Key Chain Light

A key chain with a built-in white LED comes in handy to help you at your front door or search your valuables in the dark. The intensity of white LED is 4000 to 5600 mcd (millicandela) at forward voltage of 3.6V and forward current of 20 mA.

Here’s such an LED light circuit for key chains. It comprises a toroidal transformer and two complementary transistors, and is powered by a single AAA cell. Transistors T1 (BC547) and T2 (BC558) form a relaxation oscillator with capacitor C2 (0.01 µF) in the feedback loop. The feedback is controlled by the time constant of timing components R1 and C2, which controls the frequency of operation.

The toroidal transformer steps up the oscillator output to a sufficient value to flash the white LED. The values of R1 and C1 need not be precise. Use of surface mount devices will make the unit more compact.

Fig. 2: Suggested enclosure for key chain light

A single 1.5V AAA cell gives enough brightness. For more brightness, connect two such cells in series. A good-quality white LED from a reputed manufacturer is highly recommended.

Caution. The white LED beam, when viewed directly, can harm the eyes.

Doorbell with Security Feature

This doorbell system works such that when someone presses your calling bell switch during the night, not only the bell rings but the bulb connected to it also glows. In order to turn the bulb off, just press the reset pushbutton switch provided in the circuit. Place the bulb near the calling bell switch so that you can see the person pressing the calling bell before opening the door. So you can choose not to open the door to doubtful persons. During day time, the bulb doesn’t glow and only the calling bell sounds.

When calling bell switch S1 is closed, the bell rings and simultaneously transformer X gets AC supply. The output of this 6V-0V-6V/500mA step-down transformer is rectified by diodes D1 and D2. The rectified output is filtered by 1000µF, 25V capacitor C1 and fed to the collector of transistor BC547 (T1) via 2.2k resistor R1. A light-dependent resistor (LDR) is connected to the base of this transistor.

During the day, the LDR has a very low resistance as it receives continuous light. So when the calling bell switch is pressed, the transistor conducts and its collector is pulled to ground. Thus the next section of the circuit remains inactive and we hear the calling bell only.

The next section consisting of IC 7408 (IC1) and IC 7473 (IC2) gets a separate supply voltage of 5V from regulator IC 7805 (IC3) as shown in the figure.

During the night, as no light falls on the LDR, it has a very high resistance. So when calling bell switch S1 is pressed, transistor T1 doesn’t conduct. As a result, diode D3 is forward biased to make input pins 12 and 13 of IC1 high. Since IC1 is an AND gate, its output at pin 11 will be high. This output is fed to pins 1 and 14 of JK flip-flop IC 7473 (IC2).

For a given high input to the latch made up of IC2, its output pin 12 will be high. Thus transistor SL100 (T2) receives base current and conducts. This energises a 9V, 200-ohm relay (RL) and the 100W bulb connected to the relay glows. The bulb will glow until you press reset pushbutton switch S2.

This circuit costs around Rs 150.

Anti-Theft Alarm for Vehicles

This simple and inexpensive anti-theft circuit for vehicles sounds an alarm simulating a police siren whenever someone attempts theft of your vehicle. The alarm sounds continuously for a few seconds even when the intruder switches off the ignition key. The circuit uses only a few components and can be easily assembled and installed on a car with negative grounding.

The circuit consists of an SCR-based trigger circuit and audio alarm circuit. When the ignition key of the vehicle is switched off, base voltage of transistor T1 is low and it remains turned off. When the ignition key is switched on for starting the vehicle, a positive voltage is applied to the base of transistor T1 through diode D1, switch S2, and resistor R1, which slowly charges capacitor C1. As a result, the base voltage of T1 rises. As soon as the biasing voltage crosses cut-in voltage, T1 turns on and SCR fires, giving 12V DC to the alarm circuit.

The alarm circuit is built around the siren-sound generator ROM UM3561 (IC1). It has a built-in oscillator, whose oscillation depends on resistor R5. Resistor R6 and zener diode ZD1 limit the voltage to IC1 to a safer level of 3.3V. The output from IC1 is fed to a transistor amplifier built around transistors T2 and T3.

