Traffic Baton

   In small towns, there are no traffic lights and the police regulates the traffic with hand signals. Since Fig. 1: Circuit of LED flasher their hand signals may not be visible at night, it is necessary to have some illuminated direction indicator.

      Here we present two circuits for the same. One uses 6V bulbs and the other uses bright LEDs. Both the circuits operate off a 6V, 4.5Ah rechargeable battery, which is clipped to the policeman’s waistband. Fig .1 shows the circuit of the LED flasher. It is wired as an astable multivibrator. The ‘on’ time of the LED cluster is about 108 milliseconds and ‘off’ time is around 105 milliseconds. The frequency is around 5 Hz. A diode is used in series with the base of BD140 to increase the forward voltage in order to ensure that when BD139 conducts, BD140 is cutoff. Select the LED which consumes low current (20 mA or so) but flashes bright.

     Fig. 2 shows the circuit of the bulb flasher. Timer NE555 is wired as an astable multi-vibrator. The ‘on’ period of flashing bulb is around 344 milliseconds and ‘off’ period is around 329 milliseconds. The frequency is around 1.5 Hz. Bulb-driver transistors 2N3053/ BD139 and 2N2905/BD140 are used to light up the lamp. Two diodes are used in series with the base of 2N2905 to increase the forward voltage in order to ensure that when BD139 is conducting, BD140 is cut-off. Slide switch S2 is used to change the colour status of the flashing bulb.

fig 3 :Traffic baton for LED flasher

fig 4: Traffic baton for bulb flasher

    Assemble the LED flasher and bulb flasher circuits on separate general-purpose PCBs. Enclose the LED flasher in a transparent acrylic pipe as shown in Fig. 3. The bulb flasher can be enclosed in another transparent acrylic pipe as shown in Fig. 4. Slide switches and red and green acrylic sheets are used for appropriate colour emissions. Now your traffic baton is ready to use.

ANTI-COLLISION REAR LIGHT

During poor visibility, i.e., when there is fog, or at dawn or dusk, or when your vehicle gets stalled on a lonely stretch of a highway, this flashing light will provide safety and attract the attention of people to help you out. It uses high brightness yellow LEDs. The circuit uses a dual binary counter CD4520, quadruple 2-input  schmitt trigger CD4093, 8-stage shift-and-store bus register CD4094 and some descrete components. An oscillator is built around gate A, whose frequency can be varied through preset VR1 when required. The output of the oscillator is fed to IC1 and IC3. When the circuit is switched on, the oscillator starts oscillating, the counter starts counting through IC1 and the data is shifted on positive-going clock through IC3. As a result, the four groups of LEDs flash one by one. All the LEDs will then glow for some time and switch off for some time, and the cycle will repeat. Input pins 12 and 13 of the unused gate D must be tied to ground and pin 11 left open. Preset VR1 should be of cermet type and used to change the flashing rate of each group of LEDs. The circuit works off regulated 12V. Assemble it on a general-purpose PCB and house suitably

PERSONAL STEREO

   In the output stages of most broadcast receivers and some amplifiers,there is a limit up to which maximum power can be developed without distortion. In the widely accepted output circuit, two output transistors are connected in series between the positive and ground and biasing is adjusted so that each transistor gets half the supply voltage.

   The circuit presented here is a simple audio amplifier for a personal stereo system. In this, supply voltage to each transistor can be enhanced to produce a larger output. The audio driver transformer drives the transistors adequately. A 9V-0-9V, 300mA transformer has been used in the set-up. Out of the four diodes (D1 through D4), two are used for developing the positive voltage rail (+9V) and the other two are used for developing the negative voltage rail (–9V). In the push-pull amplifier, each transistor (T2 or T3) gets double the voltage when activated.

   Connect the low audio signal from the stereo system at input terminals A and B of the audio amplifier and provide mains AC to activate the circuit. During the first half cycle of an AF cycle, transistor T2 conducts and the current flows from positive rail to ground rail (centre tap of transformer X1) via the loudspeaker coil (connected between the emitter of transistor T2 and ground) in one direction. While in the second half cycle, transistor T3 conducts and the current flows from ground rail to negative rail via the loudspeaker coil (connected between ground and the collector of transistor T3) in a direction opposite to the previous

flow. Transistors T2 and T3 of the pushpull audio amplifier should be matched correctly. If these transistors get heated, change the bleeding resistor pairs (R3 and R4 for transistor T2 and R5 and R7 for transistor T3) so that the acceptable output without overheating is obtained. You can also replace these transistors with another pair of suitable high-power transistors. For driving transistors T2 and T3, a 9V audio driver transformer having six leads is used. It is readily available in the market and reasonably matches the output and input impedances of the preceeding and succeeding stages.

   To test the quality of the audio output, connect the stereo’s outputs to the respective terminals A and B. Now increase the volume level of the stereo slowly. If you get a high-level, high-quality sound across loudspeaker L1, the amplifier is working well. If the sound quality is not good, decrease the volume level until the audio amplifier gives good results. Note that this audio amplifier works well for low-level audio signals.

