Automatic Washroom Light Switch

We turn On the lights in our washroom when we enter it and turn them off when we leave. We sometimes forget to turn Off the lights after leaving the washroom. This may lead to power wastage and also the lifetime of the lights may decrease. To avoid these problems, we are going to make a circuit which automatically turns On the lights when a person enters the washroom and it automatically turns it Off when he leaves it.
By automating this, there are many advantages like, the person need not care to turn On the light always when he is using the washroom. The circuit which we are doing does it automatically for that person. Also, the person need not turn it off after using the washroom. There is no fear that he forgets to turn it Off. The circuit is also designed to consume lesser power so that the circuit can be used in any household or public washrooms without worrying about the power bills.
 Automatic Washroom Light Switch Circuit Diagram:

The operation of the circuit is as follows. When the door of the washroom is opened and closed, the circuit turns switches On the light using a relay. When the door opens and closes for the second time, the circuit turns Off the light by turning off the relay.

The element which is used to detect the opening and closing of the door is a reed switch. There are two types of reed switches. We are using the one which will be closed in normal state and open when there is a magnetic field nearby. A reed switch electrically is just a relay kind of component but unlike a relay which activates when a coil voltage is supplied, the reed switch activates when a magnetic field is detected in the vicinity. The circuit is given a power supply of 9V. The pin-16 of IC 4017 is given 9V. The pin-8 of 4017 is given to ground.
The circuit uses IC 741 op-amp as a comparator arranged such that its output is high by default when the door is closed. The circuit is attached to the door frame whereas a permanent magnet is attached to the door in such a way that it comes closer to the reed switch when it is closed. The IC 4017 is made to alternate between each door open and door close. When the door is opened and closed for one time, the circuit turns On the relay and the the Light turns ON. When the door is opened and closed for the next time, the circuit turns Off the relay and the light turns off. The IC 4017 is capable of counting upto nine counts but we are restricting it to count only two and reset back. The ability of this IC to adjust the count value as desired helped us in this project to use it as a one bit counter.
When the door is closed, the reed switch opens and hence the op-amp output which is the 6th pin of IC 741 is HIGH. When the door is closed, the pin-6 of IC 741 is turned Off. When the door is closed back, it triggers the IC 4017 decade counter and hence the relay toggles ON and OFF for each door open and close operation.

Simple LED Blinking Circuits

LED (Light Emitting Diode) is a semiconductor light emitting diode. We know that diode allows the current in one direction and does not allow the reverse current which will affect the components in the circuit. LED also do the same function but will emit a small light when it allowed the current, which will give the sign or visual indication to the normal human that circuit is working. There are lots of applications using LEDs. They are mainly used for visual indication in any electronic devices, measuring and interacting with the process, displaying the pictures in TV or in any advertisement hoarding, etc.

Two LED blinking circuits are given below. First one is dancing bi-color LEDs (two different color LEDs) where the two color LED will run in sequence. In the second circuit, we will blink the LEDs in regular period of time.

Dancing Bi – Color LED Circuit:

Generally we use small voltage bulbs in the dancing bulbs. This circuit is mainly used in the occasions, decoration articles or in visual indication sign boards etc. In this project, we use bi-color LEDs for sequential running light.

  

Block Diagram of Bi-Color LED Circuit:

Timer is used for setting the sequential flow rate for the bi-color LED panel. The CD4017 is a decade counter which provides the timing and will make the LED ON/OFF according to the time determined.

Main Components in this Circuit:

CD4017: CD4017 is a 16 pin decade counter and only 10 pins are used for output. The 4017 will get triggered by the clock pulses. Main operation of the decade counter is as follows: When a clock pulse is taken as an input, only one output is made high for first clock pulse and remaining all output pins will be made low. For the second clock pulse, another output pin is made high and remaining all pins are made low and so on. Time period of the output pin is high according to the width of the pulse. CD4017 is used in many applications where counter is needed.

