Safety Guard For Blind

Helping in need is humanity. In order to help blind people and make their life easier here, the innovative group Dreamlover Technology designs a circuit “safety guard for the blind” using microcontroller AT89c51. This system detects obstacles within 1 meter in the path and alerts the users.

Circuit description

The entire hardware circuit of safety guard for blind is divided into four major sections. i.e. transmitter, receiver, switching section, and the voice processing section.

Transmitter section:- The logic of this section is simple and is build around most versatile IC NE555 (IC1), configured as astable multivibrator to produce frequency about 38 KHz. This is so, because IR module receiver used here works in range of 38 KHz frequency. Timing component of this circuit is resistor VR1 and R1 and capacitor C2, determine the range of oscillating. Where, formula of generated frequency (F) from transmitter section is given by

F = 1.443 / (R1 + 2R2) C2

The output frequency from pin 3 of IC1 is fed to base of transistor T1 through resistor R3. Transistor T1 is configured as Darlington pair with transistor T2 in order to drive pair of IR LED connected in series as shown in figure 2. LED1 is used as transmitted signal indicator and resistor R5 is used as current limiter.

ir transmitter

Receiver section:- When transmitted signal from transmitter is obstructed by any object lies in its path then the transmitted signal is reflected. The reflected signal is received by IR receiver module (TSOP1738) and is fed to base NPN transistor T3 through resistor R6 for amplification. Amplified signal is again amplified by pair of transistor T4 and T5 up to necessary level. The output from emitter of transistor T5 is fed through resistor R13 to pin 2 (non-inverting input) of comparator (IC3) where inverting input is connected to zener diode ZD1. The output is taken from pin 7 and is fed to I/O port P1.0 of microcontroller AT89c51 (IC4). The output of pin 7 of IC3 goes high only when non-inverting input is more the inverting input (2.2V).

receiver circuit for safety guard for the blind

Switching (Microcontroller) Section:- Here, we use microcontroller AT89c51 for only switching circuit because of availability and to show how to interface an embedded system in a homemade project. Microcontroller section is replaced by any switching circuit. Pin 1 through 4 (P1.0 through P1.3) is used as input port and pin 21 through 24 (P2.0 through P2.3) is used as output port. The output from pin 7 of IC3 is connected to pin 1 (P1.0) and its corresponding output is obtained at pin 21 (P2.0). The output from microcontroller (IC4) is fed to base of PNP transistor (T6) through resistor R18.

microcontroller connection of shefty guard for blind

The output of transistor T6 from collector is fed through resistor R19 to base of relay driver transistor T7 in order to energize the relay RL1 and is indicated by glowing LEDLED4.

When no signal is applied at input port 1.0, output port 2.0 is high and vice-versa. As transistor T6 is PNP, low at P2.0 conduct it.

Voice Processor:- The entire circuit of voice processing is designed around IC ISD1420 (IC5) which is 28-pin chip by Winbond. The voice message up to 20 seconds is recorded by this IC.

The recording process is done by pressing switch SW4 and the recorded message is played by using pin 23. The condenser microphone pickup the voice message and changed it into corresponding electrical signal connected to pin 17 and 18 via capacitor C13 and C10 respectively.

sound processing for sefty guard for blind

Pin 14 and pin 15 is connected to headphone jack or loudspeaker through coupling capacitor C14 for playing message. Volume is controlled by variable resistor VR2.

Software:- The source code for safety guard for blind is written in C-programming language.

Click here to download the source code


Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)

R1, R2, R9, R18, R19, R20, R21, R28 = 1 KΩ

R3, R6, R11, R13 = 22 Ω

R4 = 47 Ω

R5, R7 = 100 Ω

R8 = 15 Ω

R10 = 68 Ω

R12 = 4.7 KΩ

R14, R15, R16 = 470 Ω

R17a = 620 Ω

R17b, R22, R23 = 100 KΩ

R24 = 5.1 KΩ

R26, R27 = 10 KΩ

VR1 = 2KΩ (preset)

VR2 = 4.7 KΩ (preset)


C1, C4 = 100 µF/25V (electrolytic)

C2, C3 = 0.01 µF (ceramic disc)

C5, C6 = 33 pF (ceramic disc)

C7 = 0.001 µF (ceramic disc)

C8, C14 = 4.7 µF/16V (electrolytic)

C9, C10, C11, C13 = 0.1 µF (ceramic disc)

C12 = 220 µF/16V (electrolytic)


IC1 = NE555 (timer IC)

IC2 = LM7805 (+5 Volt regulator)

IC3 = LM311 (comparator)

IC4 = AT89c51 (microcontroller chip)

IC5 = ISD1420 (voice processor)

T1, T2, T3, T4, T5 = BC548 (NPN transistor)

T6 = BC558 (PNP transistor)

ZD1 = 2.2 volt, 1/4 watt (zener diode)

D1 = 1N4001 (rectifier diode)

IR LED1, IR LED2 = Infrared LED (5 mm diameter)

IRX1 = TSOP1738 (IR receiver module)

LED1, LED2, LED5 = Red LED (5 mm diameter)

LED3 = Green LED (5 mm diameter)

LED4 = Yellow LED (5 mm diameter)


B1, B2 = 9 Volt (battery)

XTAL1 = 3.579 MHz crystal oscillator

SW1, SW2 = ON/OFF SPST (Single Pole Single Throw) switch

SW3, SW4 = Push-To-On switch

MIC1 = Condenser Microphone

JACK1 = Jack for headphone connector

RL1 = 5 Volt, 100 Ω, single changeover relay


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