Speed Checker for Highways

While driving on highways, motorists should not exceed the maximum speed limit permitted for their vehicle. However, accidents keep occurring due to speed violations since the drivers tend to ignore their speedometers. This speed checker will come handy for the highway traffic police as it will not only provide a digital display in accordance with a vehicle’s speed but also sound an alarm if the vehicle exceeds the permissible speed for thehighway. The system basically comprises two laser transmitter-LDR sensor pairs, which are installed on the highway 100 metres apart, with the transmitter and the LDR sensor of each pair on the opposite sides of the road. Theinstallation of lasers and LDRs is shown in Fig. 1. The system displays the time taken by the vehicle in crossing this 100m distance from one pair to the other with a resolution of 0.01 second, from which the speed of the vehicle can be calculated as follows:
As per the above equation, for a

speed of 40 kmph the display will read 900 (or 9 seconds), and for a speed of 60 kmph the display will read 600 (or 6
seconds). Note that the LSB of the display equals 0.01 second and each succeeding digit is ten times the preceding digit. You can similarly calculate the other readings (or time).

Circuit description
Fig. 2 shows the circuit of the speed checker. It has been esigned assuming that the maximum permissible speed for highways is either 40 kmph or 60 kmph as per the traffic rule.
The circuit is built around five NE555 timer ICs (IC1 through IC5), four CD4026 counter ICs (IC6 through IC9) and four 7-segment displays (DIS1 through DIS4). IC1 through IC3 function as monostables, with IC1 serving as count-start mono, IC2 as count-stop mono and IC3 as speed-limit detector
mono, controlled by IC1 and IC2 outputs. Bistable set-reset IC4 is also controlled
by the outputs of IC1 and IC2 and it (IC4), in turn, controls switching on/off of the 100Hz (period = 0.01 second) astable timer IC5.
The time period of timer NE555 (IC1) count-start monostable multivibrator is adjusted using preset VR1 or VR2 and capacitor C1. For 40kmph limit the time period is set for 9 seconds using preset VR1, while for 60kmph limit the time period is set for 6 seconds using preset VR2. Slide switch S1 is used to select the time period as per the speed limit (40 kmph and 60 kmph, respectively). The junction of LDR1 and resistor R1 is coupled to pin 2 of IC1.
Normally, light from the laser keeps falling on the LDR sensor continuously and thus the LDR offers a low resistance and pin 2 of IC1 is high. Whenever light falling on the LDR is interrupted by any vehicle, the LDR resistance goes high and hence pin 2 of IC1 goes low to trigger the onostable.
As a result, output pin 3 goes high for the preset period (9 or 6 seconds) and LED1 glows to indicate it. Reset pin 4 is controlled by the output of NAND generator IC5. IC5 can also be reset via diode D2 at power-on as well as when reset switch S2 is pressed. IC5 is configured as an astable multivibrator whose time period is decided by preset VR3, resistor R12 and capacitor C10. Using preset VR1, the frequency of the astable multivibrator is set as 100 Hz. The output of IC5 is fed to clock pin 1 of decade counter/7- segment decoder IC6 CD4026. gate N3 at power-on or whenever reset switch S2 is pushed. For IC2, the monostable is triggered in the same way as IC1 when the vehicle intersects the laser beam incident on LDR2 to generate a small pulse for stopping the count and for use in the speed detection. LED2 glows for the duration for which pin 3 of IC2 is high.
The outputs of IC1 and IC2 are fed to input pins 2 and 1 of NAND gate N1, respectively. When the outputs of IC1 and IC2 go high simultaneously (meaning that the vehicle has crossed
the preset speed limit), output pin 3 of gate N1 goes low to trigger monostable timer IC3. The output of IC3 is used for driving piezobuzzer PZ1, which alerts the operator of speed-limit violation. Resistor R9 and capacitor C5 decide the time period for which the piezobuzzer
sounds. The output of IC1 triggers the bistable (IC4) through gate N2 at the leading edge of the count-start pulse. When pin 2 of IC4 goes low, the high output at its pin 3 enables astable clock
generator IC5. Since the count-stop pulse output of IC2 is connected to pin 6 of IC4 via diode D1, it resets clock IC CD4026 is a 5-stage Johnson decade counter and an output decoder that converts the Johnson code into a 7-segment decoded output for driving DIS1 display. The ounter advances by one count at the positive clock signal transition. The carry-out (Cout) signal from CD4026 provides one clock after every ten clock inputs to clock the succeeding decade counter in a multidecade counting chain. This is achieved by connecting pin 5 of each CD4026 to pin 1 of the next CD4026.
A high reset signal clears the deFig. 3: Power supply Fig. 4: Actual-size, single-side PCB layout for the speed checker Fig. 5: Component layout for the PCB Construction 62
cade counter to its zero count. Pressing switch S2 provides a reset signal to pin 15 of all CD4026 ICs and also IC1 and IC4. Capacitor C12 and resistor R14 generate the power-on-reset signal.
The seven decoded outputs ‘a’ through ‘g’ of CD4026s illuminate the proper segment of the 7-segment displays (DIS1 through DIS4) used for representing the decimal digits ‘0’ through ‘9.’ Resistors R16 through R19 limit the current across DIS1 through DIS4, respectively.
Fig. 3 shows the circuit of the power supply. The AC mains is stepped down by transforme X1 to deliver the secondary output of 15 volts, 500 mA. The transformer output is rectified by
a bridge rectifier comprising diodes D3 through D6, filtered by capacitor C14 and regulated by IC11 to provide regulated 12V supply. Capacitor C15 bypasses any ripple in the regulated
output. Switch S3 is used as the ‘on’/ ‘off’ switch. In mobile application of the circuit, where mains 230V AC is not available, it is advisable to use an external 12V battery. For activating the lasers used in conjunction with LDR1 and
LDR2, separate batteries may be used.
Construction and working
Assemble the circuit on a PCB. An actual-
size, single-side PCB layout for the
speed checker is shown in Fig. 4 and its
component layout in Fig. 5.
Before operation, using a multimeter
check whether the power supply
output is correct. If yes, apply power
supply to the circuit by flipping switch
S3 to ‘on.’ In the circuit, use long wires
for connecting the two LDRs, so that
you can take them out of the PCB and
install on one side of the highway,
100 metres apart. Install the two laser
transmitters (such as laser torches) on
the other side of the highway exactly
opposite to the LDRs such that laser
light falls directly on the LDRs. Reset
the circuit by pressing switch S2, so the
display shows ‘0000.’ Using switch S1,
select the speed limit (say, 60 kmph) for
the highway. When any vehicle crosses
the first laser light, LDR1 will trigger
IC1. The output of IC1 goes high for
the time set to cross 100 metres with
the selected speed (60 kmph) and LED1
glows during for period. When the
vehicle crosses the second laser light,
the output of IC2 goes high and LED2
glows for this period.
Piezobuzzer PZ1 sounds an alarm if
the vehicle crosses the distance between
the laser set-ups at more than the selected
speed (lesser period than preset
period). The counter starts counting
when the first laser beam is intercepted
and stops when the second laser beam
is intercepted. The time taken by the
vehicle to cross both the laser beams
is displayed on the 7-segment display.
For 60kmph speed setting, with timer
frequency set at 100 Hz, if the display
count is less than ‘600,’ it means that the
vehicle has crossed the speed limit (and
simultaneously the buzzer sounds). Reset
the circuit for monitoring the speed
of the next vehicle.
Note. This speed checker can
check the speed of only one vehicle at
a time.