The circuit gives sufficient time delay to switch on the alarm and to leave the vehicle. The alarm, once triggered, will sound until switch S1 is pressed to switch off the power supply.

Capacitor C2 is provided to sound the alarm even when the intruder switches off the ignition key. When the ignition key is switched off immediately, C2 discharges through R4 and keeps the alarm activated for half a minute. Reset switch S3 can be used to reset the alarm if needed.

The circuit can be assembled on a vero board. Use a small heat-sink for transistor T1. Connect point A to the ignition switch terminal that goes to the ignition coil. The hidden switch S1 is used for power on/off and switch S2 enables the circuit.

Note. Keep switches S1 and S2 on before leaving the vehicle. And don’t forget to switch off S1 and S2 before starting the vehicle.

The circuit costs around Rs 50.

Temperature-Sensing Diodes Selector

Low-cost semiconductor diodes such as 1N914, 1N4148 and 1N400X can be used as temperature sensors in applications where high accuracy is not required. They can be mounted on transistors, power diodes, transformers, heat sinks, rechargeable batteries, crystals, PCB, etc to monitor their temperature.

It is highly desirable to use temperature sensors with linear temperature characteristics and the mentioned diodes are best suited for that. To use them as temperature sensors, these diodes first need to be sorted according to their temperature coefficient. (Please refer ‘Signal Diode-Based Fire Alarm’ circuit idea published in February 2013 issue to understand how these diodes can be used as temperature sensors.) This circuit can help quickly sort different diodes based on their temperature coefficient.
Circuit and working
The circuit diagram of the device for selection of temperature-sensing diodes is shown in Fig. 1. The circuit is built around step-down transformer X1, voltage regulator 7818 (IC1), voltage regulator 7809 (IC2) and three diodes 1N4001 (D1 through D3). The mains supply is stepped down to 21V, 250mA using transformer X1. Diode D2 is sufficient to rectify it since the required output current is typically below 20mA. The linear regulator IC1 provides the 18V power supply for the diodes (connected at CON1 through CON12) to be tested. The linear regulator IC2 provides the 9V power supply for the digital voltmeter (DVM) used for measuring the voltage drop across diodes.

Utilising this device, it is possible to test the diodes at around 0.1mA of forward current. The current for their PN junctions is provided through individual resistors R1 through R12. The value of a resistor is much higher that the resistance of the tested diode’s junctions, so changing the diodes will not change the forward current significantly.
 
The number of the connectors for the test diodes can be increased or decreased. The device does not need any adjustment or calibration to operate properly. To calculate the temperature co-efficient (TKU) of each diode and to sort the diodes accordingly, we need to measure the voltage drop across the diodes under test.

To sort the diodes, ensure that all of them are connected in the circuit and the voltage drop (say U1) across each diode is measured one by one at the same temperature, say 25°C. The voltages can be read at TP3 through TP14 with respect to TP0 using the DVM as shown in Fig. 1. Now change the temperature to, say, 40°C, and take the measurements (say U2) again. We can use an electrical equipment, such as a heater, to produce the second temperature. So now the temperature coefficient of the PN junction for a diode will be:

TKU = (U1-U2)/(T1-T2), mV/°C
The voltage drop change of the PN junction due to temperature change is linear, so we should take measurements at only two temperatures for each diode. The test temperatures can be the same for all the diodes or different. Most of the diodes can be used in the temperature range of -25°C to +150°C without any problem. The temperature coefficient of most of the PN junctions is in the range of -1.3mV/°C to -3mV/°C, so we can use these diodes as sensors.

Construction and testing
An actual-size, single-side PCB for the device is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on PCB, enclose it in a suitable plastic case.

To test the circuit for proper functioning, verify correct power supply for the diodes and DVM at TP1 and TP2 with respect to TP0. Voltage drop across each diode can be verified at TP3 through TP14. 

Human-Machine Interface through Electromyography

Electromyography (EMG) is a technique of recording muscular activities by measuring the potential difference across a muscle of interest. EMG signal acquisition is the latest trend as it paves way for a new method of human-machine interface (HMI). Here, the signals sent by brain are received as a potential difference across the muscle. The signals are very weak in strength and are inherently embedded in noise and a large offset of the electrodes; therefore special procedures are often required to extract EMG signals of good quality.

