Let's Make Robots!

Big Chaser

Follow me carrying a 30kg child.





The deal is, this 'bot will (hopefully) follow me around. For me the challenge is to do something bigger than Lego scale.


So far, I have attached a car windscreen wiper motor to a pneumatic sack truck tyre. It's going to be BIG. The clue is in the name.

The wheel consists of two metal plates which are bolted together. I made a couple of extra long bolts and bolted a bit of mild steel to it. Bolted to the middle of the bit of steel is a windscreen wiper motor from a Nissan Micra.

Next weekend, I'm off down to the junk yard / scrap heap / scrappie (depending on your geographical location) to get another wiper motor. When I have two fitted to some sort of a chassis, I'll report back.

I envisage something powered by a couple of big FETs under PWM control with DPDT relays to reverse direction.


Got my second wiper motor today. Found another scrap Nissan Micra down at my second favourite place (second only to the electronics store). Got the motor for $30. That's Not too bad. I could have haggled him down to $20, but I was in a hurry.


Remembered something else about these motors: They have a momentary switch internal to them which gets a hit every rotation. Cool! BUILT-IN SPEED SENSOR!!



It would appear that my eldest son has decided that my new robot platform would make an excellent trailer for him to tow his younger brother behind his bike. As soon as I obtain custody of it, I'm ready to retrofit the motors.


I may or may not keep the plastic seat which has been bolted on top. At this stage, I'm thinking "radio-controlled pram" or maybe just automatic pram which follows me around. At last - a robot with a purpose!



Built a new platform. It turns out the trailer is proving popular. New photo above. Hit a tiny problem. Not a showstopper. I mentioned a built-in SPCO switch in the wiper motor. I had hoped to use it as a rotation/speed counter. The problem is that the the motor "ground" wire is also the "normally open" contact of the switch. The plan was to connect the "normally closed" side of the switch to ground and run the switch common to the pic as an input (with pullup resistor enabled). Great when the N/C switch is closed, but for a brief second each rotation the switch changes over and the motor "ground" becomes switched into the PIC. Not a real problem for the PIC and if the motor is running "backward" (ie, ground is powered), the PIC will be able to increment a counter based onhe rising edge. HOWEVER, if the motor is running "forward" (ie ground is grounded) then the pic will detect no change.



BTW, this is 95kg (that's about 210lbs for our American friends) of ME

standing on the robot chassis. Early experiments show that it will easily plod along with a 25kg child sitting on it!!

I'm off to work on the circuit. I have a provisional one, but I want to separate the motor 12V from the TTL 5V onto two different PCBs.


I thought about it a bit more and decided the drive board needed its own logic supply. (See next post.) The reasons will become clear as I explain further my modular robot electronics concept.


New photo, new video. OKay, I wanted to prove that the controller board worked, so I programmed it to read the outputs from a radio control receiver. These are converted to 2's compliment byte values in the PIC and fed to the PWM controllers. The result is a pair of radio controlled windscreen washer motors with pneumatic wheels.

The platform currently has a "training" wheel. It seems likely this will be a feature for a while. I want to concentrate on the modular concept first, the person following, then the balancing act.

I'm very very happy with the motor control board and now I also have a good, usable routine should I ever build an I2C radio control receiver module.

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 your Big Chaser looks very impressive. One of my dreams is, to build my own Segway.

One tip from me, although i am no electronics experts, but I know, when switching inductive loads like relays or motors is the need of flyback diodes, to kill the reverse voltage of the inductive. Without flyback diodes, you will kill your electronics! Maybe this fried your PIC.

Further information can be found here:



Greatings Peter 

Yes. I've been getting that beaten into me here over the last few weeks. Thanks!

In Nomine Patri Et Filii Spiritus Sancti,

may its little PIC soul rest in piece(s) 

Ok, communication through the shout box can be somewhat strange so I'll just post here instead.

Just to recap: BOA shouted that he is using FETs rated for 40 A and that they get very hot when running 10 A motors, and this seems a little strange to me.

After checking out your schematic on the motor controller on this page, it seems that you're driving the FET gates directly from the PIC which I would guess means that the maximum voltage at the gates are ~5V. Since you're driving the n-channel FETs as switches to turn a 12 V potential on and off, you should use 12 V to turn them on and 0V to turn them on. If you use less than 12 V to turn them on, they will only be "half open" and the resistance across the drain and source (VDS) will be far greater than when fully open and so they will dissipate a lot more heat.

