Nuts and Volts - April 2014

The 3. 5 mm jack which I used for the project requires that the sense board be mounted with small right angle brackets and holes drilled in the dryer back panel for the jack, as well as the mounting screws. Plastic brackets were fabricated and holes drilled to enable mounting. Other jacks with threaded barrels can simplify mounting. Figure 7 shows the sense board mounted to the back panel, with the current sense transformer in the foreground.

A two-conductor plus shield cable connects the sense board to the main board. There is no electrical connection to the shield at the dryer to avoid potential ground loops. Be mindful to connect the cable wires so that the collector of the transistor on the sense board connects to the signal input of the main board. The shield should connect to the common of the main board.

The Dryer Minder board is mounted next, external to the dryer. I use a shaped plastic plate with clearance for all parts and connectors of the main board. I recommend mounting the main board so that the Heartbeat and Run LEDs are easily seen. Connect the five volt power supply to the main board.

Final Testing

With the main board powered up, the Heartbeat LED should be seen to briefly flash each second. Start the dryer; the Run LED should flash in tandem to the Heartbeat. At 20 minutes less four seconds, the Minder should beep, alerting you to check the lint trap.

Other Applications

This project lends itself to many other applications besides green technology. Since the easy to edit C code includes serial output via the PIC USART Tx pin, display or telemetry applications are plentiful. For example, a running total of on time for a machine or appliance could be easily implemented using a serial LCD for display, such as the SparkFun LCD-09395 which was used during development. By decreasing the USART data rate, the SparkFun WRL- 10534 and WRL-10532 can be used for remote telemetry. I use these for wireless control of a fan system.

An alternate telemetry application would be to sense open and close times of a remote gate. The PIC has rich analog input, such as for sensing current of water heater elements or temperature with an analog temperature sensor. Port C — used for the Heartbeat and Run LEDs — has two additional inputs/outputs that can be used for alarms, moisture sensors, and other peripherals.

Whatever the application, interference with the real time clock (one second counter) connected to pins 2 and 3 or the ICSP header connections should be avoided. Be careful to not push too much code into either the interrupt handler (used by the clock) or the main loop. I found that longer messages to the serial display could have timing issues at the default (9600) baud rate of the serial display when using un-optimized code. If you need more component room for expansion, All Electronics sells a larger solderable breadboard suitable for more advanced projects.