The receiver (Fig.
5) consists of a light sensor, a signal pream-plifier, audio ampream-plifier, code detector (with audio/visual alarm) and call/tone detector with buzzer indica-tion. It uses a light-dependent resistor (LDR) as the light sensor. The resistance of LDR varies depending on the incident
light intensity, which, in turn, is a func-tion of its modulafunc-tion by the mixed output of code and tone or audio signals at the transmitter mixer stage. The output of the LDR sensor is amplified by a two-stage transistorised preamplifier.
The preamplifier output is coupled (via DC blocking capacitor C14) to:
1. The audio power amplifier built around IC LM386
2. Phase-locked loop (PLL-1) IC5 3. Phase-locked loop (PLL-2) IC6 The preamplifier output is fed to input pin 3 of audio power amplifier LM386 (IC4) through volume-control potmeter VR7. Capacitor C28 bypasses the noise signal and higher-order frequencies rep-resenting the code signal (10-15 kHz).
The audio output (comprising voice or
tone signals) from pin 5 of IC4 is coupled to loudspeaker LS1 through capacitor C30. A snubber network comprising ca-pacitor C29 and resistor R23 is used for output stability. IC LM386 is a low-volt-age audio power amplifier. Its gain is internally set to 20 to keep external part count low.
The preamplifier output, as stated earlier, is also connected to phase-locked loop IC5 and IC6 (each NE567) through capacitors C25 and C26, respectively. IC NE567 is a highly stable phase-locked loop with synchronous AM lock detection and power output circuitry. It is primarily used as a frequency decoder, which drives a load whenever a sustained frequency falling within its detection band is present at its self-biased input. The centre
frequency of the band and output delay are independently determined by external components.
Link continuity/discontinuity indi-cation. IC5 is used to detect the 10-15kHz code. In the absence of any input signal, the centre frequency of its internal free-running, current-controlled oscillator is determined by resistor R19 and capacitor C19. Preset VR5 is used for tuning IC5 to the desired centre frequency in the 10-15kHz range, which should match the frequency of the code generator in the transmitter.
The output at pin 8 of IC5 remains low as long as the transmitted code is detected by IC5. As a result, LED1 lights up to indicate conti-nuity of the optical link/path for commu-nication.
When the laser beam is interrupted due to any reason, the output at pin 8 of IC5 goes high to drive transistor T4 and its collector voltage falls to trigger monostable circuits built around IC7 and IC8 (each NE555), respectively.
As a result, the out-put at pin 3 of these ICs goes high for the predetermined time period. The time pe-riods of timers IC7 and IC8 depend on the values of resis-tor-capacitor combinations R26-C31 and R25-C34, respectively. Since output pin 3 of IC7 is connected to pin 1 of decade counter CD4033 (IC9), it provides a clock pulse to counter IC9 to increment its count, indicating interruption of the laser-light beam. The current count is shown on a 7-segment display (DIS1) connected to the 7-segment decoded outputs of counter IC9. Resistor R30 is used as a current-limiting resistor in the common-cathode path of DIS1.
For frequent interruptions of the light beam, the output of decade counter IC9 keeps incrementing the count. After the count reaches ‘9,’ the next interruption resets the counter and it starts afresh. The counter/display can also be reset manually by momentarily depressing press-to-on 6
Fig. 6: Power supply circuit
Fig. 7: Actual-size, single-side PCB layout of one-way speech communication circuit
switch S2.
As stated earlier, IC7 and IC8 are triggered simultaneously. Thus with each interruption of the light beam, the output of IC8 is pulsed high for a predetermined time to provide around 3V (determined by the output of zener diode ZD1) to melody IC UM66 (IC10). Thus IC10 generates a melodious tune whenever the light beam is interrupted. The output of IC10 is ampli-fied by transistor T5 to drive loudspeaker LS2.
For initiating a call, the person at the transmitter end depresses switch S1 to alert the remote-end person of an impending voice communication. Thus the modulated light output from the transmitter contains 1-2kHz tone com-ponent in addition to the 10-15kHz code oscillator output. After detection and preamplification, 1-2kHz tone is decoded
by PLL-2 circuit built around IC6, whose centre frequency is adjusted to match the frequency of tone/call oscillator in the transmitter.
IC6 is thus used as the call detector.
Resistor R20 and capacitor C22 decide the centre frequency of its inbuilt oscil-lator in the absence of an input signal.
Capacitors C23 and C24 serve as low-pass filter and output filter, respectively.
Preset VR6 is used for tuning the inbuilt oscillator.