CD4017 clock pulses from output pins timing diagram is shown below:

Bi-Color LED Dancing Lights Circuit Diagram:

 

Circuit Diagram Explanation:

In the Bi-color LED, it should be connected to the counter as shown in the circuit. The anode of first LED in bicolor LED is connected to the anode of second LED of 10^th bicolor LED and in the same fashion, the remaining LEDs are connected, only the second anode of first bicolor LED is connected to the reset pin of the CD4017. All the cathode of bicolor LEDs is made ground.
The main operation of this circuit depends on the 555 timer which is set in astable multivibrator mode and decade counter CD4017; the 555 timer will generate low frequency clock pulse and give input to the decade counter which will make the sequential running of the LEDs.
Variable resistor can vary the resistance which will change width of the pulse. If pulse width is changed, the time period of running the LEDs will also get changed. We run the LEDs in fast or slow. Running speed can be altered by variable resistor. The first anode of 10^th bicolor LED is made short to the reset pin of the decode counter for continuous running of lights.

LED Flasher Circuit:

LED Flasher is a simple circuit which will blink the LEDs in regular time period. This circuit can be used for decoration purpose or can be used for a signaling purpose and many more.

Block Diagram of LED Flasher Circuit:

the 555 timer is used to generate the PWM signal which will cause the LEDs to blink. The speed of the blinking by LED is determined by the potentiometer connected to the 555 timer. The PNP transistor is used to flash or blinks the LEDs.LED Flasher Circuit Diagram:

 

Circuit Explanation:

The 555 timer is made to be configured as a astable multivibrator. The potentiometer which is connected to the timer should be preset and also to adjust the blinking or flashing speed of the LEDs.
The bicolor LEDs are used in this circuit and connected to each other as shown in the schematic. The PWM signal is the output of the 555 timer given to transistor, which acts as an inverter. When the pulse generated by 555 timers is low, transistor will get ON and LEDs will get ON. When the input of transistor is high, transistor will get OFF and LEDs are made OFF. This ON/OFF of LEDs will go for every pulse width signal cycle. This mechanism will make the LEDs flashing.

LED Blinking Circuits Applications: 

Dancing LED circuit can be used for any visual sign indication in any highways or it can be used in advertisement hoarding also.

LED blinking circuit can be used in signaling purpose (It can be used as signal for help, if you are in danger)
LED blinking circuit can be used as flashing beacon.
LED blinking circuit can be used as vehicle indicator when it is broke down in the middle of the road. It can be used in operation theaters or offices as an indication that you are engaged in work.
There are lots of applications with these two circuits.

Remote Audio Level Indicator Circuit

The normal level-indicator circuits which are available in the market require connections to be made to the output of the player, which may not be easily accessible. The audio level indicator circuit described here removes this restriction as it may be placed close to the player’s speakers and yet the desired effect can be realised.
As shown in the circuit, signals are picked up by the condenser microphone, which get further amplified by the noninverting amplifier built around one of the four op-amps of LM324. The remaining three, along with four op-amps of the second LM324, are used as seven comparators to work as the level detector, giving seven output levels through seven coloured LEDs.
The sensitivity of the audio level indicator circuit may be improved by varying the 220k potentiometer. If a fine adjustment is desired, a 4.7-kilohm potentiometer may be connected in series resistors with the 220k potentiometer.
Remote audio level indicator circuit diagram

Cardiophone Circuit

The human heartbeat can be made audible by using this cardiophone circuit. It is basically an audio circuit coupled to a probe made specially for the purpose of picking up electric signal from the human heart.
To get the best signal, place the probe’s electrodes to a point close to the heart. The preferred point is just below the left breast with negative electrode pointing the the left of the sternum.
After constructing the circuit, the output A must be calibrated to null through the potentiometer P1. This is important for the circuit to function properly. The signal coming from output A can then be connected to either a low-frequency amplifier or an oscilloscope.
The signal coming from output B is a square wave in sync with the heart rhythm. This signal can be used to trigger a final stage amplifier or other circuits. The heartbeat can be heard from the final amplifier’s speaker.
The special signal probe is shown in bellow figure and the simplest way to make this probe is to use a 1 cm x 10cm blank pcb board. Following the design, the non-shaded parts of the pcb board must be etched away. The un-etched copper plate must be covered with solder to protect it from corrosion and to facilitate good contact with the skin. Take note that two of the probe’s electrodes are marked negative and positive respectively. It is of utmost importance to use a shielded twisted pair wire for the cable connecting the probe with the cardiophone circuit.
Cardiophone circuit diagram

