Electronic Idea

Earth Movement Sensor

2009-03-12T02:51:00.000-07:00

A seismometer is an intriguing device. It's hard to believe that a sensor could be sensitive enough to detect ground vibrations halfway around the earth. A seismometer or earth-movement sensor, is shown in Fig. 1

The seismometer is an uncomplicated but sensitive magnetic pendulum detector. While this sensor will not pick up earth- quakes on the opposite side of the globe it has been used to pick up nuclear underground testing a few states away. The heart of the pendulum detector is a 3-foot-long x 3-inch diameter PVC tube that houses the pendulum detector. The pendulum must be free to swing unaffected by air currents or movements of the PVC outer tube. The pendulum must be placed in a draft-free, quiet, stable location to operate correctly. The easiest solution to these requirements is a PVC housing mounted to a large round plywood base that is anchored to a concrete slab Place a cap on the top end of the PVC tube. Next, drill a hole near the cap to accept a long screw that goes through the tube as shown to adjust the pendulum height above the detector. Drill a second hole near the bottom of the tube to provide viewing and sensor alignment. Place a small clear plastic cover over this hole to prevent air currents from affecting the pendulum. Attach a large round plywood base treated with a protective coating, to the PVC. Use three right-angle brackets to attach the tube to the plywood base. Secure the wooden base to a concrete slab to prevent the wooden base and sensor from "dancing" about on the floor in the case of a strong motion event.

The electronics of the earth-movement sensor include two Hall-effect sensors (see Fig. 2). Place tile two or three-lead Hall sensors at the base of the PVC tube assembly. Place both sensors flat-side down next to each other. Place one face up, i.e., writing up, and flip over the other sensor and connect the leads as shown. Sensor A has a positive output while sensor B produces a negative output with a zero magnetic field. At the center position of the pendulum, the two outputs cancel. These two outputs provide a difference signal, which is further amplified via a TI TLC251 op amp. The gain of the op amp is set for 20. Connect the op-amp's output to a zero-centered voltmeter or bridge circuit. The earth movement will be displayed as a plus and minus swing of the meter.

Next, attach a small magnet to a length of fine string or thread. Wind the free end of the thread around the long screw at the top of the PVC tube. Allow the magnet to hang freely over the Hall sensors at the base of the PVC tube. Adjust the pendulum length so that the magnet is just over but not touching the sensors. Most seismic sensors use a damping scheme to quench excessive swinging movement. This can be accomplished either mechanically or electronically. One approach to damping the pendulum is to place a glass petri dish, filled with mineral oil on top of the Hall sensors. Adjust the pendulum so that the magnet cart swing freely in the mineral oil. The earth-movement sensor is now ready to sense ground movement. This sensor arrangement is very sensitive and therefore it should be located in a quiet site away from active hiving spaces, because it will pick up passing trucks and general nearby movement.

The electronics portion of the earth-movement sensor can also be used as a compass to detect magnetic north. Mount the sensors and the electronics in a small plastic box. Affix a compass rose to the top of the plastic box. Mount the sensors next to each other perpendicular to the top of the box; i.e., the first side of the sensors are perpendicular to the around. The meter could then be read as the box is rotated. An indication will be given as the box is moved through north, and the compass rose will indicate the other directions. An LED could be substituted for the meter in this application.

Earth-Movement Sensor Parts List

Quantity	Part	Description
2	R1, R2	4.7 KΩ, ¼W resistor
1	R3, R10	100 KΩ, ¼W resistor
1	R4	1 MΩ, ¼W resistor
1	R5	24 KΩ, ¼W resistor
1	R6	50 KΩ, ¼W trim pot
2	R7, R8	20 KΩ, ¼W resistor
1	R9	330 Ω, ¼W resistor
1	D1	LED
2	U1, U2	UCS3503 Hall cells (Allegro Microsystems)
1	M	100-0-100 μA meter
1	BZ	Piezo buzzer

Smoke Detector

2009-03-12T02:26:00.000-07:00

A compact photoelectric smoke-detection system can be constructed using the Cherry Semiconductor CS235 integrated circuit. A pulsed infrared LED and a silicon photodiode are used together as the light source and detector pair. With the addition of a few discrete components, you can create a sensitive and reliable smoke detector. By cascading a number of these detector modules, a complete house smoke-detector systems can be fabricated.