Thus when the 1-2kHz tone compo-nent is detected by IC6, its output pin 8 goes low to light up LED3 as also sound piezobuzzer PZ1 to alert the receiver-end person. Since the 1-2kHz tone component at the output of the preamplifier also passes through LM386 power amplifier, the tone is heard from loudspeaker LS-1 as well.
Voice communication. For voice communication, the person at the trans-mitter end speaks into the mike while call switch S1 is open. The modulated light beam contains the 10-15kHz code frequency and voice components. Af-ter demodulation at the receiver, the 10-15kHz code component is largely bypassed by capacitor C28 at the input of LM386, while the voice component (up to 3400 Hz) is attenuated insignifi-cantly. Thus speech is reproduced at the
output of LM386 via state. Resistor R1 acts as the current-lim-iting resistor for LED.
An actual-size, single-side PCB layout of the laser-based one-way speech communication circuit (comprising the transmitter, receiver and power supply units) is shown in Fig. 7 and its com-ponent layout in Fig. 8. For two-way (duplex) communication, you will need two PCBs.
Precautions
Take the following precautions while handling laser diodes:
1. For observing laser beams, always use safety goggles that block laser beams.
Laser diodes up to 5mW output are ranked as Class III A products.
2. Laser diodes use gallium-arsenide (GaAs), which is potentially hazardous to the human body. Therefore, never crush, heat to the maximum storage temperature or put the laser diode in your mouth.
3. Semiconductor laser diodes are highly sensitive to electrostatic discharge, so be extremely careful while handling these. Don’t touch the leads of the laser diode directly. Wear cotton gloves or ESD-protection gloves and handle the laser diode as shown in Fig. 9.
Fig. 8: Component layout for the PCB
Fig. 9: Laser diode handling
deviCe switChing using
on/off the devices. The circuit can switch on only one device at a time, out of a maxi-mum of nine connected devices. To switch on/off the device, you need to enter a correct 4-digit password via the keypad.Fig. 1 shows the block diagram of the device switching system using password.
It mainly comprises a keypad, DTMF tone generator, DTMF decoder, demultiplexer and password circuit. Four DIP switches (DIP1 through DIP4) are used to set up the password.
The DTMF decoder-and-password setup unit is connected to the devices to be controlled. The DTMF generator (transmitter) is connected to the rest of the circuit through a two-core cable to enable device switching from a remote location.
1. The transmitter section. The transmitter circuit is built around DTMF encoder IC UM91214B (IC1). The DTMF encoder is commonly used as a dialler IC in telephone sets to generate DTMF tones. For its time base, IC UM91214B requires a 3.58MHz quartz crystal, which is connected between pins 3 and 4 of IC1 to form an internal oscillator. The oscillator output is converted into an appropriate DTMF signal through frequency division and multiplexing by the control logic of IC1.
A telephone type keypad is connected to ICI via 4-row and 3-column lines. Pins 15 through 18 of IC1 are row pins and pins 12 through 14 are column pins. Of the twelve keys on the keypad, we’ve used keys ‘1’ through ‘9’, ‘0’ and ‘*.’ The ‘#’ key is not used here. Keys ‘1’ through ‘9’ are used for controlling the device, key ‘0’ is used to switch off the device and key ‘*’ is used to reset the circuit.
As stated earlier, we’ve used here a telephone-type keypad (also used for cash/debit card purchases) with twelve push-to-on switches to enter the password to control the devices. fre-quency band of 300 to 2400 Hz and are chosen such that interference with any other frequency existing in the normal speech simultane-ously is minimised. To mini-mise interference, a lower frequency from the rows (697 Hz, 770 Hz, 852 Hz or 941 Hz) is paired with a higher frequency from the columns (1209 Hz, 1336 Hz, 1477 Hz or 1633 Hz).
Thus a valid DTMF tone is the sum of a lower-fre-quency tone (697 Hz, 770 Hz, 852 Hz or 941 Hz) and
PARTS LiST Semiconductors:
IC1 - IC UM91214B DTMF dialler IC2 - KT3170/MM8870 DTMF
decoder
IC3 - 74LS154 4-to-16-line decoder/
demultiplexer IC14, IC15 - 74LS04 hex invertergate D1-D9 - 1N4007 rectifier diode T1-T9 - BC548 npn transistor ZD1 - 3.3V zener diode Resistors (all ¼-watt, ±5% carbon, unless stated otherwise):
DIP1-DIP4 - 4-way DIP switch - Keypad
Fig. 1: Block diagram of the device switching system using password