Cardiophone probe

Cardiophone PCB layout

Mobile Phone Multipower Unit

Most of the mobile phone complaints are related to the power supply. This is mainly because of improper charging and use of non-recommended chargers and low-quality batteries. Depending upon the make and model, the charging time of mobile phones varies from 1 hour to 3 hours. The charging current is also different for different models. So it is better to use the charger specified by the company only.

Equipment for measuring the capacity or the backup time of a battery are not readily available in the market. But by measuring the charging and discharging currents, the approximate backup time of a battery can be found out. For example, charge a battery of 4.8V, 400mA rating for 1 minute and check whether it can discharge for 1 minute through a 400mA torch bulb.

If the battery is discharged fully, it would not get charged again through a normal charger. The battery would require an initial charge or  boosting.

Here’s a multipurpose circuit (see Fig. 1) for battery boosting as well as normal cellphone battery charging. You can boost the battery at 400 mA for two minutes and then charge using a normal charger or this car charger. Other features of this circuit are a variable regulated DC voltage output (0-12V), voltage display panel meter, provision to measure charging/discharging current, ammeter, and micro soldering iron.

The supply voltage for the whole unit is given by a 230V/18V, 2A transformer. This is rectified by a bridge rectifier (1N5408 x 4), filtered, and given to the ICs (IC1, IC2, and IC3) of regulator 7812. Another regulator IC (7805) gives regulated 5V to the voltage display panel meter. The center tapping of the transformer is connected to LM7805. The panel meter displays the variable output voltage (0-12V). It is a 3½-digit LED display module, which is readily available in the market.

 Since pin 2 of IC1 (IC 7812) is grounded through a 2-kilo-ohm preset (VR1), it produces an output voltage of 13V (12V+voltage drop across the preset). You can increase the output of IC1 up to 18V by varying the preset.

The output voltage of IC1 is given to transistor T1 (S8050) through 1kilo-ohm potentiometer VR4 and 2.2-kilo-ohm resistor R1. Potentiometer VR4 acts as the boosting voltage controller. The function of 2.2 kilo-ohm resistor is to limit the boosting current. Transistor T1 acts as the pre-current amplifier.

Power transistor T2 (3055) works as the current amplifier, while 1-kilo-ohm resistor R2 acts as a current limiter to transistor T2. The emitter of T2 is connected to point C of a 12V, 200-ohm relay.

In normal condition, discharge switch S2 is opened and points ‘A’ and ‘a’ of the relay are closed to ‘C’ and ‘c’, respectively. Hence the boosting out terminals get a supply of 12V maximum. This voltage can be varied from 0 to 12V by using boosting voltage controller VR4. The mobile phone battery is boosted from this variable DC output. The boosting voltage is also given to the digital voltmeter or panel meter for display of the variable DC output.

A volume unit (VU) meter is used for measuring the charging and discharging current. It works from 0.1V to 1V (max.). Within this voltage range, it reads a load current of maximum 1 amp. The maximum current reading can be set with the help of 10-kilo-ohm preset VR5 connected to the VU meter.

The VU meter, boosting terminal, and car charger are connected to ground through 1-ohm, 5W resistor R4. So the VU meter displays the current taken while charging and discharging according to the voltage drop across this resistor.

When discharge switch S2 is switched on, relay RL energises and points ‘A’ and ‘a’ come in contact with points ‘B’ and ‘b’, respectively. Now if a battery is connected to the boosting terminals, it discharges through the discharge bulb and the V-U meter reads the discharging current.