WIRELESS TV HEADPHONE CIRCUIT

This circuit allows you to watch your favourite TV programmes late at night without disturbing other family members. As against imported stereo wireless TV headphones available in the market for around Rs 1200, it costs just Rs 30, or even less, if the components are taken from a discarded transistor receiver, with no compromise on performance. The unit is basically a simple FM transmitter housed in a plastic or metal enclosure. Transistor T1 acts as an audio reamplifier. Transistor T2 works as an FM oscillator and modulator in conjunction with
other passive components. Trimmer capacitor VC1 connected across inductor L1 can be varied to achieve the desired frequency. Inductor L1 comprises 4 to 6 turns of closely wound 25SWG enamelled copper wire on a 4mm dia. air core. A 20-30cm long wire serves as an antenna.
Most modern TVs are nowadays equipped with audio-in/out and video-in/out RCA sockets. Using an RCA-to-RCA cord, connect the audio output of your TV to the transmitter’s input. Adjust the gain of the audio preamplifier with the help of preset VR1 for clear reception in a portable FM receiver equipped with an earphone socket. Use a good-quality earphone.
This transmitter draws only a few milliamperes of current and doesn’t require on/off switch. It can be fabricated on a small piece of stripboard. All connectors should be firm and as short as possible to prevent unwanted oscillations. The circuit operates off two AA-size penlight torch cells. The circuit is meant for mono reception.

