Month: February 2021

It was 2008, I was very excited and I had just brought home my first commercial domestic robot: the Roomba Discovery 4220.

It was second-hand from an Ebay seller who claimed it just needed a new battery, but after getting one, cleaning it up and having it scuttle around my house and workshop cleaning I tried to put it onto its home-base to charge. This of course didn’t go well, and when I attempted to wake it up the next day, it was dead flat despite charging for over 12 hours.

It was the dreaded burned-out U2/U4 MOSFET transistors that were very underrated for the current and heat they would handle to charge the battery. For some time I charged the battery with an external charger and popped it back in to make him clean, but eventually I had to tackle the problem.

At the time, there wasn’t a huge amount of info about this problem, so the advice from many at the time was to just replace these two tiny transistors with an equivalent match. It was difficult, and I wasn’t super across surface-mount components but I managed to change both out with the same replacement. The advice was to just make sure it never ran completely flat, or pre-charge the battery for a bit before putting it in the robot to charge, and the transistors shouldn’t burn out again.

Of course Roombas would sometimes get stuck somewhere for long periods of time, and it only took a couple of years before it burned them out again with a flat battery after it was wedged under a couch all night.

For the next 5 years I charged the battery externally, and this dance went on until I’d had enough, and put it away until a few months ago, when taking it apart I put the vacuum and side brush plugs the wrong way around on reassembly which burned out their transistors. I’d had enough.

The Onboard Charging Fix:

I was determined this year, and with the advice of those who had solved the problem on the Robot Reviews website forum years earlier, I set about putting huge MOSFET transistors that should never overheat or blow no matter the state of the battery. Instead of using tiny surface-mount components, I sourced much larger TO-220 form factor transistors: FQP27P06 (rated for 60v 27amps, way WAY higher than will ever be experienced by the bot). I found space above the battery compartment where they would fit inside the plastic top shell and set about gluing them first to small pieces of flat aluminium (to act as small heatsinks) and then gluing these two assemblies to the plastic case.

I carefully removed the mainboard, taking photos to ensure I get the leads in the right sockets again after (some have the same sockets, they’ll burn your transistors that drive things like the vacuum and side brush out). I carefully de-soldered U2 and U4 transistors from the board (on opposite sides). I like to snip the legs off them using super sharp tiny side-cutters, and then heat the head of them to remove them, to avoid pulling the pads off the board.

Using appropriate thickness wire I then ran the 3 pads that were connected to each of the legs out to the transistors I had glued in place (making sure to match the specs sheets for gate, drain, and source). I made sure to use enough to move the mainboard around, but not so much that it’s hard to coil it back inside the robot (maybe 5cm or so?).

Without the case on, I plugged the Roomba directly into the power supply and bingo – the transistors got warm and it was charging. Or so I thought, as they quickly cooled back down and I wasn’t so sure. So how can we know what the robot is actually doing?

Learning to speak robot:

Turns out the Roombas all have a great serial interface called SCI that has been around since the first models. It’s pretty well documented, but the most useful thing I’ve found is simply the feedback you can get from it charging with highly detailed info about battery voltage and charge.

But to do so, we need a method of plugging this interface into our computer. Computers use RS232 for their serial (or USB serial) which is -12 to +12 volts. The Roomba however runs at TTL levels with is 0v to 5v signalling. It would probably do damage to simply try and directly plug this in. So we need to make a cable and method to plug in.

First up the cable is a mini-din8 (technically the roomba has mini-din7 but din8 will plug into it, and din7 is hard to find). I found in a box of old cables for Apple macs there was a mini din8 that was for Appletalk between machines in the 80s/90s. I was lucky, but if you can’t find this you can look for these through suppliers, or even whole cables on ebay.

We’ll cut one end off and strip the wires carefully apart, and strip the insulation from their tips. Using a multimeter we need to find the TX, RX and Ground pins. When looking at the male connector on the other end of the cable, turn the connector so that the single notch is upright, and the flat part of the connector is at the top. The pins are numbered starting at the bottom left, and from left to right. So the bottom row is 1,2, next row up is 3,4,5, top row is 6,7,8. Use your multimeter on continuity mode (where it will beep to show connection between the two leads) and carefully pick your way through the pins and match these with the coloured leads coming out of the freshly stripped area. On my lead yellow was TX, red was RX, and blue and purple were ground.

Now we need to interface with the computer. I always have on hand for other projects the handy little FTDI Basic boards from Sparkfun. These are great because you can plug in TTL level devices to interface via USB.

So with my FTDI Basic board at the read I simply soldered little pins onto the ends of the leads we identified earlier, plugged the TX line into the Rx of the FTDI, then the RX into the TX of the FTDI, and the two ground leads linked together into the GND of the FTDI.