PC Multimedia Speakers

      This circuit of multimedia speakers for PCs has single-chipbased design, low-voltage power supply, compatibility with USB power, easy heat-sinking, low cost, high flexibility and wide temperature tolerance. At the heart of the circuit is IC TDA2822M. This IC is, in fact, monolithic type in 8-lead mini DIP package. It is intended for use as a dual audio power amplifier in battery-powered sound players. Specifications of TDA2822M are low quiescent current, low crossover distortion, supply voltage down to 1.8 volts and minimum output power of around 450 mW channel with 4-ohm loudspeaker at 5V DC supply input. An ideal power amplifier can be simply defined as a circuit that can deliver audio power into external loads without generating significant signal distortion and without consuming excessive quiescent current. This circuit is powered by 5V DC supply available from the USB port of the PC. When power switch S1 is flipped to ‘on’ position, 5V power supply is extended to the circuit and power-indicator red LED1 lights up instantly. Resistor R1 is a current surgelimiter and capacitors C1 and C4 act as buffers. Working of the circuit is simple. Audio signals from the PC audio socket/headphone socket are fed to the amplifier circuit through components R2 and C2 (left channel), and R3 and C3 (right channel). Potmeter VR1 works as the volume controller for left (L) channel and potmeter VR2 works for right (R) channel. Pin 7 of TDA2822M receives the left channel sound signals and pin 6 receives the right-channel signals through VR1 and VR2, respectively. Amplified signals for driving the left and right loudspeakers are available at pins 1 and 3 of IC1, respectively. Components R5 and C8, and R6 and C10 form the traditional zobel network. Assemble the circuit on a medium-size, general-purpose PCB and enclose in a suitable cabinet. It is advisable to use a socket for IC TDA2822M. The external connections should be made using suitably screened wires for better result.

Battery-level indicator

      Normally, in mobile phones,the battery level is shown indot or bar form. This lets youeasily recognise the battery level. Herewe present a circuit that lets you knowthe battery level of a device from thenumber of LEDs that are glowing. Ituses ten LEDs in all. So if three LEDsglow, it indicates battery capacity of 30 per cent. Unlike in mobile phones where the battery-level indicator function is integrated with other functions, here only one comparator IC (LM3914) does it all. The LM3914 uses ten comparators, which are internally assembled in the voltage divider network based on the current-division rule. So it divides the battery level into ten parts. The circuit derives the power supply for its operation from the battery of the device itself. It uses ten LEDs wired in a 10-dot mode. The use of different coloured LEDs makes it easier to recognise the
voltage level on the basis of the calibration made. Red LEDs (LED1 through LED3) indicate battery capacity of less than 40 per cent. Orange LEDs (LED4 through LED6) indicate battery capacity of 40 to less than 70 per cent and green LEDs (LED7 through LED10) indicate battery capacity of 70 to under 100 per cent. The brightness of the LEDs can be adjusted by varying the value of preset VR2 between pins 6 and 7. Diode D1 prevents the circuit from reverse-polarity battery connection. The tenth LED glows only when the battery capacity is full, i.e., the battery is fully charged. When the battery is fully charged, relay-driver transistor T1 conducts to energise relay RL1. This stops the charging through normally-open (N/O) contacts of relay RL1. For calibration, connect 15V variable, regulated power supply and initially set it at 3V. Slowly adjust VR1 until LED1 glows. Now, increase the input voltage to 15V in steps of 1.2V until the corresponding LED (LED2 through LED10) lights up. Now the circuit is ready to show any voltage value with respect to the maximum voltage. As the number of LEDs is ten, we can easily consider one LED for 10 per cent of the maximum voltage. Connect the voltage from any battery to be tested at the input probes of the circuit. By examining the number of LEDs glowing you can easily know the status of the battery. Suppose five LEDs are glowing. In this case, the battery capacity is 50 to 59 per cent of its maximum value. Assemble the circuit on a generalpurpose PCB. Calibrate it and then enclose in a box.

Simple Room Temperature Monitor Circuit

A very simple yet highly precise air temperature sensor gauge circuit has been presented here. The use of the highly versatile and accurate IC LM 308 makes the circuit respond and react superbly to the smallest temperature changes happening over its surrounding atmosphere. Diode 1N4148 is used as an active ambient temperature sensor here.

The electronic air temperature sensor gauge circuit presented here is very accurate in its function, categorically due to its minimum level of hysteresis. Complete circuit description and construction clues included herein.

Circuit Description

The present circuit of an electronic air temperature sensor gauge circuit is outstandingly accurate and can be very effectively used to monitor the atmospheric temperature variations. Let’s briefly study its circuit functioning:

Here as usual we use the very versatile “garden diode” 1N4148 as the sensor due to its typical drawback (or rather an advantage for the present case) of changing its conduction characteristic in the influence of a varying ambient temperature. The diode 1N4148 is comfortably able to produce a linear and an exponential voltage drop across itself in response to a corresponding increase in the ambient temperature. This voltage drop is around 2mV for every degree rise in temperature. This particular feature of 1N4148 is extensively exploited in many low range temperature sensor circuits.

