OK, I'll admit it - I tend to kill plants. I set them on a sunny window sill, give them an initial drink of water, and promptly forget to water them ever again. Having grown up in a world where objects tend to beep or blink for my attention, the stubborn silence of the average houseplant dooms it to an early death under my care.
But no more! In an effort to solve this problem, and have a bit of fun at the same time, I created the Plant Whisperer. This little device measures the soil moisture every two hours, and reports its status to anyone who might be within earshot - that's right, it talks! The message content depends on the soil condition. For example, if the soil is too dry, the plant might say, "I'M THIRSTY! WATER ME!" If the moisture is okay, it might say "I'M A HAPPY PLANT!" - or maybe something funny like "I SUGGEST YOU BECOME A CARNIVORE." It is also capable of detecting the ambient light level and commenting on that, as well.
Did I mention the cool retro robot voice? Yeah, who wouldn't want that on their houseplant?
The Plant Whisperer is based around a Parallax Propeller, which makes real-time text-to-speech easy to implement. Other major components include a two-channel ADC, a two-hour timer IC, a load switch and an audio amplifier. In total it costs about $30 to build.
Step 1: The Design & Schematics
Included below are the schematics and PCB artwork, so that you may build the PCB required for this project. It is a double sided board (no getting around that, I'm afraid!) I made my board using the photo-etch method, which is the only home method capable of getting good results for surface mount parts.
How it Works:
At the heart of the Plant Whisperer is a Parallax Propeller. It's one of the only microcontrollers out there that is powerful enough to handle real-time text-to-speech. Upon start up, the Propeller initializes the external ADC (since it has no ADC of its own) and the audio amplifier. After a brief period of time, the ADC is polled for soil and light level information. Depending on the results, a random phrase is selected from a list and spoken to anyone who might hear it. However, it will not speak if it's dark - this plant might be needy, but it won't wake you up when you're sleeping! When the Propeller has spoken its message, it sends a signal to the timer to restart its timing cycle.
In order to conserve as much power as possible, an LTC6991 "Timerblox" low-frequency oscillator is used to turn the entire circuit on and off. It is set for a period of about two hours, though it is capable of timing up to 9 hour intervals! So, every two hours the timer output goes high, which turns on a load switch. This activates the rest of the circuit. When just the timer is running, the Plant Whisperer draws only 75uA (yes - microamps!) The timer allows the rest of the circuit to stay on for up to an hour, though it's usually on for only a few seconds.
The moisture probe is pretty straight forward. A voltage is pushed through the soil, resulting in a small current. That current is amplified and converted to a voltage by a transistor. The voltage produced will be somewhere between 0V and 3V, depending on the soil conductivity. That voltage is measured by a Microchip MCP3202 ADC, which then relays the value to the Propeller.
A small 0.25W amplifier is included to drive the speaker. It is activated by the Propeller when needed.
A single LED is used for status; it turns on at the command of the Propeller.
There is a single switch, which is used to manually turn on the Plant Whisperer. When you press the button, it overrides the timer's internal timer, and allows the rest of the circuit to power up and do its thing.
Two AAA batteries are used for power. With a capacity of 1250mAh, the Plant Whisperer should have enough juice to run for months.
My original design was square and BORING. I hope you like the paisley-type swoosh I came up with. The Plant Whisperer is designed to mount directly onto the front of a small planter, with the moisture probes mounted permanently into the side of the planter. If you'd rather not do it this way, two small connection points are provided to connect a set of test probes with wire.