The detector system consumes very little power. It pulses the system once every 10 seconds for 20 milliseconds. During the second half of the 20-ms period the LED is pulsed and the detector samples for a smoke condition. After the first alarm signal is detected a sample rate increases to a 2-second interval. After three consecutive alarm level samples are accumulated, the logic signal drives the output latch on pin 3. This synchronous detection method has very high noise rejection and provides excellent reliability.

The internal latch is capable of sinking 100mA of current and will drive a low-current relay directly. The CS235 operates from a 12Vdc supply. The latch current is determined by resistor RL, as shown in Figure, and depends upon the relay that you choose in your particular system.

The smoke detector system uses a two-ground system. A high-level ground is present on pin 4 and the low-level ground is connected on the voltage divider on pin 5. The CS235 smoke detector system is easy to construct and will provide a reliable protection system. The smoke detector can be operated on a 12V battery trickled-charged from a 110V source.

Photoelectric Smoke Detedor Parts List

Quantity	Part	Description
1	R1	20 MΩ, ¼W resistor
1	R2	75 KΩ, ¼W resistor
1	R3	6 KΩ, ¼W resistor
1	R4	510 Ω, ¼W resistor
1	R5	100 KΩ, trim pot
1	R6	18 KΩ, ¼W resistor
1	R7	12 MΩ, ¼W resistor
1	R8	36 Ω, ¼W resistor
1	R9	270 KΩ, ¼W resistor
2	R10, R11	150 KΩ, ¼W resistor
1	RL	Current Limiter Adjusted for 100 mA
1	C1	0.47 μF, 25V capacitor
1	C2	150 μF, 25V capacitor
1	C3	0.0068 μF, 25V capacitor (disk)
2	C4, C5	0.47 μF, 25V capacitor
2	C6, C7	0.001 μF, 25V capacitor (disk)
1	D1	Status LED
1	D2	Infra Red LED
1	D3	Photodiode
1	U1	CS235 (Cherry Semiconductor)
1	RL-1	9-12Vdc SPST relay

Lead-Acid Battery Charger With Voltage Analyser

2009-03-12T02:09:00.000-07:00

Nowadays maintenance-free lead-acid batteries are common in vehicles, inverters, and UPS systems. If the battery is left in a poor state of charge, its useful life is shortened. It also reduces the capacity and rechargeability of the battery. For older types of batteries, a hygrometer can be used to check the specific gravity of the acid, which, in turn, indicates the charge condition of the battery. However, you cannot use a hygrometer for sealed-type maintenance-free batteries. The only way to know their charge level is by checking their terminal voltage.
The circuit presented here can replenish the charge in a battery within 6-8 hours. It also has a voltage analysing circuit for quick checking of voltage before start of charging, since overcharging may damage the battery. The voltage analyser gives an audio-visual indication of the battery voltage level and also warns about the critical voltage level at which the battery requires immediate charging.
The charger circuit consists of a standard step-down 12V AC (2-amp) transformer and a bridge rectifier comprising diodes D1 through D4. Capacitor C1 smoothes the AC ripples to provide a clean DC for charging the battery.
The battery voltage analyser circuit is built around the popular quad op-amp LM324 that has four separate op-amps (A through D) with differential inputs. Op-amps have been used here as comparators. Switch S2 is a pushswitch, which is pressed momentarily to check the battery voltage level before charging the battery.

Battery Voltage Status of LEDs Comments
Red Green Yellow Orange
<> 9.8V On Off Off Off Danger Level
11.5V On On Off Off Low Level
12.0V On On On Off Normal Level
12.5V On On On On High Level

The non-inverting terminals of op-amps A through D are connected to the positive supply rail via a potential divider chain comprising resistors R1 through R5. Thus the voltage applied to any non-inverting input is the ratio of the resistance between that non-inverting terminal and ground to the total resistance (R1+R2+R3+R4+R5). The resistor chain provides a positive voltage of above 5V to the non-inverting inputs of all op-amps when battery voltage is 12.5V or more. A reference voltage of 5V is applied to the inverting inputs of op-amps via 5V zener diode ZD1.
When the circuit is connected to the battery and pushswitch S2 is pressed (with S1 open), the battery voltage is sampled by the analyser circuit. If the supply voltage sample applied to the non-inverting input of an op-amp exceeds the reference voltage applied to the inverting inputs, the output of the op-amp goes high and the LED connected at its output lights up.
The different levels of battery voltages are indicated by LED1 through LED4. All the LEDs remain lit when the battery is fully charged (above 12.5V). The buzzer connected to the output of IC1 also sounds (when S2 is pressed with S1 kept open) as long as the voltage of battery is above 9.8V. If the voltage level goes below 9.8V, the buzzer goes off, which indicates that it’s time to replace the battery. The status of LEDs for different battery voltages is shown in the table.
The circuit can be assembled on a general-purpose PCB or a veroboard. Use 4mm wire and crocodile clips to connect the charger to the battery. A 2.5-amp fuse connected to the output of the charger protects the analyser circuit against accidental polarity reversal.

