Modernizing old FischerTechnik 27MC remote control

[pcprogrammer]

I finally got over procrastinating and many hours of playing Generation Zero and finished part one of my project to modernize an old 27MC remote control produced by fischertechnik.

Quite a while back I draw up some schematics for prototype boards and soldered them together. Started with writing the software, but it went slowly and playing games took over. 

But I managed to setup a proof of concept and created a working system. To make it proper I decided to do the transmitter first and made a schematic for the final version. Designed a PCB for it and send it of to JLCPCB. The package arrived a couple of days ago and I assembled it with new parts and some switches from the old transmitter.

The design process took a bit of time with printing out tests to see if the holes aligned with the housing, getting the parts from Aliexpress and making sure everything was correct.

Had to make a bracket to fix the antenna cable in the housing, allowing the real antenna to be external.

Today I finished the assembly and it works. Leaves still some work to be done. For the receiver the final schematic and PCB needs to be made, and the software needs a good cleanup and pouring into a neat set of projects with shared files and proper defines for the IO pins and other settings.

Once all is finished I will open source it.

See the photos below to get an impression of it all.

[pcprogrammer]

Routing the PCB for the receiver is going to be a challenge. 

Attached is a screen capture of the component layout I made in EasyEDA. It is zoomed in. The area where there is still room lies underneath the transceiver module and not a good place to place part of the design.

I know that with SMD it is possible to put parts on both sides of the PCB, but the housing it needs to be placed in has some obstacles that make it a bit tricky to see if it fits.

[coromonadalix]

    continue the good work

[pcprogrammer]

I will.

Had a mate of mine, who is much better at electronics then I am, look at the schematic and he mentioned that the clamping diodes on the motor driver output are not needed. Saves a bit on PCB real estate making it a little bit easier.

To make sure he was right I removed the diodes from my test board and it works just the same. Tried with a lot of motor actions and no crashes on the micro.

[pcprogrammer]

Finished the layout of the receiver PCB, but my gut says it can be optimized by changing part of the component layout. Something that comes when you are routing the traces and you start thinking what if, so I'm going to do that. Can keep parts of the routing as is.

I tried to do a copper pour on the top layer, but it does not look very good and I wonder if soldering the SMD components becomes harder with the cross connection of the ground pads. Maybe more sophisticated software can do a better job on that. Not a big issue for this project, and the transmitter works fine without it.

Attached is another screen cap of what it looks like now in EasyEDA.

[pcprogrammer]

And here is the second version. To my feeling it is better, but feel free to critique.

[pcprogrammer]

While waiting for the receiver PCB to show up, a bit of background on the fischertecknik remote control 30270 and motor and servo set 30375 from ~1982 for which they now ask crazy prizes on the internet by the way.  I saw one set with original boxes on ebay for 719,99 euro.

I got mine many years ago for more acceptable prices when I was buying a lot of fischertechnik from ebay. Regulars will know it is also a hobby of mine.

Since they are having problems with intermittent behavior I decided to modernize them, but now I'm thinking to leave one set to be fully original. Can't play with two of them at the same time anyway. Bringing the transmitter back to original is no problem, but for the receiver it is necessary to remove a bit of plastic in the case to make room for the antenna connector.

Tests showed me that the problem lies in the potentiometers and replacing them will most likely make them work as intended again, but finding the right parts is tricky. With the digital solution it also is far less of a problem.

Attached are scans of the manuals I got from the net, but also have the originals as can be seen on the photos. Also a look into the guts of the things.

[pcprogrammer]

Unfortunately the receiver has problems I need to solve.

The motor driver chosen for the job might be to light weight and problems with back EMF when the big yellow motor is driven showed up. Did not see it in the prototype at first, but today more tests caused it to hang at some point. Have to reconnect the diodes across the motor driver outputs to see if it makes a difference.

So it might be that the diodes across the motor are needed after all, even though the datasheet shows the MOSFET's having their own parasitic diodes, or the power supply setup is not good enough. It does differ from the prototype in that noise suppression has been added and instead of diodes for reverse voltage input protection P-channel mosfets are used. Or with everything crammed on the same board it reacts differently.

In another thread I wrote about the problems I'm having with the MX1508 motor driver(s).