REMOTE CONTROL FOR HOME APPLIANCES

Connect this circuit to any of your home appliances (lamp, fan, radio, etc) to make the appliance turn on/off from a TV, VCD or DVD remote control. The circuit can be activated from up to 10 metres. The 38kHz infrared (IR) rays generated by the remote control are received by IR receiver module TSOP1738 of the circuit. Pin 1 of TSOP1738 is connected to ground, pin 2 is connected to the power supply through resistor R5 and the output is taken from pin 3. The
output signal is amplified by transistor T1 (BC558). The amplified signal is fed to clock pin 14 of decade counter IC CD4017 (IC1). Pin 8 of IC1 is grounded, pin 16 is connected to Vcc and pin 3 is connected to LED1 (red), which glows to indicate that the appliance is ‘off.’
The output of IC1 is taken from its pin 2. LED2 (green) connected to pin 2 is used to indicate the ‘on’ state of the appliance. Transistor T2 (BC548) connected to pin 2 of IC1 drives relay RL1. Diode 1N4007 (D1) acts as a freewheeling diode. The appliance to be controlled is connected between the pole of the relay and neutral terminal of mains. It gets connected to live terminal of AC mains via normally opened (N/O) contact when the relay energises

Remote-Operated Musical Bell

This infrared lightcontrolled 12-tone musical bell can be operated using any TV remote control. It can be operated from up to 10 metres, provided the remote control is directed towards the sensor. The circuit uses the popular 3-lead IR sensor TK1836 to trigger musical bell built around IC UM3481(IC1). (You can also use IC UM3482, UM3483, or UM3484 in place of IC UM3481.)
The sensor responds only to 36 kHz. Most TV remote controls transmit this frequency.
When any button on the TV remote control is pressed, the sensor’s output pulses low. Transistor T1 conducts to apply a triggering pulse to IC1 at its pin 4. After playing one musical tone, the circuit automatically resets. If you again press any of the remote’s buttons, another music is heard. This way, twelve different musical tones can be generated. The circuit works off a 5V power supply. Regulator IC 7805, powered from a 9- 12V DC source, provides regulated 5V.

SCHOOL/COLLEGE QUIZ BUZZER

Manual buzzers used for quiz competitions in schools and colleges create a lot of confusion in identifying the first respondent. Although there are circuits using PCs and discrete ICs, they are either too expensive or limited to only a few number of players. The quiz buzzer circuit given here can be used for up to eight players, which is maximum in any quiz competition. The circuit uses IC 74LS373 and a few passive components that are readily available in the market. The circuit can be divided into two sections: power supply and quiz buzzer.

Fig. 1 shows the power supply section. The regulated 5V power supply for the quiz buzzer section is derived from AC mains. The 230V AC mains is stepped down to 7.5V AC by transformer X1, rectified by bridge rectifier BR1, filtered by C1 and regulated by regulator IC1. Capacitor C2 bypasses ripples in the regulator output.
Fig. 2 shows the quiz buzzer section. At the heart of this section is IC 74LS373, an octal latch that is used to transfer the logic state at data input pins D0 through D7 to the corresponding Q0 through Q7 outputs. Data pins D0 through D7 are normally pulled low by resistors R1 through R8, respectively.
One terminal of push-to-on switches S1 through S8 is connected to +5V, while the other terminal is connected to the respective data input pins. The switches are to be extended to the players through cord wire. The torch bulbs BL1 through BL8 can be housed in boxes with the front side of the boxes covered with a white paper having the name or number of the contestant written over it for easy identification. Place the boxes above the head level so that these can be seen by the audience also. When the power is switched on using switch S9 (provided terminals ‘A’ and ‘B’ of both the power supply and quiz buzzer sections are interconnected), the circuit is ready to use. Now all the switches (S1 through S8) are open and Q0 through Q7 outputs of IC 74LS373 are low. As a result, the gates of silicon-controlled rectifiers SCR1 through SCR8 are also low.
As soon as a contestant momentarily presses his respective switch, the corresponding output data pin goes high. This triggers the corresponding SCR and the respective bulb glows. At the same time, the piezobuzzer (PZ1) sounds as transistor T1 conducts. Simultaneously, the base of transistor T2 becomes high to make it conduct. Latch-enable (LE) pin 11 of IC2 is tied to ground to latch all the Q0 through Q7 outputs. This restricts further change in the output state due to any change in the state of switches S1 through S8 by any other contestant. Only one of the eight torch bulbs glows until the circuit is reset by on/off switch S9. Note. The complete kit is available at Kits ‘n’ Spares outlet.