Submitted by:
G.MANIKANTA
L.THARUN KUMAR REDDY
B.VIKRAM RAMA SWAMY

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Chapter-1
INTRODUCTION
The MAX-232 of PCs is a versatile tool to use. Using it, you can develop several PC based applications. Controlling a triac to switch on/off the gadgets is one of them, where the computer decides the sequence of switching. The circuit given here can be used to eight devices through MAX-232 data lines.
MAX-232:
The MAX232 is an integrated circuit that converts signals from an RS-232 serial port to signals suitable for use in TTL compatible digital logic circuits. The MAX232 is a dual driver/receiver and typically converts the RX, TX, CTS and RTS signals.
The drivers provide RS-232 voltage level outputs (approx. ± 7.5 V) from a single + 5 V supply via on-chip charge pumps and external capacitors. This makes it useful for implementing RS-232 in devices that otherwise do not need any voltages outside the 0 V to + 5 V range, as power supply design does not need to be made more complicated just for driving the RS-232 in this case. The receivers reduce RS-232 inputs (which may be as high as ± 25 V), to standard 5 V TTL levels. These receivers have a typical threshold of 1.3 V, and a typical hysteresis of 0.5 V. The later MAX232A is backwards compatible with the original MAX232 but may operate at higher baud rates and can use smaller external capacitors – 0.1 μF in place of the 1.0 μF capacitors used with the original device. The newer MAX3232 is also backwards compatible, but operates at a broader voltage range, from 3 to 5.5 V.
VOLTAGE LEVELS:
It is helpful to understand what occurs to the voltage levels. When a MAX232 IC receives a TTL level to convert, it changes a TTL Logic 0 to between +3 and +15 V, and changes TTL Logic 1 to between -3 to -15 V, and vice versa for converting from RS232 to TTL. This can be confusing when you realize that the RS232 Data Transmission voltages at a certain logic state are opposite from the RS232 Control Line voltages at the same logic state. To clarify the matter, see the table below. For more information see RS-232 Voltage Levels.
RS232:
In telecommunications, RS-232 (Recommended Standard 232) is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors.
Chapter-2
CIRCUIT DESCRIPTION & WORKING
2.1 Circuit description
Fig. 1 shows the circuit for multiple device switching through the serial port. The circuit comprises zero-crossing optically-isolated triac driver MOC3041,triac BT136 and a few discrete components. Parallel-port pins 2 through 9 are connected to pin 1 of IC1 through IC8 via current limiting resistor R1 through R8, respectively.Pin2 of IC1 through IC8 is joined together and connected Pins 18 through 25 of the parallel port.
The zero crossing, optically isolated triacdriver is an effective solution for interface application between the parallel port and AC power loads. When pin2 of the parallel port is low and the internal LED of IC1 is ‘off’, the AC line voltage appears across main terminals of both the triac and the triacdriver. When pin2 of the parallel port goes high and sufficient current Ift is supplied to the internal LED, the triac driver latches ‘on’. This action introduces a gate current in the main triac (BT136), triggering it from the blocking state into full condition. Once triggered, the voltage the main terminals drops to a very low value lower than its holding current, thus forcing the triac driver into ‘off’ state, even when sufficient current for the internal LED is applied. The power triac remains conducting until the load current drops below its holding current-a situation that occurs every half cycle. The actual duty cycle for the triac drive is very short(in the range of 1 to 3µs).When sufficient current for conduction of internal LED is present, the power triac will re-trigger every half cycle of the AC line voltage until the internal LED is switched off and the power triac has gone through a zero current point(see Fig2)Use of resistor R9n is not mandatory when the load is resistive since the current is limited by gate-trigger currentIgt of the power triac. However, resistor R9 prevents possible destruction of the triac driver in those application where the load is highly inductive. Unintentional phase control of the main triac may happen if current-limiting resistor R9 is too high in value. The function of this resistor is to limit the current through the triac driver in case the main triac is forced into the non-conductive state close to the peak of the line voltage and the energy stored in ‘snubber’ capacitor C1 is discharged into the driver.