The schematic is attached as png and pdf. Would appreciate any feedback from the specialists.

All capacitors are ceramic smd types, where as on the motor driver modules electrolytes are used. Not sure if that can make a big difference. The connections to the control inputs of the motor drivers are done in the same way as in the prototype, but on different pins. Maybe they need some resistors or filters in between to avoid problems. PWM control frequency is 100Hz.

The prototype can be seen on the first photo in the first post. Only one motor driver module installed on the photo at that time. The other one is plugged in right next to the other one.

A photo of the populated receiver PCB is also attached. The soldering on the motor drivers is not the best, but had to de-solder and re-solder the IC's a couple of times, as can be read in the thread about the MX1508 versus the MX1508RX.

[pcprogrammer]

Never mind. Found the problem, and I chuck it up to inexperience with motors and motor drivers.

What happened is that the big yellow fischertechnik motor needs a lot of current to start, and this causes the voltage to drop all over the system. The buck regulator needs more then 4V on the input to keep on working and it dropped to just under 4V. The MCU sees this as a power down and up again and resets. Receives the signal from the transmitter and tries to start the motor again, causing another reset, and so on.

The 1508S IC's did not like this and died. Tried it with a MX1508 today, and it survived, but as it can only handle peak current of 2.5A it warms up. The prototype works due to a slightly different power supply setup, and during most tests in the beginning the MCU was powered from USB. 

When supplied from a battery there is a diode, followed by a 47uF capacitor to ground, in series with the buck converter so the voltage drop on the input is dampened by it allowing the MCU to keep on going. Once the motor is running the current drops down to ~700mA, which is fine for the MX1508, but when the speed is reduced it does starts to heat up more.

By using the FET Q1 as a diode due to disconnecting R1 and placing it in the place of ZD1 and adding a bigger capacitor across C2 the MCU stays up and running, and the motor can run. The power supply needs to be capable of delivering the start up current though.

All in all a redesign is needed. As is, it can work with small motors but not with the one it basically is all about. Not a big deal. The MX1508's are always handy for other projects with smaller motors and the PCB's costed less then 4 euros, not a big loss.

I should have looked more into the original receiver. It uses a H bridge made up of BD433's and BD434's. These can do 4A continuous and 7A peak. 

A good learning experience.

Attached are a couple of screen capture of the measurements on motor startup. The first one has the input voltage on the yellow trace, the MCU voltage on the cyan trace, the MCU analog supply on the green trace and the reset line on the magenta trace. The second one has the input voltage on the green trace, the buck converter input on the cyan trace and the MCU voltage on the yellow trace. The magenta trace is not used in this case.

[pcprogrammer]

I have found a new motor driver and mocked up the board to test it on one channel. It does the job so now it is time to design a new board and have that produced.

As can be seen in the pictures I also added a bunch of pins to attach the probes for looking at the voltages around the board.

[pcprogrammer]

Finally, the receiver board is finished. The long delay was caused by the postal services. I needed 100uF ceramic capacitors and I ordered a bunch from Aliexpress. Unfortunately that order was refused to be processed by the shipping company and was returned to the seller. 

So I ordered from another shop, this time with choice and fast shipping. (13 days) And again something got f.....d up. This time the local postal services. The package arrived at a local distribution center on the 11th of March and I'm still waiting for it. 

Thus again an new order on Aliexpress with 9 day delivery, and that arrived yesterday. Took only 10 days from placing the order to arrive, 5 days before the estimated delivery day. 

Today I soldered the last components to the board and loaded the firmware. Tested it on the bench and it works.

Still need to work on the servo control code a bit because there is some overshoot. Some improving on the transmitter for the range and center point of the joysticks is needed too. Also have to add the receiver select code to the project and give it a final test, but that is easy.

Attached are some pictures of the finished receiver.

[pcprogrammer]

Made a short video of the working model. Almost drove it of the table.   



Also made a repository for the project. So far only added the hardware parts. Still have to tweak bits of the software.

Edit: corrected a spelling error.

[pcprogrammer]

Uploaded the source code to the repository even though not completely finished.

The transmitter needs some code for battery voltage monitoring, but that is not crucial for the operation of the system.

For now I call the project done.