# Is this simple circuit correct?

I made this circuit for my school (they are going to use it if it all works on an open day) so could be nice if it does.

Would the motor rotate anti-clockwise (so if battery was normal way around it would rotate motor clockwise) and the rheostat allows anyone to control and see the speed change?

I was thinking of attaching an LED too so that they can adjust brightness too!

• Do you think a pot sitting between + and - with no other connection fills a purpose? As for how the motor works, you have to post a datasheet or it's anyone's guess. Nov 8 at 14:53
• No, R1 does not do anything useful for you as shown. Nov 8 at 14:54
• your rheostat/potentiometer is in parallel to the motor and doesn't affect it in any way
– Ilya
Nov 8 at 14:54
• Yeah that makes sense, usually Miss does the circuit stuff, I just wanted to help. Nov 8 at 15:10
• Would the motor spin the other way as the battery is flipped please? Nov 8 at 15:11

No, it needs to be in series with the motor: -

But, the resistor/rheostat could get very hot and burn so, you need to design this with care. Also, if you are unsure about the motor direction, you can fit a reversing switch and lock it in place for your open-day.

• Hi, thank you! What’s a reversing switch please? It needs to be safe as kids will use it. P.s. that was really cool how you edited the image! By the way this is on school budget Nov 8 at 15:03
• Wouldn’t flipping the battery + and - make motor spin the other way please? Nov 8 at 15:12
• Yes, after flipping it will spin the other way. What voltage is the motor so we can add a led brightness function? Nov 8 at 15:20
• @Rafael in my experience, motor direction is one of those things that, by bad-luck or wrong thinking inevitably is gotten the wrong way round so, I was suggesting a reversing switch should this happen. If it's easy to swap the battery terminals then no problems. Edited using that old PC tool called paint. Nov 8 at 15:26
• This probably doesn't make any sense either; there should be a series resistor in addition to the pot. Nov 8 at 15:29

Roughly, for simple calculation, the no load impedance of a motor is about 60ohm (3V/50mA)(from ohms law).

To decrease current to half to see speed decrease you need to add another 60 ohms in series (your rheostat) (use 47ohm/1W).

When the rheostat is turned to 47ohms the motor voltage will be also halved (1.5v).

Now, the red led parameters are 1.8v/10mA.

You want the Led to be at full brightness at 3v so calculate the Led current limiting resistor R_lim = (3-1.8)/10mA = 120ohm (use 100ohms).

All together:

simulate this circuit – Schematic created using CircuitLab

As you are turning the rheostat you should see the motor speed changes and also the the change of led brightness.