Wow! Servo Surprise!

I’ve read some concern here about the DC motor bit not being reversible without using a hardware switch. That may one of the reasons I haven’t bought one. Well, that and the lack of precision - they just spin. But servo motors are a completely different thing! They are precise, and repeatable. They (generally) don’t spin, but rather they move.

So last weekend, while shopping at Fry’s I came across a device called a ‘Parallax continuous rotation servo motor’. Now that just seemed weird - cause “continuous” and “servo” seem incompatible. None-the-less, I bought the cheaper one, a medium sized motor, with plastic gears and a standard 3 wire cord and connector for about $17.

Now comes the cool part: The connector looked like it would fit in the same socket as the smaller littleBits motor on the servo Bit. So … I removed the motor from the Bit, and plugged in the Parallax continuous motor. - NOTE: be sure it’s plugged in correctly. Looking at the ‘front’ of the bit (away from the printed name) the signal wire is on the right.

In a simple circuit with a slide dimmer, and it worked! It spun around and around - not to fast, but no slowpoke either. AND when the input voltage was reduced with the slide dimmer, it slowed, stopped at about the midway point, then reversed and spun backward as the voltage was further decreased!

In short, this motor turned my servo Bit into a reversible DC motor Bit! here is a movie to demonstrate:

Ok, so one of my concerns was that this larger motor would draw way too much current, so I tested the current draw using a proto module and a milliammeter. It required about 100mA regardless of the direction or speed. It used about 20mA when at rest. In contrast the standard servo uses about 20mA at rest, and 85mA when moving, so the new motor does use about 18% more power. And this may indeed have an effect on the overall circuitry. But for now, it’s my new reversible motor!


@alexpikkert, I know you were asking about this. Get a look! :eyes:


I did, it is a great solution for the reversible motor.
I have checked the Parallel servo type and I can buy it here in the Netherlands from a local webshop!
My only (small) concern is the power consumption which is +15 %…
@chris101 can you maybe do a test to check if the green servo control bit stays at an acceptable temperature level ? I really hope it will not be smoked… :stuck_out_tongue_winking_eye:

Hey Alex,

I thought I had come across something unique, but then I did a bit of “youtube research”. :wink:

It turns out that continuous rotation servos have been done for a while. Parallax is the first company to jump on it and make one commercially. You make a servo by removing the feedback mechanism, or rather by replacing a potentiometer with a couple resistors, which “fools” the mechanism into thinking the motor has not rotated at all. Here are two 'tubes to look at, first a dry corporate looking one that explains the theory of servo motors:

The second is a guy (and his daughter on the camera - very funny!) who actually mods a motor:

These vids make it look easy, so I’m gonna actually hack one of my little servos (I have 4 little ones - the littleBits motor, and three more from RadioShack.) I’ll post here when it’s done.

ps, @alexpikkert, I ran my Parallax for about 20 minutes, and nothing got hot. However when I added a number (not a plus) bit to monitor the voltage, the servo became erratic. My analog meter showed that the voltage from the power bit had dropped below 5 volts. Perhaps the higher current usb power bit would not have this problem (the supply I use with the regular power bit on my bench is limited to 350mA total.)

1 Like

Hey Chris, thanks for the tubes, I will take a look at them.
About the power drain:
Maybe it is the same issue as with the cloudbit not working on the P1 power and fully functional with the P3.
My P1 also limits on 350 mA and the P3 I use with a wall adapter of 2000 mA.
I dismantled my magic marble machine today and reconnected all my servos (4) to P1 and then to P3 power. In both situations it was ok…
This is real funny so I will investigate further… there must be a reason for erratic behaviour.

I think I found the reason…
I initiated erratic behaviour of the servos in two situations:

Test circuit:
Power-button-number-4 servos-motor-long LED-RGB LED

Erratic behaviour:using Power P1 with wall outlet max 350 mA
Non erratic behaviour: using Power P3 with wall outlet max 2000mA

Erratic behaviour: adding a proto bit before the button with removed VCC contact and connected analog ammeter. This results in erratic behaviour with BOTH P1 or P3 power bits…

My conclusion:
It becomes erratic when there is not enough power or if there is too much resistance in the VCC line.
I also have a protobit with a 100 Ohm resistor in the VCC line (originally designed to block noise in the synth speaker). With this added totally nothing happened. No movement at all on the servos.and no LEDs lighting and silent motor…


1 Like

Thanks for doing the ‘legwork’ Alex. This makes sense. I saw something somewhere about the maximum current draw of the power bit, and I think it was pretty low.

Have you tried your experiment using a fresh 9v alkaline battery?

( @alexpikkert for notification purposes)



First I extended the test circuit with a bargraph, a bright led, another long led and a buzzer.
So I sent my whole green army to the battlefield.

Now the funny conclusion:
Only P3 Usb power is working well.
P1 and fresh alkaline battery initiate erratic behaviour.
Removing the number bit and reconnecting it at the end of the chain made it NON erratic with ALL types of power, P3 with 2000 mA wall outlet, P1with 350 mA wall outletand P1 with fresh battery.
There is something funny with mister number bit I think.

1 Like

@alexpikkert, that IS odd! Perhaps something about how the number bit works makes it interfere with … ?? Pulses the signal line perhaps? I wish we had the oscilloscope bit already!

1 Like


With my little analog ammeter connected in the VCC before the button it was again fully erratic with all 3 power sources, I mean with the number bit at the end.

So now I will dust off my old Leader oscilloscope and do some surgery.
Will be continued. :smiling_imp: