to be in a ready state if it is turned on. Minutes or hours perhaps before it activates the fan.

#### Audioguru

Sep 24, 2016
3,650
Yes, a Cmos timer will begin timing when a momentary switch closes for a moment. The motor will run at full speed then turn off when the timing finishes.

Here is a circuit with a single pushbutton that turns on the relay when the button is pushed then turns off the relay when the button is pushed a second time. The circuit will work with your 3.7V battery voltage and the relay coil can be your motor:

#### Attachments

• Alternate-action with relay.GIF
7.7 KB · Views: 130

#### HPS USA

May 2, 2017
23
Yes, a Cmos timer will begin timing when a momentary switch closes for a moment. The motor will run at full speed then turn off when the timing finishes.

Here is a circuit with a single pushbutton that turns on the relay when the button is pushed then turns off the relay when the button is pushed a second time. The circuit will work with your 3.7V battery voltage and the relay coil can be your motor:

Okay, we are getting closer. ;-)

The circuit needs to operate upon a momentary close. This is because the 'event' that starts the sequence is the recoil of a rifle briefly closing an inertia switch (no, really).

#### Audioguru

Sep 24, 2016
3,650
I was wrong. A Cmos 555 timer has a very small supply current of about 200uA (0.2mA) all the time.

#### HPS USA

May 2, 2017
23
I was wrong. A Cmos 555 timer has a very small supply current of about 200uA (0.2mA) all the time.

That small draw might not be a deal killer. The device has an on/off rocker switch. The user would not expect to leave it on all the time (days) and not deplete the batteries. The batteries are (2) CR123 3v.

And regarding voltage, the 3.7 volt motor is actually being fed with 6 volts... the overspeeding is working well on the prototypes.

#### HPS USA

May 2, 2017
23
Can the CMOS 555 be powered by a 6v battery? Or does the power need to be regulated?

#### Audioguru

Sep 24, 2016
3,650
Can the CMOS 555 be powered by a 6v battery? Or does the power need to be regulated?
Why not look at its datasheet that tells you all about it.
Maximum supply voltage is 15V, every circuit shown in the datasheet has a 0.1uF capacitor supply bypass capacitor to smooth the supply voltage. Supply voltage regulation is not needed.
The datasheet shows that its output current is too low to drive a motor so an additional transistor is needed.

#### HPS USA

May 2, 2017
23
Why not look at its datasheet that tells you all about it.
Maximum supply voltage is 15V, every circuit shown in the datasheet has a 0.1uF capacitor supply bypass capacitor to smooth the supply voltage. Supply voltage regulation is not needed.
The datasheet shows that its output current is too low to drive a motor so an additional transistor is needed.
Thanks for the reply, sorry I did not look at the datasheet... I am simply out of my area of expertise.
So, the CMOS 555 can accept 6VDC. Excellent.
The transistor acts like a relay, or switch?

#### Minder

Apr 24, 2015
3,335
Probably able to use a 2n7000, less current still.
M.

#### HPS USA

May 2, 2017
23
Probably able to use a 2n7000, less current still.
M.

What is a 2n7000?

Apr 24, 2015
3,335
A Mosfet.
M.

#### Attachments

• FET2n7000.pdf
831 KB · Views: 44

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