You can use optocouplers to bridge between the 5V from the PIC and the 12V from the motor voltage, but these have an upper limit to how fast they can be pulsed.

Ah, bother. This is a feature of FETs which I never understood properly. Is it the case that the drain/source voltage can only equal the gate/source voltage?

I don't really get this. I thought the FET would operate like a big switch if I supplied the +12V to the motor direct and used the FET to periodically ground the motor.

Assuming I've understood you correctly, couldn't I just use a transistor to open the FET gate?connect the base to the PIC, the emitter to the FET gate and the collector to +12?

1) Yes when using n-channel MOSFETs as switches I think you would want turn it on with a voltage at least as high as what is seen on the drain. If you'd like to read some stuff on how these things work , try wikipedia and search for "mosfet" - great description although a bit cryptical at times.

2) Well it is in fact doing that right now, but it's just getting a wee bit too hot :-). As far as remember it has to do with the n-channel not forming tight and narrow when the gate-source voltage is lower than the drain-source. I'm not really an expert on MOSFETs though.

3) I would definitely try that solution just to see if it works - but again I'm no expert. BJTs are current controlled whereas MOSFETs are voltage controlled and the MOSFET gate is a small capacitor so after a very short time when capacitor is charged, there will flow no more current into the gate. I don't know if it's a problem or not.

TransFET.jpgI've now tried this on the breadboard with and without the transistor.

Without the Tx, the FET delivers only 5V as predicted. (I don't know why I hadn't the sense to scope this before).

With the Tx, the FET delivers the full 12V. Trouble is it doesn't tun off. I guess there's a path through source-> gate-> emitter and some emitter/collector leakage to keep the gate open.

Sometimes transistors suck.

Tomorrow's experiment is 12V through a resistor to collector, gate connected to collector. So, +logic to the base should open the gate. 

Please excuse me if I am adding redundant info:
There is no path through source->gate-> emitter in the original drawing, FETs use a capacitor to change states.
That is why the diagram is always drawn with G separate from the rest of the FET. What is happening in the original drawing, is you are charging the Gate capacitor when you turn it on. With your design their is no way to Discharge the Gate, so it remains open until at some point it will leak away.

The way I understand it is, FETs operate by charging and discharging of the Gate capacitor, vs Transistors which work by a small current flow. FET Gates need to be all the way on or all the way off, Transistors can accept more variability in the Base current flow.

When Transistors heat up they conduct more through the Collector -> Emitter.

When FETs heat up they conduct less through the Drain->Source. That is why you can use FETs in parallel to build massive switches, because they are stable, in a parallel Transistor circuit, you would get run-away heat/voltage in one of the transistors, vs the auto-balancing behavior of the FETs.

I have mostly seen N channel FETs and NPN Transistors connected to the low side of motors. Typically when I put a NPN on the high side it will generate more heat vs attaching it on the low side, I assumed this was the case with the N channel FETs, but have no empirical data.


Is the last diagram the one the fried your PIC? I don't see why it should unless the resistor off of the PIC is not biased enough to prevent the over-sourcing the pin. 

Thanks for another piece of the puzzle!!

No the last diagram didn't fry the PIC. In fact, it was teh one which gave me full ooomph and no heat.

I only applied the rework to one of the two channels on my PCB, then having discovered the sweet smell of success, I applied it to the second channel and that's when teh PIC fried. I'm looking for a solder spash or something just now.

PS - found nothing. Removed the PIC. Applied +5V to the output pin holes in turn with +12V applied. Found no strange feedbacks. Decided to risk a new chip. Plugged it in and it works fine. I guess it was just his time to go.

12V -> resistor -> collector, collector -> gate? That sounds like a pull-up resistor on the gate meaning it will always be on ;-).

Using an optocoupler you could probably use the circuit I attached below. R2 is a pull-down resistor to keep the gate closed when there is no activity through the optocoupler. The PC817 optocoupler is a cheap and small (4-pin dip) chip that also gives you the benefit that it isolates your PIC completely from the noisy motor circuit. As previously mentioned (in another post) there is an upper limit to how fast this can be pulsed so you'd probably have some gap between stop and slow forward and between fast forward and full throttle. The PC817 has rise and fall times of about typical 5 us.