The circuit described here helps in making the analogue front end (AFE) of a simple and cost-effective EMG acquisition system. The circuit is specially designed for battery-operated portable systems. You can see the acquired EMG signals on a PC-based oscilloscope or a normal oscilloscope for simplicity. One such PC-based oscilloscope (freeware) can be downloaded from http://www.zeitnitz.eu/scope/scope_141.exe.

Circuit and working
The circuit diagram of HMI through EMG is shown in Fig. 3. Starting from the left, two electrodes attached to a muscle are connected to a single-supply instrumentation amplifier INA122 (IC1), which amplifies the voltage difference between the two electrodes, rejecting any signal that is common to both—thereby removing much of the noise and 50Hz AC EMI. Resistor R1 determines the gain of the output of IC1.

It is difficult to deal with the bi-polar nature of the bio-signals. To overcome this problem, we use virtual-ground topology or split-rail topology, which splits the main power rail into two rails—one having Vcc and the other with half of Vcc (virtual ground). The created virtual ground acts as the pseudo-zero voltage and thus makes the output to swing between 0 and Vcc with virtual ground as zero volts, which makes the entire portion of the EMG wave available and also in the positive range only, thus making this topology ideal for interfacing with digital systems or microcontrollers.
IC1 is specifically designed for single-supply operations, for which it includes voltage translators and overvoltage protection. The electrodes are directly attached to the muscle. Prior to attaching the electrodes, clean the patient’s skin with alcohol for removing dirt and apply an ultrasound gel. Two electrodes are attached over the muscle of interest and the third one (reference) is connected to an area preferably in the joints where there are less muscles. The reference electrode should only be connected at pin 3 of CON1. It is vital as it provides the common-mode signal.

The reference is connected to the virtual ground and pin 5 (Vref) of IC1, so care must be taken to isolate the patient from ground. The patient must not touch any grounded metal, which can cause drainage of common-mode voltage and thus raise the amplitude of the output and inject a lot of noise.

The output from instrumentation amplifier IC1 is fed to a non-inverting amplifier built around OPA2241 (IC2) via  capacitor C1, which removes the DC offset from the electrodes. The amplified output signal from IC2 is sent to the PC oscilloscope via an audio jack to get the display of the waveform.

Construction and testing
An actual-size, single-side PCB for the HMI circuit is shown in Fig. 4 and its component layout in Fig. 5. After assembling the circuit on PCB, enclose it in a suitable plastic case. 

Fix a three-pin connector for the electrodes and two-pin connector for output. Also fix a two-pin connector for power supply on the front side of the panel. Please note that electrode connected to pin 3 of CON1 is the reference electrode and it should be in contact with a joint where there are less muscles. We used ECG electrodes during testing as they are easily available in medical stores. For troubleshooting, verify the voltages at different test points shown in the table.

---

Garage Light and Security Control

Useful for vehicle owners, this gadget automatically turns on indoor/outdoor garage lights and raises an alert when an automobile enters the garage.

Assume switch S2 is in ‘on’ (closed) state. When power switch S1 is turned on, the complete circuit is energised by the 12V DC supply. LED3 lights up to provide power-on indication. Simultaneously, IC3 (CD4017B) is instantly reset by the power-on-reset circuit formed by the combination of capacitor C4 and resistor R5, and green LED2 lights up as a standby indicator. As per the physical arrangement, IR rays from IR-LED fall on phototransistor T1 and it conducts to pull up the inputs of NAND gate N1 (used here as an inverter) to logic 1. As a result, the output of gate N2 goes high to make the monostable built around IC2 inactive.

Now, when a vehicle moves through the door, the IR beam is interrupted and the output state of gate N2 changes from high to low state, which triggers the monostable and red LED1 (Rx on) lights up briefly. The output of monostable provides clock pulse to IC3, which changes its output state, with its pin 2 going high and pin 3 going low. As a result, standby indicator green LED2 goes off and relay driver pnp transistor T2 gets forward biased via gate N3 to energise relay RL1. The contacts of relay RL1 can be used to switch indoor and outdoor garage lights.

