A Mowing Concern

Everybody wants to have the nicest, greenest lawn on the block (although it always seems greener on the other side). But nobody in their right mind actually wants to do all the work needed to make their lawn look perfect. This is especially true when it comes to mowing in the summertime. The heat, the sweating, the precious weekend hours wasted: yuck! It is almost enough to make a person want to wish away the beautiful summer days so that a thick blanket of snow can cover up their troubles.

But these days, there is a lot of help available to the turf enthusiasts of the world. Robot lawn mowers, for instance, are coming way down in price — and some of them even do a pretty decent job. They still cost considerably more than a comparable push mower, however. But with more of them turning up on the second-hand market these days, there is a good chance of scoring one at a good price.

GitHub user eelcogg has noticed a lot of used Robomow RX and RT series robot lawnmowers, in particular, popping up lately. These are some very capable mowers, but the bargains do have a very significant downside — they are frequently missing their base stations. These base stations not only charge the mowers, but they also generate a signal that determines the boundaries of your lawn (and any areas that should be avoided), so that your neighbors’ bushes don’t get an unwelcome “trim.” Without the base station, the Robomow is effectively useless.

Buying a replacement from the manufacturer is an option, but not a very sensible one. Replacement parts for these entry-level models are priced surprisingly high. The base station alone could set you back half the cost of a brand new robot mower. After factoring in the price of the mower itself, what is the point of even buying used at that point? Yet tossing these otherwise perfectly good mowers in the landfill seems horribly wasteful.

So eelcogg did what hackers do and started reverse engineering the system. After connecting an oscilloscope to a working system and capturing the boundary wire signal, a waveform just above 3 kHz at around 2 volts peak-to-peak was observed. By experimenting with resistors, it became clear that the original base station was designed to push a steady current (roughly 150 to 200 milliamps) through the perimeter loop. That current creates the magnetic field the mower relies on to orient itself.

Once the signal was understood, the next step was recreating it with inexpensive hardware. This was done with an ESP32 microcontroller and a trusty LM386 audio amplifier. The measured waveform, stored in a byte array, is continually fed into the ESP32’s onboard DAC. The amplifier boosts the output signal enough to drive the boundary wire, and just like that, the mower happily follows along as if nothing were missing.

By flipping the polarity of the signal, the guide wire can also be used to specify keep-out zones. This is helpful if you need to protect a flower bed or fresh grass seed.

The long-term goal of this project is to pair the homemade base station with a charging circuit so it behaves just like the real thing: broadcasting the boundary while the mower works, shutting off the signal when charging, and toggling back when it is time to mow again. And with that, the worst part of summer could be taken over by affordable robotic assistants.