On adjusting 2-kilo-ohm preset VR2, IC2 (IC 7812) gives an output of 16.5V. This voltage is given to transistor T3 (3055), which works as a current amplifier, through 1-kilo-ohm potentiometer VR3 and 1-kilo-ohm current-limiting resistor R3. Potentiometer VR3 works as the voltage control for micro-tip soldering iron. A standard micro-tip iron needs 16V DC maximum to heat up to 300°C. The micro iron current amplifier drives a micro iron of 1W to 25W.

Regulator IC3 (IC 7812) produces an output of 13V by making use of diodes D5 and D6 connected in series at pin 2 towards ground for dropping 1V. This output is given to power transistor T4 (3055), which works as a current amplifier. An output of 12.5V is obtained at the collector of T4, which is given to the car charger.

The car charger works on DC and it has an inbuilt voltage regulator and current limiter. The input of car charger varies from 4V to 12V. The outputs of different car chargers depend on the make and model. Each charger has its own connector for connection to the mobile phone. The charger holder given here can be used to connect any model of car charger for charging a mobile phone battery.

Normally, mobile phones have a voltage rating of 2.4V to 4.8V. Be careful while connecting a substitute power supply, as even a slight increase in the applied voltage can damage the phone.

Some phones go dead due to a shorted RF power amplifier. If a battery is connected to such a handset, it may suddenly get fully discharged and become dead.

Thus it is advantageous to verify the overall loading of the handset before connecting an external power supply or battery. For the purpose, you can use an ohmmeter. The battery terminal of the handset reads 5 to 50 ohms in one direction and 1 kilo-ohm to 150 kilo-ohms in the other direction. If a wide difference is noted, the circuit is either open or shorted.

A Miniaturized Photoelectric Twilight Switch

Controls the ignition of lamps or other electrical loads when the brightness of the environment falls below an adjustable threshold.

There are circuits that never get old and withstand time, progress, and emergence and spread of a variety of microcontrollers and boards, perhaps because there is always a need of something simple, cheap and that gets the job done.

Among those there’s the twilight switch, that will never get old, at least until  there will be the need for something to turn the lights on and off based on ambient light.

A lot of twilight switches are available on the market, what to invent then? This time we tried to act on size and we propose a project that, despite being a classic, adapts to one of the needs of modern times: miniaturization. In fact, what we present is a magnificent twilight switch a with relay output to control in low voltage loads (to handle loads operating at 220 VAC just use an  adequate capacity relay to control the exchange) whose printed circuit board, complete with all components, measures just 29x29x15 mm!

Diagram

The diagram is very simple: an operational amplifier mounted as a comparator and a photoresistor we use to detect the level of lighting in the environment. To complete the circuit you’ll find also the actuator, which in our case is a small relay. As said, to detect the ambience illumination we use of a photoresistor dubbed FR1, which has maximum resistance in the dark (about 1 Mohm) and the minimum (some hundreds of ohms) at the exposure to a strong intensity light: this allows to detect the level of illumination of the environment on the basis of the value taken by the resistive component. To do this we insert a photoresistor in a voltage divider, doing so by referring to the voltage obtained at the output from the latter.

By using a divider we can use a comparator with a defined voltage threshold corresponding to a certain brightness value: in correspondence of the threshold the will be energized. The inclusion of a trimmer in the comparator network leaves us free to define the brightness level at which the relay must be activated.

Let’s see the operation in detail, assuming we start from a total darkness condition, with the FR1 resistance much higher than that of R3 and R5, and then the voltage present between the node formed by it with R3 and R6 being approximately equal to that which is detected downstream of the D1 diode and thus the same that feeds the U1.

the cursor of the RV1 trimmer is far from the positive line (ie, the cathode of D1), the voltage present on the inverting input of the operational amplifier is lower than that localized on the non-inverting input. In this way, the U1 output goes to logic high and gets polarized basing on T1. T1 current collector conducts current and simultaneously feeds the relay coil and the R2/LD1 bipole, illuminating of the LED (thus signaling the activation of the twilight switch) and energizing RL1. RL1 switch closes between the C and NO, closing the circuit of the load connected to them.

