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Teardown: Automatic Garden Timer

By All About Circuits

Courtesy of All About Circuits

Every garden needs some water every once in awhile. With this digital timer valve, watering can be an automated chore. This is a low cost, battery powered, electronic valve that has various easy to program settings. Let's take a look at what makes it suited for the application!

Opening It Up

The Back of the Timer

The Back of the Timer

Opening this timer was pretty simple compared to many of today's electronics. Four self-tapping Phillips head screws held the back panel on. With the screws removed, the back shell made of ABS plastic pulled off easily. With the back panel removed, the electronic valve was exposed as well an another panel with several more screws.

The Valve With the Back Panel Removed

The Valve With the Back Panel Removed

The inner panel had a water sealing silicon-like compound to keep water out of the electronics compartment. This soft adhesive was removed with a small flat head screwdriver and the inner panel was removed to reveal the electronics. To remove the circuit, a few more Phillips head screws were taken out and the board was ready to be removed. Removing the circuit board revealed the back side of the LCD and an elastomeric connector. The display was attached to the front of the case with an adhesive.

The Display Under the circuit Board

The Display Under the Circuit Board

Waterproofing

One of the Two Sealing Gaskets

One of the Two Sealing Gaskets

There were 3 main forms of water proofing found in this gadget. The first we are going to take a look at are gaskets. There were gaskets on the battery drawer and under the front dial. These were relatively small with a width of about 3mm and made of a soft silicone.

The White Sealing Adhesive

The White Sealing Adhesive

The second method of waterproofing in this timer was silicon based adhesive. This was used to seal the panel separating electronics from the valve portion of the device and filling the wire passage used to connect between the two compartments. This adhesive was easy to remove for disassembly.

The Thin Layer of Conformal Coating

The Thin Layer of Conformal Coating

The third form of waterproofing used is a thin coat conformal coating that is applied to many of the components. This is a very thin layer that appears to be selectively hand applied.

Power Supply

The Battery Tray

The Battery Tray

This timer is powered by two AA batteries. The batteries are placed into a tray that is then slid into the device. There are two through hole spring contacts robustly soldered on the circuit board.

The Switching Regulator

The Switching Regulator

The board has a boost power supply on it made from discrete components. On the back side of a PCB, there is a through hole bobbin inductor, L1. A blocking diode, D2, is used to prevent the capacitor, C16, from flowing back through the switching transistor, Q7. The C16 capacitor is located on the back of the circuit board and is a 2200uF 25V through hole capacitor used to smooth the output voltage. The switching transistor, Q7, is switched by the microprocessor and has a pull-down resistor, R14, tied to the gate. On the output of the voltage regulator is a zener diode, D3, that clamps the voltage to around 4.7V.

User Interface

The Display Powered On

The Display Powered On

To display information to the user there is a large LCD display that is always energized. This is a custom display with many segments. The 15 connections required by this LCD are connected to the circuit board through an elastomeric connector.

One of the Dome Switched Taped Down

One of the Dome Switched Taped Down

There are four dome switches used on for user input on this timer. The contacts are exposed copper on the circuit board with the dome contacts taped in place. There is a rubber keypad that depresses the dome.

The Contact for the Dial

The Contact for the Dial

To change the mode and scroll through various settings, a dial is used. This dial uses exposed copper and a wiper to act as a rotary switch. A plastic spring and detents are built into the molded ABS components.

The Decedent and Wiper Mechanism of the Dial

The Decedent and Wiper Mechanism of the Dial

Valve Assembly

The Water Valve

The Water Valve

The valve of this timer is comprised of a solenoid, an ABS tube, a male GHT fitting, and a female GHT fitting. This valve latches open and latches closed by alternating the polarity. The solenoid is switched by an H-Bridge comprised of Q8, Q9, Q10, and Q11.

The FETs in of the H-Bridge

The FETs in of the H-Bridge

Circuit Board

The Top of the Circuit Board

The Top of the Circuit Board

The circuit board in this timer is a 2 layer board with green solder mask and white silk screen on both sides of the board. It has an electroless nickel immersion gold (ENIG) finish over the copper. Both sides of the board have a hatched copper ground plane pour. Various testing points and programming headers are located on the board. This board has a combination of through hole and surface mount components.

The Back of the Circuit Board

The Back of the Circuit Board

Conclusion

A lot of work went into this design to ensure that it is suited well for the application. This is a pretty well-made device, that for the price point is pretty impressive. With the growing popularity of IoT devices, this timer could easily be upgraded to support WiFi or Bluetooth control in the coming years.