After parking the vehicle, when the owner moves through the passage to interrupt the light beam once again, the monostable (IC2) is retriggered and the output state of IC3 changes again. This time, the output at pin 2 of IC3 goes low, while the output at its pin 4 goes high. This output resets IC3, after a short delay determined by components R8, C5, and D3. Standby LED2 again lights up.

Before the resetting function, the security system drive circuit is activated via gate N4 as follows: During retriggering, both inputs (pins 12 and 13) of gate N4 are at high logic level, taking its output pin 11 low to forward bias pnp transistor T3 via resistor R11. As a result, the SCR (BT169) is triggered via R12 and latched. Now the DC supply is extended to the rest of the circuit via the SCR until it is reset by disabling switch S2.

Door switch S3 is N/O type and it opens only when the door is opened. This triggers the regenerative pair of transistors T4 and T5, and relay RL2 is energised (and latched). Contacts of relay RL2 may be connected to an emergency beeper, a high-power signalling device, or an automatic telephone dialer, as desired by the user.

Resistor R7 and capacitor C6 have been deliberately added to delay the switching off of relay RL1. This extends the lamp’s ‘off’ time (for a short duration), allowing the owner to move in while the light is on. The delay can be increased by increasing the value of capacitor C6. (Note. A high-value capacitor will also increase the delay in turning the lights on, which is not desirable.)

This circuit costs around Rs 150.

Infrared Proximity Detector

This proximity detector using an infrared detector (Fig. 1) can be used in various equipment like automatic door openers and burglar alarms. The circuit primarily consists of an infrared transmitter and an infrared receiver.

IR proximity detector

The transmitter section consists of a 555 timer IC functioning in astable mode. It is wired as shown in the figure. The output from astable is fed to an infrared LED via resistor R4, which limits its operating current. This circuit provides a frequency output of 38 kHz at 50 per cent duty cycle, which is required for the infrared detector/receiver module. Siemens SFH5110-38 is a much better choice than SFH506-38. Siemens SFH5110-38 is turned on by a continuous frequency of 38 kHz with 50 per cent duty cycle, whereas SFH506 requires a burst frequency of 38k to sense. Hence, SFH5110-38 is used.

The receiver section comprises an infrared receiver module, a 555 monostable multivibrator, and an LED indicator. Upon reception of infrared signals, 555 timer (mono) turns on and remains on as long as infrared signals are received. When the signals are interrupted, the mono goes off after a few seconds (period=1.1 R7xC6) depending upon the value of R7-C6 combination. Thus if R7=470 kilo-ohms and C6=4.7µF, the mono period will be around 2.5 seconds.

Proposed arrangement for separation of IR LED and receiver module in the proximity detector

Both the transmitter and the receiver parts can be mounted on a single breadboard or PCB. The infrared receiver must be placed behind the infrared LED to avoid false indication due to infrared leakage.

An object moving nearby actually reflects the infrared rays emitted by the infrared LED. The infrared receiver has sensitivity angle (lobe) of 0-60 degrees, hence when the reflected IR ray is sensed, the mono in the receiver part is triggered. The output from the mono may be used in any desired fashion. For example, it can be used to turn on a light when a person comes nearby by energising a relay. The light would automatically turn off after some time as the person moves away and the mono pulse period is over.

The sensitivity of the detector depends on current-limiting resistor R4 in series with the infrared LED. Range is approximately 40 cm. For 20-ohm value of R4 the object at 25 cm can be sensed, while for 30-ohm value of R4 the sensing range reduces by 22.5 cm.

(Note. The author procured the samples of Siemens products from Arihant Electricals, New Delhi, the distributor of Siemens in India.)

This circuit costs around Rs 125.

Intelligent Water Pump Controller with Water-level Display

Most of the circuits for multi-level indication/control of water in tanks employ a bunch of wires running between the circuit and the overhead tank, which accounts for almost half the cost of the entire project. Here is an intelligent scanned water-level indicator-cum-pump controller circuit (Fig. 1) that utilises just four wires to the overhead tank to indicate nine different levels. The connection arrangement for the overhead tank (OHT) and the underground tank (UGT) is shown in Fig. 2. Two wires from the circuit in Fig. 1 run to the underground/ground-level tank (to output line K and return line J, respectively) to check the availability of water in the tank before operating the pump, thereby guarding the pump against the damage due to dry running.