When the light in the environment increases, the voltage brought from R6 and D3 to pin 5 of the U1 begins to lower, due to the fact that the resistance of the photoresistor starts to fall progressively, in relation with the intensity of the light that hits the sensitive surface. At some point in this process, the non-inverting input is at a lower potential than that brought on the inverting one from the RV1 trimmer and the comparator inverts the state of its output, which switches to low level and leaves the transistor T1 inhibited.

Now, the LED turns off and the movable load of the relay falls. If the brightness of the environment drops back, pin 7 of the U1 gets back to high and the relay is energized again (also the LED turns back).

The point at which the relay is energized and the LED lights up is regulated by RV1 trimmer. By bringing the cursor of this component to ground reduces you reduce the voltage at which the comparator returns to rest: plenty of light it is required to deactivate the relay. On the contrary, moving towards the D1 diode cathode, the voltage that pin 5 must reach grows and to trigger the relay you must submit to the photoresistor higher resistance values (and therefore a darker ambience).

Looking at the comparator circuit you can notice that the D3 serves to bring R6 potential to the operational, avoiding that C3 to discharge through it. D3 is inserted to achieve, together with the C3 capacitor, a kind of anti-commuting network indispensable to avoid the comparator to switch at the occurrence of a very short light variation (due for example to overflight of a bird or the movement of a person or a car). Both in the transition from dark to light and vice versa, the relay will start to ring because the comparator switches repeatedly since the resistance value taken from the photoresistor oscillates in the neighborhood of the one which determines the switching. The latter situation could also be avoided by retroacting U1 in positive, thus realizing a circuit hysteresis (with two different switching thresholds): in this case we opted for the normal comparator, filtering the voltage supplied by the divider that comprises the photoresistor by means of a RC network.

Having said that we should only look at the power supply circuit, consisting of the D1 diode (which protects the input terminals from reverse polarity) is located downstream of the power supply and the C1 and C2 capacitors (the purpose of which is to filter the power supply, especially if taken from a power line.

The circuit requires DC voltage to work, better if stabilized (otherwise the comparator may oscillate in the vicinity of the threshold voltage, despite the network RC filter), at values between 9 and 12 volt. The required current is of the order of 40 milliamps, thanks also to the adoption of a sub-miniature relay whose coil absorbs very little (about 15 mA).

A final detail concerns the D2 diode antiparallelly placed respect to the RL1 coil and therefore interdicted in normal conditions; this component is used when the transistor, by interdicting, interrupts the current in the coil of the relay while, due to the inductive nature of the inductances, the same reacts by generating an reverse extravoltage. This tension, if not suppressed by the fact that the diode, inversely polarized, short circuits it, would damage the collector junction of T1.

Localizer with SIM908 module

The device is based on a GSM/GPRS module with included GPS. Its main function is to detect and communicate its own geographical position using, on the choice, the cellular phone reference system or the GPS. Its small dimensions are due to the use, for the first time, of a GSM/GPRS module integrating the GPS receiver. That is the SIM908 a recent product by SIMCOM.
Circuit schematic of the localizer

The circuit of the localizer is build around two boards, one with the SIM908 on board and the second one including the microcontroller and the battery charger for the lithium battery. To get the GPS working will be necessary to complete the localizer circuit with an appropriate antenna.
The circuit includes the mother board, mounting the microcontroller and its circuitry, and the daughter board mounting the communication module, the block named GSM in the schema. The reference numbers associated to the contacts on the GSM block correspond to the pins of the connector linking the daughterboard to the motherboard.

