Our three add-on circuits for the SMS Controller are as follows: (1) a test jig; (2) a PIR sensor interface; and (3) a low-battery alarm. Let’s start with the test jig.

Test jig

After suitable message programming, all of the controller’s inputs and outputs can be tested with little more than a length of wire and a multimeter. However, if you want to do some serious bench testing or just want to demonstrate your completed project, a simple test jig with LED indicators can be constructed to make life easier.

As shown in Fig.1, push-button or toggle switches can be connected between each of the inputs and ground. Closing any switch pulls that input down to a logic low (0V) level. When the switch is opened again, the input returns to a logic high.

On the output side, the LEDs are used to provide a visual indication of the state of each channel. All LEDs are powered from +12V via individual 1kΩ current-limiting resistors. When any output is programmed to be "low", the open-collector driver for that channel is switched on, illuminating the respective LED.

Fig.1: this simple test jig uses four pushbutton switches, eight LEDs and eight resistors to demonstrate that the SMS Controller is working correctly.