You’ll then need to use a serial program to show the output from the robot. I use Ubuntu Linux on my computer so I used GTKTerm with the serial port settings of:
port: /dev/ttyUSB0 (this could change depending on what you have plugged in)
baud rate: 57600
parity: none
bits: 8
stop bits: 1
flow control: none

Bingo – you should now be receiving data from the Roomba when it’s plugged into the charger. It’ll report every 1 second what’s happening. The important piece of info here is the charge rate. It should be something around 1500ma when fast charging, something like 280ma when slowing down, and maybe 100ma when trickle charging. If you see negative numbers, you haven’t fixed your transistor problem properly and the robot is discharging.

A first charge can sometimes be something like 16 hours as it attempts to recondition the battery, but as mine was externally charged, I simply unplugged and ran the roomba for a short bit before plugging in again to snap it out of this mode and charge fast. I wouldn’t recommend leaving the roomba alone to charge overnight until you’re sure it’s safe and happy as you could cause a fire if something is wrong and the battery overcharges.

It should just charge on the home base right?

So I excitedly unplugged the charger from the Roomba, and plugged in the home base, then put the Roomba back on the home base, and….. not much. Exercising patience…

So what was up with the home base? Plugging in my serial interface from above showed me that when on the home base the charge rate was in negative numbers. It was actually discharging while on it.

It turns out the home base also has a switching MOSFET transistor inside that turns on the power to the pads only when the Roomba has made contact – it was the exact same type that had failed in the bot, so I replaced it in the same way also, squeezing the bigger transistor to the bottom of the case with glue. This time, the home base worked (be careful on assembly and disassembly, there are screws in 8 places underneath pads and foam).

So it can charge now for the first time since 2010 or so.

But the vacuum fan and side brush is running all the time?

From previous adventures in assembly and disassembly, I’d mixed some of the connectors up and burned some regular bipolar junction transistors (BJT) out, meaning they were shorted (switched on) all the time. Obviously not ideal.

I had to locate the transistors in question: Q35, Q36, Q17

Then replace them with something similar: BC337

BUT: make sure you follow the data sheets, the legs of the new transistors were reversed to the original (originally SS8050), so these needed to be flipped.

Soldered together, reassembled, with the dance complete, finally the vacuum and side brush have stopped, and they start when you start the robot up. Excellent.

Put it back together: pull it back apart

I assembled the casing following all of this testing, and…. the vacuum motor won’t run. Why? A multimeter doesn’t show any power coming from the prongs on the side.

Let’s take it apart again.

On closer inspection we find that all of this plugging and unplugging has made the poor little connectors quite loose, and tracing back the vacuum lead to the mainboard shows that when in test mode for the vacuum (that’s a whole other story to get there) wiggling the lead starts the vacuum.

This is the same kind of connector you’ll find on all of these kinds of things. I’ve found them in my other Neato XV-21 robots, when their wheels start to misbehave and drive erratically.

It’s a simple fix. There isn’t anything as drastic as corrosion, it’s simply the internal prongs in the connector have bent apart and no longer squeeze the pins when plugged in.

All we need to do here is gently use a sewing pin to lift the super micr0-tiny plastic tab on the side of the connector for each pin, and gently slide the pin out of the connector by pulling the cable slowly. DO THIS ONE AT A TIME SO YOU DON’T MIX THEM UP. Even take photos so you don’t accidentally reverse the polarity. This takes some practice and skill so take your time. When you have the connector out, use tiny pliers to squeeze the tiny prongs back together, but don’t be rough. The last thing you want to do is try and make a new connector.

Slide them back together and plug it back in. You should notice straight away that it’s now quite tight.

Test the other connectors to feel if they feel tight. If they feel loose it’s better to do this maintenance now than later.

So, does it work now?

Yes. Yes it does. OH MY GOD IT WORKS. And it works very well.

Of course normal maintenance now applies, and for this model that’s the usual Roomba brush deck clearing and cleaning, wheel and cliff sensor blowing out with air, and troubleshooting when you see odd things happen (like circle dances etc). They can be a little rougher than newer models, and sometimes docking with the home base can take a couple of goes, but they still clean very well and do it reliably.

The biggest thing though with this model of Roomba is that the front wheel is a non-swivel castor, which can be be rough for the little wheel, so I thoroughly recommend cleaning the wheel, making sure it spins easily, and tightly pulling electrical tape around it. Winding it around a few times means that it has a protective layer that you can replace time to time so that it doesn’t grind off. If you’re cleaning concrete like mine does my workshop, definitely paint it with glossy concrete paint because otherwise you’re going to just sand that wheel off.

Let’s keep these things working for as long as we can. It’s something that can reduce so much waste, but also can be another cleaning buddy to keep your lungs healthy indoors. If you don’t want yours, don’t throw it away, offer it for very cheap in online trading websites, or give it to someone who will put it to use again.