The program running in the microcontroller U1, one PIC18LF6722, waits for an incoming event or for the button P1 being pushed. While the button is pressed the line RB1, provided with internal pull-up resistor, switches from logical level 1 to the logical level 0,
In case of incoming of an SMS message, the program reacts depending on the content of the message that could be a configuration message or a geographical position request.
Let focus on the process aimed to retrieve the geographical position, that is quite the same in both cases of manual request and P1 pushing (alarm or S.O.S.). After the request has been detected, the program in the PIC microcontroller sends commands to the cellular module in order to have it connected to Internet in GPRS mode. Then connects to the Google Maps server and sends a request of position based on the identification of the cell the SIMCom module is connected to; then again loop waiting for data on the RX channel of the UART. While got data back with the position (Latitude and Longitude) and accuracy, it is a composed string with the appropriate link to Google Maps and sent to the requesting phone, or to the phone number stored in memory coupled to the alarm function.
If the cellular phone is an Android Smartphone or an iPhone, the link received in the SMS can open Google Maps directly on the area where the localizer is present. In the other cases the message contains the coordinates and other data.
The GSM module is managed by the microcontroller using the lines: RF1 (pin 8 on connector) through which it detects the incoming calls through the Ring Indicator (RI), RC7/RX1 (pin 14 on the cellular board); these last two are the lines, respectively, of reception and transmission of the UART used for receiving and sending SMS messages. The same two lines are used for managing the SIM908, unless the reset and power supply lines. Power supply is controlled by line RC2 that affects pin 1 of the cellular module in order to turn ON and OFF the SIM908 and to enable the phone after initialization. Lines cited before are common to the GSM and GPS section of SIM908.

Both the boards are powered by the switch SW1 from the 3.6 volt Li-ion battery connected to the + and – poles of the PWR connector.
Many capacitors inserted along the positive power line filter noises coming from the cellular during transmission, that could lock the microcontroller.
To save power, there is the features to “hibernate” the localizer for a maximum period of 240 seconds. This limits the power consumption of the micro and enables the possibility to put also the cellular in standby mode and reduce the system clock speed. In standby mode the cellular soaks only 6 mA of power. On SIM900 the Slow Clock can be enabled using the AT extended command AT+CSCLK=2. This command enables the Slow Clock mode automatically when there is no traffic on serial port and disabled it while new data comes in.
The microcontroller exits the “hibernate” mode when a new call come in or at the end of the period (240 seconds), in this case the microcontroller checks for possible SMS received. In the case, it executes and delivers the requests and, at the end, turn back in “hibernate” mode. While in “hibernate” mode the microcontroller can’t detect incoming SMS, this way a possible urgent request will be delayed until the microcontroller will wake up. To overcome this situation could be suitable to anticipate a phone call, may be of just one ring, and then send the SMS. The call will awake the microcontroller and the SMS will be detected immediately.
One specific application of the locator is its use as a motion detection sensor. In this mode the detection is based on the change of the cells the cellular is connected to, the microcontroller stores in memory the current cell and the neighbour cells; if the cellular commutes within these cells it means that it is almost standing still; if it commutes on cells outside the range in memory it means it is moving and, for instance, an alarm can be activated.

This approach can be little sensitive in case of scarcely inhabited lands with a little number of cells: in this case the motion, to be detected, requires a movement quite long.
The power supply comes from a 3.6 volt battery that can be charged by a miniUSB plug that allows recharge from any PC. The power regulator is the chip MCP73831T in SMD version (package SOT-23), it can supply up to 550 mA at 3.6 – 3.7 volt to fully charge a lithium or Li-Po battery with an input supply of 3.75 – 6 volt.
The chip charges the battery with a constant current. The charging current (Ireg) is set by the value of the resistor connected to pin 2, whose value is calculate as:

Ireg = 1.000/R

where the value of R is in ohm and Ireg in Ampere.
As an example with R of 4.7 kohm the current will be 212 mA, while with an R of 2.2 kohm the current will be 454 mA. While pin 5 is opened the chip goes into sleep mode and soaks only 2 µA (therefore the pin 5 can be used as enable).

The LED LD3, while ON means the battery is in charge, and when turns OFF, it means the battery has been fully charged.

The battery charger circuit is completed by the capacitors C1 and C2, while C1 filters high frequency noises and C2 filters alternate noises and stabilize the power to 5 volt.

The cellular board

The SIM908 is mounted on the board by a male connector of 20 pins, (two rows of 10 pins each) step 2 mm.

