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SMS Controller Add-ons by PETER SMITH Did you build the SMS Controller published in the October & November 2004 issues? The universal nature of the design means that it can be used in a huge variety of applications. As a result, external interface circuits will sometimes be required. Here are three handy add-ons that we’ve devised after reading a lot of your emails! O UR 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 1kW 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. PIR sensor interface To eliminate the need for a fullblown alarm system, some constructors have asked if it would be possible to connect the output terminals of a PIR sensor (or similar) directly to one of the SMS Controller’s digital inputs. While a typical sensor can be connected directly to the controller, its output will trip many times when an intruder is detected, causing the controller to send multiple messages. A simple solution to this problem is to connect a monostable circuit between the sensor’s output and the controller’s input. The circuit shown in Fig.2 provides a 114s (approx.) positive pulse at its output, measured from the time of the last pulse at the input. Additional input pulses that occur within this period retrigger the monostable via transistor Q1, discharging the timing capacitor (C1) and restarting the timer. The effect is to produce one long positive pulse for the controller, meaning just one alarm message. R1 and C1 can be altered to change the pulse width for your particular application. The additional circuit in Fig.3 can be inserted ahead of the power supply inputs of the monostable (or any other add-on interface that you devise) to protect against transient voltages when reliability is important. Fig.1: this simple test jig uses four pushbutton switches, eight LEDs and eight resistors to demonstrate that the SMS Controller is working correctly. 78  Silicon Chip siliconchip.com.au Fig.2: you can use this circuit to interface the output of a PIR sensor (or some other sensor) to the SMS Controller’s digital inputs. Note that the jumper for the associated 3.3kW pull-up resistor on the controller input should be removed, as it is not required when driven from the 555’s totem-pole output. Low-battery alarm Several constructors have requested a low-battery alarm add-on for the SMS Controller. Although a number of circuits would be suitable for this job, perhaps the easiest approach is to modify the Micropower Battery Protector, published in the July 2004 edition of SILICON CHIP. The original project is designed to disconnect a battery from its load when the terminal voltage drops below a preset value. In this case, we require only the voltage monitoring circuit and can dispense with the Mosfet switch (Q1) and a few other components (see Fig.4). The circuit is based around the MAX8212 Voltage Monitor (IC1), which compares a scaled-down version of the input voltage (set by R1, R2 & VR1) on the THRESH pin with an internal 1.15V reference. When the input (battery) voltage is above the Fig.3: this circuit can be inserted between the power supply and the supply rails to Fig.2 to protect against transient voltages. preset value, the open-drain output on pin 4 is grounded. Conversely, when the input voltage falls below the preset value, the output goes open circuit. Although the circuit could be constructed on a prototyping board, the easiest route would be to partly assemble the original Micropower Battery Protector PC board. A matching overlay diagram appears in Fig.5, showing how to populate the PC board for the low-battery alarm function. The fuse (F1), Mosfet (Q1), 220nF capacitors and zener diode (ZD3) that were part of the original design are all omitted. Two links are added in place of the fuse and Mosfet and a 100W resistor is substituted for the 1MW value to the left of the existing 100W resistor. The battery to be monitored connects to the input terminals and the “+” output connects to one of the inputs of the SMS Controller. The jumper for the associated 3.3kW pull-up resistor on the controller input should remain in place, as the low-battery alarm’s output is open-drain. Where to get stuff Copies of the July 2004 issue are available from our subscription department. A kit of parts for the Micropower Battery Protector is available from Dick Smith Electronics (Cat. No. K3132). Alternatively, blank PC boards can be obtained from RCS Radio (board no. 11107041) while MAX8212 ICs are available from Wiltronics, on the web SC at www.wiltronics.com.au Fig.5 (below): the original PC board for the Micropower Battery Protector can be used to build the low-battery alarm. This diagram shows the revised parts layout. Fig.4: a low-battery alarm add-on is a handy feature. This circuit is based on the Micropower Battery Protector (SILICON CHIP, July 2004). When the battery voltage falls below a preset value, pin 4 of IC1 goes open circuit. siliconchip.com.au April 2005  79