Trigger a Microwave RF Sensor

OK, before you think I’m really going down the rabbit hole here… I bought one of these Auto-Flush Kits from with the intention of either hacking it or “using it as inspiration” to produce something similar, shall we say.

It is very nicely designed with a rotary servo motor that is (Schmitt) triggered and pulses a voltage in two directions to get a forward and reverse rotation. A (nicely placed) SMT hall effect sensor with a corresponding RE magnet on the armature are used to stop the rotation during the “un-flush”. A PIC MCU orchestrates everything, the device works well.

My impression is that it is extremely well constructed. The electronics are potted in the lid of the enclosure in such a way that I cannot see what kind of sensor is used without completely destroying the unit. My assumption is that it is using a microwave/RF sensor. It seems reliable and I am very impressed with the quality of the unit, the sensor reads through the ceramic tank top quite easily and reliably.

So onto the point… I have a vacation home which can be vacant for some time. I have a friend who will come into the house on occasion (which is to say he remembers usually at least once in the few days that precede our return) with the intention of keeping the toilets’ seals working and keep water in the bowl (prevents critters and gasses from coming out of that nasty open hole if the water evaporates away).

So, Particle to the rescue. I can easily make one of these, but this would be a lot easier if I could just spoof the microwave sensor on this unit. Would anyone know an easy way to accomplish this? I can easily add another hall effect sensor to restart a timer for a manual flush, but I want to be able to trigger this programmatically.

Any advice out there on triggering this type sensor without tearing the unit apart? I’d prefer it to be electronic (i.e. I don’t want a servo on the top/inside of the tank with some kind of flapper to activate the sensor).

Any ideas would be appreciated

@BulldogLowell, you could use an optical proximity sensor, however for the microwave type, you may want to look into these:

Not sure about this one but for the price, could be worth trying:

with an accompanying review:

There is also the HB100 module but it requires external analog circuitry to work. This module does the job:


Thanks, but I am (right now) looking for a way to hack the device I posted without destroying it in the process. For example, could I simply put a simple coil above the sensor and trigger it? I’m not wanting to affect its function in normal “manual” operation. I’m also trying not to irreversibly damage this $40 gadget whilst I figure this out!

I am going to build one (multiple toilets there) so I’ll take you up on your advice re: sensors, thanks!

@BulldogLowell, doh! The sensor looks for motion so putting a “flap” on a servo and moving it over the area of the sensor would most likely do it. However, you will need something that reflects microwave like foil I suspect.


i think all that is needed is a splice into the motor that actuates the flush. then a method to energize the power circuit to the motor. and finally a method, such as a switch, that is activated by a signal from the photon. this method would essentially bypass the microwave controls of the touchless flush system. wire the motor that does the work to the existing battery pack and place a photon activated switch inbetween. when photon gets signal power is applied to flush motor, toliet flushes.

Yes but the motor is controlled by the PIC and Schmitt trigger with feedback from the Hall effect sensor, so merely powering the motor, which is agreed quite trivial, won’t signal the PIC to take over.

My question is simple… can I spoof that microwave sensor in a solid state fashion?

I think I’m going to try a coil of wire and see if a magnetic field can create enough disruption in the microwave field such that it signals…

Will report back.,

@BulldogLowell, the microwave is emitting at gigahertz frequencies and the module is looking for a rebound from an object. I highly doubt a coil and magnetic field will do anything. I look forward to your experiment!