The scanning section employs an NE555 timer (IC1) wired as an astable multivibrator to oscillate at around 1 kHz. The output of NE555 is connected to CLK inputs of two CD4017 Johnson counters (IC2 and IC8). (IC8 is placed near the overhead tank in Fig. 2.)

Suppose at a given time, there is some specific water level in the OHT. The clock from NE555 keeps advancing the Q outputs of IC2 and IC8 starting from Q0. Only when the Q output of IC8 corresponding to the first (starting from top) water-submerged probe goes high, the OHT RET line goes high through water in the OHT. This causes pins 2, 5, 10, and 13 of quad AND gate ICs (IC3 and IC4) as well as one input of AND gate A2 to go high via emitter-follower transistor T2. The identical Q output of IC2 goes high simultaneously to light up the corresponding LED (LED1 through LED9) to indicate that particular level.

Similarly, upon reception of the next clock, the next lower level is indicated by the next LED, and so on. Scanning at a very high speed gives the illusion that all LEDs up to the one corresponding to the actual level in the OHT are continuously lit. This is due to the persistence of vision.

When the water level in the OHT is high enough to light up LED1, both the inputs of AND gate A1 also go high simultaneously. As a result, the output of AND gate A1 goes high to reset the flip-flop IC (IC5). The output pin 2 of IC5 goes low to de-energise relay RL1. Now when the water level in the OHT goes low such that LEDs 1 through 9 are off, the output of AND gate A3 goes high to set RS flip-flop (IC5), thereby making its output pin 2 high. Only when there is enough water in the UGT, pin 12 of AND gate A4 will be at logic 1 to provide forward bias to relay driver transistor SL100 (T3) to energise the relay to switch on the pump motor. The motor will switch off only when the water level reaches the uppermost level or when the UGT gets empty. LED10 through LED12 indicate ‘motor off’, ‘motor on’, and ‘UGT empty’, respectively. IC8 is powered separately, using a 9V battery that lasts long enough.

This circuit costs around Rs 200.

Musical Bell

This circuits uses very few component and gives melody sound. It uses 3 terminal IC UM66 and can be build small enough to be placed inside a greeting card and operated off a single 3V flat button cell.
    There is not much to the circuit. The UM66 is connected to its supply and its output fed to a transistor for amplification. Any common speaker can be used or a “flat” piezoelectric tweeter like the one found in alarm wrist watches. If you use the piezo, then it can be connected directly between the output pin 1 and ground pin 3 without the transistor.
    The UM66 looks like a transistor with 3 terminals. It is complete miniature tone generator with a tune. Now they come with wide variety of different tunes.
    For amplification we have used a NPN transistor which is BC548. Here BC548 makes a common emitter circuit. For limiting the base current we have used a resistance of 220 Ohms so that transistor will not get damaged by excess current.

          Circuit Diagram of Musical Bell

PROCEDURE :
1. Draw circuit diagram on ply board and make hole with compass or broader for component pin insertion.
2. Identify emitter base collector of transistor and pin no. of IC UM66
3. Solder all parts according to the circuit. You will need soldering iron, Soldering flux and flexible wire.
4. Make sure all points are well soldered according to the Circuit Diagram and no dry solders. Wrong connection of IC may heat up and get damage.
5. After loading battery power ON the circuit. Now you can check the function of the project.

Rain Alarm

GIVES BEEP WHEN WATER IS IN CONTACT WITH THE WIRE

Water is a conductor of electricity. When water is in contact with the probe then there is a flow of current which reaches to the base of Q1. Transistor Q1 is a NPN transistor which conducts. With the conduction of Q1 electron reaches to Q2 which is a PNP transistor .Q2 also conducts and current flows through the speaker. In a speaker there is inductive coil which causes motion in one direction and also produce induce current which is in opposite direction to the flow of current this induce current in the form of pulse flows through a capacitor, resistance and switches off Q1 and relax .this process repeats again and again till probe is in contact with water or we can say there is a oscillation in the circuit thus speaker diaphragm vibrates and gives a tone. Frequency of the circuit depends on the value of Speaker Coil impendence, Capacitor and Resistance Value.