GATE 2015 - Electrical Engineering

Security System Switcher

An audio signal can be used as a form of input to control any security system. For example, an automatic security camera can be configured to respond to a knock on the door. The circuit described here allows the security system to automatic in on state. It uses a transducer to detect intruders and a 5V regulated DC power supply provides power to the circuit. 

As shown in Fig. 1, a condenser microphone is connected to the input of small signal Pre- amplifier built around transistor T1. Biasing resistor R1 determines to a large extent the microphone sensitivity. A microphone usually has an internal FET which requires a bias voltage to operate. The sound picked up by the microphone is amplified and fed to input pin 2 of IC1 (LMC555) wired in monostable configuration.

Fig. 1: Security system switcher

IC2 (CD4538B) is a dual, precision monostable multivibrator with independent trigger and reset controls. The output of IC1 is connected to the first trigger input pin 4 of IC2(A) through switch S1. If an intruder opens or breaks the door, IC1 is triggered by sound signals; the timer output pin 3 of IC1 goes high and enables first monostable multivibrator IC2(A). IC2(A) provides a time period of around 5 to 125 seconds, which is adjusted with preset VR1. 

Another monostable multivibrator IC2(B) also provides a time period of around 25 to 600 seconds, which is adjusted with preset VR2. The output of IC2(B) is used to energise relay RL1. Indicator LED1 is provided to display the relay activity. Any AC/DC operated security gadget is activated or deactivated through a security switch. Thus, the security switch of the gadget is connected in the n/o contacts of the relay.You can also operate high power beacons, sirens or hooters in place of the security switch for any AC/DC operated security gadget.

Assemble the circuit on a general-purpose PCB and enclose it in a cabinet as shown in Fig. 2 along with 5V adaptor for powering the circuit. Connect the security switch according to the circuit diagram and use appropriate AC/DC power supply required to operate the security gadget.

Fig. 2: Proposed cabinet 

Warning! All relevant electrical safety precautions should be taken when connecting mains power supply to the relay contacts. With the help of single pole double throw (SPDT) switch S1, internal or external trigger input (active high signal) can be selected.

Water Pump Controller

A water pump controller senses the level of water in a tank and drives the water pump. The circuit described here is built around timer IC1 (555). When the water level of tank goes below the low level marked by 'L' the voltage at pin 2 of IC1 becomes low. As a result, internal SR-flip-flop of IC1 resets and its output goes high. This high output pin 3 of IC1 drives relay driver transistor T1 (BC547) and energises relay RL1. Water pump gets mains power supply through n/o contacts of the relay and is powered on. It starts filling water in the tank.

When the water level goes high and reaches the high level marked by 'H', the voltage at pin 6 of IC1 also goes high. As a result, internal SR-flip-flop of IC1 sets and its output goes low. This low output pin 3 of IC1 cuts off relay driver transistor T1 and de-energises relay RL1. Water pump disconnects from the mains power supply through n/c contacts of relay and goes off. This stops the water flow. 

Use 12V battery or 12V adaptor to operate the circuit. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Use three-sensor (thick conductive wire) and fit into the tank ensuring that 12V sensor touches the bottom and low-level sensor remains just above the bottom. High level sensor should be placed at a height till where the water needs to be filled. The circuit is economical, highly reliable and can be easily constructed.

Battery-Low Indicator

Battery-Low Indicator  

Rechargeable batteries should not be discharged below a certain voltage level. This lower voltage limit depends upon the type of the battery. This simple circuit can be used for 12V batteries to give an indication of the battery voltage falling below the preset value. The indication is in the form of a flickering LED.

At the heart of the circuit is voltage comparator IC LM319 (IC1). It is a dual comparator with a TTL-compatible output. We have used only one comparator here. A reference voltage of 1.2 volts generated by band-gap reference diode D1 (LM385) is applied to the non-inverting input (pin 4) of the comparator. The inverting input (pin 5) of the comparator is fed a voltage generated from the potential divider arrangement built around resistors R2 and R3 and preset VR1. That means, if you are using a 12V battery and want an indication as soon as the battery voltage goes below 10.5V, adjust the voltage at the inverting input using reset VR1 so as to get a voltage of 1.2 volts (with battery voltage at 10.5V).

Initially, when the battery is fully charged, the voltage at the inverting input of IC1 is higher than the non-inverting input and output pin 12 of IC1 remains low. The reset pin (pin 4) of IC2 connected to pin 12 of IC1 also remains low and the astable multivibrator built around IC2 does not oscillate. As a result, LED1 does not flicker.

When the battery voltage falls below 10.5V, the voltage at the inverting input of IC1 becomes lower than the non-inverting input and the output of IC1 goes high. The reset pin of IC2 connected to pin 12 of IC1 also goes high and the astable multivibrator built around IC2 starts oscillating. LED1 flickers to indicate that the battery voltage is low and the battery needs to be charged before further use. Both IC1 and IC2 operate off regulated +5V DC generated by voltage regulator IC 7805 (IC3).