The active contacts of the connectors are:

the power supply, VCC on pins 17 and 19; the power on control line (ON/OFF);
the serial communication lines to and from the GSM module (TXD and RXD);
the ground (GND) on pins 18 and 20;
the Ring Indicator.

In the electrical schema can be seen that the line ON/OFF is used by the microcontroller to manage the switching on and shutting down of the GSM1 module, that it is always under power, delivered by Vcc line on pins 55, 56 and 57 62 and 63; the line includes an internal pull-up resistor and goes ON at logical 0. Therefore, to switch ON the cellular module, the microcontroller have to put high the line ON/OFF (pin 1 on connector). This saturates the T2 transistor, that drive to low the line PWR of GSM1.
The control of reset is at switch-on time, therefore there is no reset line and the jumper J2 must be left open.
Now take a look at the lines reserved to communication. The SIM908 module has two different serial ports on board, one for the cellular section of the module and one for the GPS section. The first UART uses the pins 12, 14 and 10 of the connector; the serial port of GPS communicates on GPSTXD and GPSRXD lines (contacts 4 and 5). Actually the serial port on cellular allows the full management of SIM908 module, therefore it can be used to configure and communicate with the GPS receiver, in order to call for data about satellite status and geographical positioning, and to transfer them to the microcontroller. This is the approach followed in the design of this project.
From the GPSTXD/GPSRXD serial port flows a continuous stream of data in NMEA format, if the microcontroller would have used this source of data for the GPS, it would have been overloaded by data, loosing the possibility to perform the other functions.
Apart from serial communication lines, the IR line (pin 18) of SIM908 module it is used to keep the microcontroller informed about incoming calls.

There are also four audio lines on the module, two for the microphone, MIC1P and MIC1N, (pin 19 and 20) and two for the loudspeaker SPK1N and SPK1P (pin 21 and 22) that are not used in this project.

The antenna for the GSM module is connected directly to its own connector on SIM908 module. The module has a second antenna connector for the GPS antenna. Both active and passive antennas can be connected to the SIM908 module, in the first case the antenna can be powered directly by the module, by closing the jumper J1.
The transistor T1 it is used to drive the signal power LED, its base gets polarized by the logical level on the pin 52 (NETLIGHT) of GSM1 module. The collector of the transistor is connected to the pin 3 of the connector through which the microcontroller gets informed on the presence of the GSM network and on the quality of the connection.
At last the description of the SIM of the cellular phone, named SIM1 and positioned in the classical housing; the contacts on the card are SIM_CLK (clock), SIM_RST (reset) and SIM_DATA (data channel) while the line SIM_VDD (filtered by the capacitor C1) is used to switch on and off the SIM by the SIM908 module. The first three lines have resistors in series to protect the SIM908 module in case the SIM would be inserted incorrectly, short-circuiting the contacts.

Part list SIM908 breakout
[code]
C1: 220 nF (0805)
C2: 100 nF (0805)
C3: 470 µF 6,3 VL (CASE-D)
C4: 470 µF 6,3 VL (CASE-D)
C5: 100 nF (0805)
LD1: LED (0805)
R1: 15 ohm (0805)
R2: 15 ohm (0805)
R3: 15 ohm (0805)
R4: 10 kohm (0805)
R5: 4,7 kohm (0805)
R6: 10 kohm (0805)
R7: 330 ohm (0805)
R8: 10 kohm (0805)
R9: 4,7 kohm (0805)
T1: BC817
T2: BC817
GSM1: GSM SIM908
SIM1: SIM-CARD
- Strip male 2×10 2mm
[/code]

Setting and commands

Once completed and programmed, the localizer must be appropriately configured, using a common cellular phone. Some commands are password protected while other commands will be executed if coming from one out of the eighth qualified phone numbers stored in memory.

The same eighth phone numbers are the only qualified to ask for geographical positioning.

The predefined password (automatically set at every system reset) is 12345; it is possible to change the password with one of choice (five digit long) sending the command by SMS containing the text PWDnewpwd;pwd, where pwd is the current password and newpwd is the new one.