Referring to the  figure we see that, IC1 is wired as an inverting amplifier and forms the heart of the circuit.

Its non inverting pin # 3 is held at a particular fixed reference voltage with the help of Z1, R4, P1 and R6.

Transistor T1 and T2 are used as a constant current source and helps in maintaining higher accuracy of the circuit.

The inverting input of the IC is connected to the sensor and monitors even the slightest change in the voltage variation across the sensor diode D1. These voltage variations as explained, is directly proportional to the changes in the ambient temperature.

The sensed temperature variation is instantly amplified into a corresponding voltage level by the IC and is received at its output pin #6.

The relevant readings are directly translated into degree Celsius through a 0-1V FSD moving coil type meter.

Parts List

R1, R4 = 12K,

R2 = 100E,

R3 = 1M,

R5 = 91K,

R6 = 510K,

P1 = 10K PRESET,

P2 = 100K PRESET,

C1 = 33pF,

C2, C3 = 0.0033uF,

T1, T2 = BC 557,

Z1= 4.7 V, 400mW,

D1 = 1N4148,

IC1 = LM308,

General Purpose Board as per size.

B1 and B2 = 9V PP3 battery.

M1 = 0 – 1 V, FSD moving coil type voltmeter

Setting Up the Circuit

The procedure is a bit critical and requires special attention. To complete the procedure you will need two accurately known temperature sources (hot and cold) and an accurate mercury-in-glass thermometer.

The calibration may be completed through the following points:

Initially keep the presets set at their midways. Connect a voltmeter (1 V FSD) at the output of the circuit.

For the cold temperature source, water at about room temperature is used here.

Dip the sensor and the glass thermometer into the water and record the temperature in the glass thermometer and the equivalent voltage outcome in the voltmeter.

Take a bowl of oil, heat it to about 100 degrees Celsius and wait until its temperature stabilizes down to about 80 degrees Celsius.

As above, immerse the two sensors and compare them with the above result. The voltage reading should be equal to the temperature change in the glass thermometer times 10 mill volt. Didn’t get it? Well, let’s read the following example.

Suppose, the cold temperature source water is at 25 degrees Celsius (room temperature), the hot source, as we know is at 80 degrees Celsius. Thus, the difference or the temperature change between them is equal to 55 degrees Celsius. Therefore the difference in the voltage readings should be 55 multiplied by 10 = 550 mill volts, or 0.55 volts.

If you don’t quite get the criterion satisfied, adjust P2 and continue to repeat the steps, until finally you achieve it.

Once the above rate of change (10 mV per 1 degree Celsius) is set, just adjust P1 so that the meter shows 0.25 volts at 25 degrees (sensor held in water at room temperature).

Night alert

    Idea of this circuit came to me at midnight when my pet dog started barking continuously on sensing a moving shadow, perhaps that of an intruder. Dogs have a night adaptation capability to maximize the sensitivity of vision in low light. They are well adapted to see moving objects rather than stationary ones in darkness. 

    This circuit turns a lamp ‘on’ for a short duration when the dog barks, giving an impression that the occupants have been alerted. 

The condenser microphone fitted in the dog’s cage senses barking sound and generates AC signals, which pass through DC blocking capacitor C1 to the base of transistor BC549 (T1). Transistor T1 along with transistor T2 amplifies the sound signals and provides current pulses from the collector of T2. 

The input trigger pulse is applied to the collector of transistor T3 and coupled by capacitor C3 to the base of transistor T4 causing T4 to cut off. The collector voltage of transistor T4 forward biases transistor T3 via resistor R8. Transistor T1 conducts and capacitor C3 discharges to keep transistor T4 cut-off. Transistor T4 remains cut-off until capacitor C3 charges enough to enable it to conduct. 

    When transistor T4 conducts, its collector voltage goes low to drive transistor T3 into cut-off state. Resistor R9 andcapacitor C3 are timing components. When fully charged, capacitor C3 takes about two minutes to discharge. So when sound is produced in front of the condenser mic, TRiAC1 (BT136) fires and the bulb (B1) glows for about two minutes. 

    Assemble the circuit on a general-purpose PCB and enclose in a plastic cabinet. Power to the circuit can be derived from a 12V, 500mA step-down transformer with rectifier and smoothing capacitor. Solder the triac ensuring sufficient spacing between the pins to avoid short circuit. Fix the unit in the dog’s cage, with the lamp inside or outside as desired. Connect the microphone to the circuit using a short length of shielded wire. 

Enclose the microphone in a tube to increase its sensitivity. 

    Caution. Since the circuit uses 230V AC, many of its points are at AC mains voltage. it could give you lethal shock if you are not careful. So if you don’t know much about working with line voltages, do not attempt to construct this circuit. EFY will not be responsible for any kind of resulting loss or damage.