Construction

In the solution I had in mind, the feed is provided by a small storage tower that drops the food from the top with a little help from gravity through an opening in the aquarium cover. You can build a feeder tower like this with parts from your local hardware store or order the materials online from craft stores. In my case, a desk pen holder served as a starting point; this part is readily available in many stores and are often made of transparent plastic, which is ideal for monitoring the fill level.

Now it's time to saw and drill holes. After that, I added the servo and the mechanical system. The whole thing was then mounted on a base plate, which also bears the Zero W and a small circuit board with a status LED and control buttons (Figure 3).

Figure 3: The feed tower has two separate chambers, which are operated by a mechanical system.

The feed outlet is operated by a specially developed mechanism. Depending on the direction of rotation of the servo, one of the two tubes of the pen holder opens. For this purpose, a straight shaft passes through both tubes, with a closure for each of the two openings sitting on the shaft.

Because of the angular offset, only one of the two shafts can open at any given time during a short left or right turn, and it is closed again automatically a short time later. Small paddles act as the closures to avoid a shaft remaining permanently open and ensure that the desired amount of feed is delivered (Figure 4).

Figure 4: The outlet mechanism: Depending on the direction of rotation, feed is delivered from one of the two chambers.

The 160-liter aquarium with low fish stock levels does not need more than a heaped teaspoon of dry food every day. The supply in the vending machine will be fine for a three-week vacation. If you need a larger reserve, adjust the setup to your needs in terms of diameter and height of tubes.

Control

The feeding process starts automatically after applying the supply voltage. As specified in the Raspberry Pi crontab, the vending machine feeds the fish once a day. On Saturdays, feed is taken from the second shaft by rotating the servo at the same angle, but in the opposite direction. Once a minute the housing LED flashes a visual ready signal. To track all events on the device, the LED uses various flash codes (Table 1).

What you can't see in the circuit diagram (Figure 5) are the two push buttons on the case, which are connected to the Zero W general purpose input output (GPIO) pins to trigger a feed in the first shaft and for a controlled system shutdown.

Figure 5: The Zero W controls the servomotor and the LED from the GPIO pins. The circuit diagram shows the wiring setup.

Because the feeder is on the local network, it can be maintained over the network, as well. To do so, log in to a terminal over SSH with:

ssh pi@feeder

You can then make changes to the crontab or control script. If needed, you can disable the automatic feeder by calling:

python3 /home/pi/scripts/feed.py 4

The 4 option tells the device to enter maintenance mode without any further actions.

Observations

One interesting and positive side effect is the ability to see whether the fish are getting any feed from the feeder. The control script triggers a snapshot through the aquarium glass with a delay of 10 seconds after operating the servomotor. The simple Pi Cam Rev 1.3 serves this function well. Because I only need to grab an image of a section of the front of the aquarium, image quality is secondary.

The connection from the vending machine on the top of the aquarium to the Pi cam on the front of the aquarium requires a long camera cable (about 30cm for my setup) that depends on the height of the aquarium cover. A narrow aluminum plate fixes the small camera housing a few centimeters in front of the screen.

To get sharp images, you need to set the shortest possible focal length on the Pi cam. To do this, turn the adjustment ring of the small lens very carefully with a tool and grab a few test images to check the setting.

The connection to the network is primarily designed to post the image grabbed after feeding, so I can check it online. It also opens up some possibilities for remote operation. This setup requires an HTTP server on the Zero W. The installation of the lean lighttpd web server [2] and PHP on the Raspberry Pi is well documented online, so I will not go into further detail here.

The feeder's web page (Figure 6), where you can check the last feeding image and trigger a feeding along with a snapshot with three buttons, is implemented in PHP and resides in the Raspberry Pi's www directory. The code for this and the Python script to control the feeder is on the project's GitHub page [3], and an English version is on the Linux Magazine download site [4].

Figure 6: Monitoring over the web: The feeder releases the food, and the fish flock to devour it.

To access the web interface on the local network, simply enter the local IP address of the Zero W in a web browser. To monitor the system remotely while on vacation, you need to enable the server on the Zero W in your router for the web and assign a permanent IP. However, if you already use a website with FTP access, you can alternatively upload the current feeding image to your own web space and view it there.