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.

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. 

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.

Auto Switch-off Staircase Light

In areas like staircase or porch of your home, lighting is required only for a short period of time at night. We often forget to switch off these lights, which results in considerable wastage of electricity. Here is a simple circuit that switches off the lights automatically after a predetermined time. The circuit consumes no power when inactive.

Producing a small DC voltage from AC mains to run an electronic control requires a step-down transformer or a voltage-dropping capacitor circuit. Here a tricky and easy solution is adopted. Bulb B1 gets power via the diode of bridge rectifier BR1 and zener diode ZD1. The voltage drop across zener diode ZD1 is filtered by capacitor C1. This voltage is sufficient to run the rest of the circuitry.

Working of the circuit is simple. Press switch S1 momentarily to turn bulb B1 ‘on.’ The bulb remains ‘on’ for around 20 seconds and then turns off automatically. This duration is long enough for you to find your way up or down the staircase in the dark. It can, however, be varied by changing the values of timing components R2 and C2.

Fig. 1: Circuit of staircase light controller

Construction and testing
An actual-size, single-side PCB for the staircase light controller is shown in Fig. 2 and its component layout in Fig. 3. Assemble the circuit on a PCB to minimise time and assembly errors. Carefully assemble the components and double-check for any overlooked error.

Fig. 2: An actual-size, single-side PCB for staircase light controller

Switch S1 should have current rating corresponding to the load. Multiple switches can be installed in parallel to switch S1 to turn on the bulb from different places, say, from top and bottom of the stairs. The circuit runs directly from mains power. So take utmost care while assembling.

To test the circuit for proper functioning, check test points TP1 and TP2. These should be at around 5V once switch S1 is pressed.

Solar Battery Charging Indicator

Here is the circuit of a simple charging monitor that indicates whether the storage battery of a solar power unit is being charged or not. It, however, does not tell the state of the solar panel. 

Fig. 1: Block diagram of solar battery charging indicator

The circuit consists of two common ICs, an npn transistor, ten 5mm red LEDs and a few discrete components. It can be divided into two parts: voltmeter and display controller.

The voltmeter, built around IC LM3914 (IC1), is a low-power, expanded-scale type LED voltmeter that indicates small voltage steps over the 7-16V range for 12V solar panels. The meter saves power by operating in a low-duty-cycle 'flashing' mode where the LED indicators are on (and hence consuming power) briefly. The circuit may be switched to steady mode where the active indicator remains on at all times.
The input for IC1 (LM3914) is derived from the solar panel voltage via a potential divider network comprising preset VR1 and resistors R1 and R2. This variable input is about 3V for a DC potential of 12V.
The display range depends on the internal voltage reference and resistors R3-VR2-R4. The lowest LED (LED1) glows when the input voltage at pin 5 of IC1 is 1.8V and the top most LED (LED10) glows when the voltage exceeds 4V. as the input signal is divided by 4, the display ranges should be multiplied by this figure. So the actual display range is 7-16V, i.e., 1V per LED.
The display controller is built around IC LM555 (IC2) that is wired in astable (free-running) mode with a narrow-pulse output. The duty-cycle of IC2 is controlled by the ratio of resistors R6 and R7. If you want faster blinking, use a smaller value of resistor R7. A preset may be substituted for R7 if a rate adjustment is desired. Increase the value of resistor R6 to get a longer 'on' time for LED indicators. The frequency of oscillations is determined by the combination of capacitor C4 and resistors R6 and R7.
The output of timer IC2 is fed (through current-limiting resistor R5) to transistor T1 ,which, in turn, controls the power to IC1. Capacitor C1 filters the control voltage input to IC1 and capacitor C3 provides DC filtering for the entire circuit. When you press switch S1 across capacitor C4, the output of IC2 remains high, and the display switches to steady mode from flashing mode. Switch S2 is the master power-on/off switch.
Assemble the circuit on a small, general-purpose printed-circuit board (PCB) and enclose in a suitable plastic box. After necessary calibration, connect the circuit to the output cable of the charge controller unit with correct polarity.
For calibration, lock preset VR1 at the centre position and then set VR2 to its maximum resistance with the help of a digital multimeter. Now close both the switches (S1 and S2) and connect the circuit to a variable-voltage DC power supply unit with its output level set to 12V (1%). Adjust VR1 until LED6 (at pin 14 of IC1) lights up. Finally, lock presets VR1 and VR2 using glue.  

Fig. 2: Circuit of solar battery charging indicator 



 

Cellphone-Based Remote Controller for Water Pump

nconvenience in switching on a water pump installed in a remote farm is a common problem faced by farmers. Many circuits have been developed to solve this problem. Most of them are expensive and microcontroller-based. Here we present a cellphone-based remote controller for water pump. By calling the cellphone attached to the controller, the water pump can be directly activated.

Circuit and working
Fig. 1 shows the block diagram of cellphone-based remote controller for water pump. Fig. 2 shows the circuit. The circuit is built around DTMF decoder IC MT8870 (IC1), timer NE555 (IC2) wired as monostable multivibrator and a few discrete components. The main component of the circuit is IC MT8870. This DTMF decoder has band-split filter and digital decoder functions. It offers the advantages of small size, low power consumption and high performance.

Fig. 1: Block diagram of cellphone-based remote controller for water pump 

 Fig. 2: Circuit of cellphone-based remote controller for water pump 
Once monostable timer IC2 is triggered, its output goes high for the preset time period. The time period depends on the values of resistor R7 and capacitor C4. It can be adjusted between 8 and 50 minutes using pot-meter VR1. The high output at pin 3 of IC2 energises relay RL1 to switch on the water pump.

The triggering pulse for IC2 is generated by DTMF decoder IC1 and the arrangement of diodes D1 through D5. Std pin of IC1 provides a high pulse when a valid tone-pair is received. Transistor T1 conducts only when outputs Q0 through Q2 and Std are high simultaneously. This can be achieved by sending digit ‘7’ through DTMF.

The water pump controller is connected to a dedicated cellphone through connector J1 with auto-answering mode enabled. The DTMF signal sent from the user end is decoded by the DTMF decoder and the corresponding binary-coded decimal (BCD) value appears on outputs Q0 through Q3. In this circuit only three of them are used.

Working of the circuit is simple. To switch ‘on’ the water pump, call the cellphone connected to the controller circuit and press ‘7’ once the ring stops. LED1 will glow to indicate that the water pump is switched on. The water pump turns off automatically after the preset time. LED1 turns off simultaneously.

Construction and testing
An actual-size, single-side PCB for cell-phone-based remote controller is shown in Fig. 3 and its component layout in Fig. 4. Suitable connector is provided on the PCB to connect the cellphone. Assemble the circuit on a PCB to minimise time and assembly errors. Carefully assemble the components and double-check for any overlooked error. Use suitable IC socket for MT887 and NE555 ICs.

Fig. 3: An actual-size, single-side PCB for cellphone-based remote controller

Fig. 4: Component layout for the PCB

Use relay RL1 with contact current rating capable of carrying the water pump’s current.

To test the circuit for proper functioning, press switch S1 and verify 5V at TP1 with respect to TP0. Connect the cellphone to the controller using connector J1. Call this cellphone and press ‘7’ once the ring stops. At the same time, verify high-to-low triggering pulse at TP2. TP3 now should be high for the preset time period.