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Piezobuzzer PZ1 is used as an audible indicator for true, fake, random touches and system halt. It is controlled from port pin PD6 of the microcontroller with the help of transistor T1. The iButton socket is connected to port pin PD2. Pull-up resistor R1 is used as required from the 1-wire protocol. Port pin PD4 controls the relay operation through optocoupler PC817 (IC2). The door locking mechanism, say, for a slide door, is connected to the contacts of relay RL1, which closes after some time automatically.

Whenever someone attempts to gain access by touching the iButton (IC5) using his own iButton, the firmware inside the AVR reads its unique ID and matches with the ID in the firmware. If the ID matches, port pin PD4 goes high, the internal LED of the optocoupler (IC2) glows and relay RL1 energises for the predefined time. Simultaneously, the buzzer sounds to indicate grant of access.

Thereafter, relay RL1 de-energises. The buzzer gets the PWM signal to produce sound from port pin PD6 of the microcontroller. The 1-wire communication is done through interrupt port pin PD2 (INT0), so the AVR does not have to poll the pin but respond when any change in signal is detected on the interrupt pin. Switch S1 is used for manual reset.

Port pin PD1 of the microcontroller is used to interface with the hyper terminal of the PC through RS-N 232 interface MAX232 IC (IC3) for iButton verification and checking. The microcontroller provides a transmit channel for serial data transfer. Transmit data pin (TXD) is specified at port pin PD1. The microcontroller is connected to T1 IN (pin 11) of MAX232. T1 OUT (pin 14) of IC3 is connected to pin 2 of the Comport connector. The signals provided on these pins are TTL-level and must be boosted and inverted through a MAX232 converter to comply with the RS-232 standard.

The MAX232 has two internal charge pumps that convert +5V into ±10V (unloaded) for RS-232 driver operation. The first converter uses capacitor C6 to double the +5V input to +10VM on capacitor C8 at pin 2. The second converter uses capacitor C5 to invert +10V to -10V on capacitor C4 at pin 6.

The power supply for this circuit is derived from 230V, 50Hz AC mains. Transformer X1 steps down 230V, 50Hz AC mains to deliver a secondary output of 9V, 300 mA. The transformer output is rectified by a full-wave rectifier comprising diodes D1 through D4, filtered by capacitor C1 and regulated by IC 7806 (IC4). Capacitor C2 bypasses the ripples present in the regulated supply. LED1 acts as the power indicator and R5 limits the current through LED1.

Construction and testing

An actual-size, single-side PCB for the secure digital access system is shown in Fig. 5(View as PDF) and its component layout in Fig. 6(View as PDF). Assemble the circuit on a PCB as it minimises time and assembly errors. Carefully assemble the components and double-check for any overlooked error. Connect the assembled circuit to the COM port of the computer. The iButton access code and message is transferred is to the PC through the COM port using the Hyper Terminal program.

Download PCB and component layout PDFs (Fig. 5, 6): click here

Software

The software for this project is given at the end of this article. It is written in ‘Basic’ language and compiled using Bascom-AVR compiler. The source program is well commented and easy to understand.

Burn the finally obtained Intel hex code file into the AVR’s flash memory using a suitable programmer. The microcontroller uses an 8MHz internally generated clock. To activate, program fuse bytes as follows:

Fuse low byte = D4
Fuse high byte = 99

The source program is developed quickly with the help of the BASCOMAVR library function. The 1wrest, 1wread and 1wwrite functions are used for checking the presence of 1-wire device, reading of the unique ID of iButton and writing to the 1-wire device, respectively. Sound function is used for generating the beep sound from the buzzer. The iButton is connected to hardware interrupt pin INT0 so that any change on the pin can be detected and further processing for 1-wire done.

The program execution starts by initialising the input/output ports, the interrupt pin and its level of detection. Global interrupts are enabled and the interrupt service routine is ready to be executed when any interrupt is received. As soon as the normal program execution is interrupted, an interrupt is issued and the control of execution enters the interrupt service routine, where iButton is given reset command first and then the ROM command to fetch the unique ID of the iButton. This fetched ID is compared with the unique ID programmed in the firmware. If there is a mismatch between the two IDs, a fake parameter counter is incremented and the relay connected to port PD4 de-energises. If the two IDs match, the relay connected to port PD4 energises and simultaneously the buzzer connected to port PD6 sounds. If the unique ID is wrong sequentially five times, the system enters the major warning state where the buzzer sounds continuously and it can only be stopped by resetting the system.

EFY note. The source code of this project is available on the link given below.

Download source code: click here


Dr H.N. Pandya is a reader and head of Department of Electronics at Saurashtra University, Rajkot, and Ph.D guide of the coauthor. Chirutkar Harshadkumar Govindrao is a research scholar in Department of Electronics, Saurashtra University.

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