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Mount LED1 and switch S1 on the front side of the case. Connect a 12V battery to check its voltage level. 

Over-Speed Indicator

This circuit is designed for indicating over-speed and direction of rotation of the motor used in mini hand tools, water pump motors, toys and other appliances.

A 12V DC motor (M1) is coupled to the rotating part of the appliance with a suitable fixing arrangement. When the motor rotates, it develops a voltage.

This over-speed indicator is built around operational amplifier CA3140 (IC1). Set the reference voltage (depending on the desired speed) by adjusting preset VR1 at pin 2 of IC1. When the voltage developed at pin 3 of IC1 is higher than the reference voltage at pin 2, output pin 6 of comparator IC1 goes high to sound piezobuzzer PZ1 and light up LED3.
The rotation indicator circuit is built around AND gate 74LS08 (IC2). Pin 2 of gate N1 goes high when the motor rotates in forward direction, while pin 1 of gate N1 is pulled high via resistor R2. When both pins 1 and 2 are high, output pin 3 of gate N1 goes high to light up LED1. Similarly, pin 5 of gate N2 goes high when the motor rotates in reverse direction. When both pins 4 and 5 are high, output pin 6 of gate N2 goes high to light up LED2
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Simple Stereo Level Indicator

Usually, low-priced home stereo power amplifiers don’t have output level indicators. An output power level indicator can be added to each channel of these stereo power amplifiers. As low levels of the output power are not disturbing and damaging to the people, there is no need to add a preamplifier and low-level detector before IC LM3915. But you should know when the output power becomes considerably high.

Here we present a very simple, low-cost stereo-level indicator circuit for home power amplifiers with power rating of around 0.5W. The circuit is built around two LM3915 dot/bar display driver ICs (IC1 and IC2). LM3915 senses analogue voltage levels to drive ten LEDs, providing a logarithmic 3dB/step analogue display.
The voltage levels below 1V are not important because these correspond to a low level of the audio signal. Similarly, input voltage levels above 30V correspond to too high levels of the output power, which are not applicable for home power amplifiers. So the voltage levels of our interest are 1V to 30V, which can be handled directly with LM3915. LM3915 needs no protection against ±35V inputs, which simplifies the circuit.

Most audio power amplifiers can drive 2-ohm to around 32-ohm loads. A load of several kilo-ohms will not change the conditions for the amplifier. CON1 is the input connector and CON2 output connector for the loudspeaker or headphone. Each channel has its own LM3915 and ten LEDs to indicate the power level. To indicate the different audio levels, select LEDs of three colours as per your liking. For example, you can have five green LEDs, three yellow LEDs and two red LEDs.

If appropriate signal generators and measuring equipment are not available, the level indicator can be calibrated based on personal observations. For example, the audio signal should be in the green LEDs zone when the signal is strong enough but not irritating, in the yellow zone when it is disturbing or starting to get distorted, and in the red zone when it is heavily distorted or too strong to listen to in the room.

Calibration can be done with the help of potentiometers VR1 and VR2, which are optional. Switches S1 and S2 let you select between two modes of LM3915 operation—the bar mode and the dot mode. These too can be removed or replaced with jumpers, if not required. When the switches are removed, leave pins 9 of IC1 and IC2 open.

The circuit works off regulated 12V. You can also power it through a 12V rechargeable battery.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Fix all the LEDs in two rows on the front side of the cabinet. Also affix the two terminals for input and output on rear side of the cabinet
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Water level indicator

Here’s a simple water-level indicator for overhead tanks that uses three LEDs (LED1, LED2, and LED3) to indicate minimum, middle, and maximum water levels in the tank.

The sensor probes comprise A, B, C, and D, where A is the common probe and B, C, and D are meant for sensing the minimum, middle, and maximum levels, respectively. When water in the tank touches sensor wires A and B both, a small current passes from A to B through water and to the base of transistor T1 via resistor R1. As a result, transistor T1 conducts, causing LED1 to glow. Similarly, when water touches sensor C, LED2 glows to indicate that the water has reached the middle level. Finally, when water touches sensor D, LED3 glows to indicate the maximum level of water. Thus all the three LEDs glow when the tank is full. At this stage, the motor should be switched off manually.
The circuit can be easily assembled on a general-purpose PCB and enclosed in a wooden box. The three LEDs should be mounted on the front panel of the box with a spacing of about 4 cm between them. Short lengths of four 18 SWG copper wires may be used for sensor probes. For the common sensor A, a bare copper wire of 18 SWG should be used. For sensors B, C, and D three single-core PVC wires should be used, with their insulation removed to a length of one centimetre towards the ends. All the four wires may be tied around a 12.5mm dia. PVC tube with nylon thread at different heights, without touching each other (not shown in figure).

The sensor probes should be kept in the tank vertically and connected to the main circuit using four flexible PVC wires of different colours.

The circuit is powered by a battery eliminator or a 6V battery and kept near the motor switchboard. The current drawn by the circuit, when all the LEDs glow, is up to 50 mA, which is less than the current drawn by a 6V bed-lamp.