Hi YouTube, my name’s Geoff and I’m the
VegOilGuy. I recently posted a video of some homemade
caterpillar tracks and I’ve had a few people ask how I was controlling the motors. Well
it wasn’t by radio control, though that’s coming up soon, it’s actually a very simple
electronic process involving switching polarity. Polarity refers to the direction of a magnetic
or electrical field. Let’s look at a simple battery. It’s got
two metal ends that are called terminals. The electrical field travels in a certain
direction and it’s this direction that determines which terminal is which. So each terminal
of the battery is said to have a ‘polarity’ and this polarity is either positive or negative.
This is fairly important information when you’re plugging batteries into devices as
the way electricity flows has a direct effect on electric items and the way they work.
For instance, if we connect the positive and negative from this battery to this motor,
the motor spins in a certain direction. That’s because this is a standard brushed motor and
the electrical current flows through a coil and reacts with magnets that are inside.
Now, if we swap these wires around, the motor spins in the opposite direction. Why? Because
we changed the way the electrical current flowed within the motors coil. We’ve reversed
the polarity… Convinced? No… I wouldn’t be either on
the strength of that footage. But stick with a few more seconds…
In a nutshell reverse polarity is changing the positive to a negative and a negative
to a positive. So when I was controlling my caterpillar tracks,
that’s all I was doing. The tracks are driven by two motors, one per track, and by changing
the polarity I was able to change the way the motors spun and the direction the tracks
travelled. Of course it wouldn’t be very practical
to manually swap the battery connections over whilst trying to drive this model, so I used
a couple of switches instead. You just need switches like this – they’re
called DPDT switches, which stands for Double Pole Double Throw. They’ve usually got on
off on settings which, if memory serves me right, allows them to control four independent
circuits I think… but let’s not worry about that.
The great thing about these cheap, easily available switches is that they can reverse
polarity. They can do all that wire swapping bit for you.
To achieve this, you need to wire up the switches in a particular way. Here you’re looking
at the underside of the switch, where the terminals are. Look at the way the red and
blue wires cross over each other and connect all the end terminals. As long as you remember
that simple cross, you can wire one of these switches.
I’ll do one for you now. The positive goes from the battery to one
of the terminals, then it’s joined to its opposite end terminal. The same thing happens
with the negative. This achieves the cross over and it really helps to think of this
as the REVERSING part. The two remaining centre terminals connect
to the device that needs powering, such as the motors on my caterpillar tracks… but
let’s try and make this a bit clearer using a multimeter.
With the probes of the meter touching the two centre terminals of the switch, we can
measure the voltage across them. At the moment it reads zero as the switch is in the centre
“off” position. With one click, the meter reads 1.62 volts
and as this is a 1.5volt battery, that’s great. Another click turns this off and back
to the centre. Clicking the other way now and the meter read minus 1.62 volts. The voltage
has changed from positive volts to negative volts. The switch has reversed the polarity.
And that’s exactly what I did with my caterpillar tracks. It’s a little harder to see at a
glance what’s goings on, but there’s obviously only two switches involved and the crossover
on each switch is plain to see. Let me show you a simple diagram of what’s
happening here: Just like in the earlier diagram we can see
the underside of the DPDT switches with the crossover. The input is from the battery and
the output of each switch goes to a separate motor – which means the motors can work
independently and even go in opposite directions. So there you go… how did I control my caterpillar
tracks? By reverse polarity switching. Easy as that.
Now, with all that said guys I’d really like to stress an important point here…
I wouldn’t recommend controlling large motors like this. I know I’ve done it and I’ve
told you how I did it, but this really was temporary and just for testing.
Large motors draw a lot of current. When taken quickly from one direction to another, they
can draw massive amounts of current. For this reason it’s ideally necessary to use a speed
controller. This is an electronic device specifically designed to deal with these current peaks
and handle them safely. It’s safer for you, it could also save blowing up your expensive
motors. I’ll be talking more about speed controllers when I convert my caterpillar tracks to Radio
Control. This DPDT switch circuit is fine for tiny
motors, but not bigger items. So that’s it guys… reverse polarity switching
using a simple DPDT switch. I hope you enjoyed this quick tutorial and
if you did please Like it. If you’ve got any questions, drop me a line and if you haven’t
subscribed yet guys, please do. Take a look out for my other videos and above all folks,
thanks for watching.