Voltage Based Controller For Switches

2009-02-20T23:22:00.000-08:00

Here’s a simple circuit for controlling four switches from a distance through just a pair of

wires.

In the circuit, the inverting inputs (pin 2) of operational amplifiers IC1 through IC4 are set to refrence voltages of +12V, +9V, +6V, and +3V, respectively, through a chain of four 1k resistors (R1 through R4).

The reference voltage (VREF) can be simply calculated by the following relationship:

Total applied voltage × Resistance across reference voltage

V_REF =

Total resistance

For example, reference voltage V_REF3is calculated as follows:

12V + (R3 + R4) 12V x 2K

V_REF3 = =

R1+R2+R3+R4 4K

= 6V

The non-inverting inputs (pin 3) of the four op-amps ( IC1 through IC4) are tied together and connected to a pair of wires that provide +3V to +12V input voltage for controlling the switches.

Four 12V, 200-ohm, single-changeover relays are connected to four BC548 relay driver transistors (T1 through T4) via resistors R5 through R8, respectively. These relays energise depending on the voltage present at the controlling voltage input terminal; for example, relay RL4 energises when controlling voltage input of +3V is available at non-inverting pin 3 of IC4. Four electrical equipment can be connected to the terminals of the relays through the 220V AC, 50Hz mains.

Ultra Bright LED Lamp

2009-02-20T23:16:00.000-08:00

This ultra-bright white LED lamp works on 230V AC with minimal power consumption. It can be used to illuminate VU meters, SWR meters, etc. These LEDs emit a 1000-6000mCd bright white light like welding arc and work on 3 volts, 10 mA. Their maximum voltage is 3.6 volts and the current is 25 mA. Anti-static precautions should be taken when handling the LEDs. The LEDs in water-clear plastic package emit spotlight, while diffused type LEDs have a wide-angle radiation pattern. This circuit (Fig. 1) employs capacitive reactance for limiting the current flow through the LEDs on application of mains voltage to the circuit. If we use only a series resistor for limiting the current with mains operation, the limiting resistor itself will dissipate around 2 to 3 watts of power, whereas no power is dissipated in a capacitor. The value of capacitor is calculated by using the following relationships: XC = 1/(2πfC) ohms ————— (a) XC = VRMS/I ohms ————— (b) where XC is capacitive reactance in ohms, C is capacitance in farads, I is the current through the LED in amperes, f is the mains frequency in Hz, and Vrms is the input mains voltage. The 100-ohm, 2W series resistor avoids heavy ‘inrush’ current during transients. MOV at the input prevents surges or spikes, protecting the circuit. The 390-kilo-ohm, ½-watt resistor acts as a bleeder to provide discharge path for capacitor Cx when mains supply is disconnected. The zener diode at the output section prevents excess reverse voltage levels appearing across the LEDs during negative half cycles. During positive half cycle, the voltage across LEDs is limited to zener voltage. Use AC capacitors for Cx. Filter capacitor C1 across the output provides flicker-free light. The circuit can be enclosed in a CFL round case, and thus it can be connected directly to AC bulb holder socket. A series combination of 16 LEDs (Fig. 2) gives a luminance (lux) equivalent of a 12W bulb. But if you have two series combinations of 23 LEDs in parallel (total 46 LEDs as shown in Fig. 3), it gives light equal to a 35W bulb. 15 LEDs are suitable for a table-lamp light. Diode D1 (1N4007) and capacitor C1 act as rectifying and smoothing elements to provide DC voltage to the row of LEDs. For a 16-LED row, use Cx of 0.22 µF, 630V; C1 of 22 µF, 100V; and zener of 48V, 1W. Similarly, for 23+23 LED combination use Cx of 0.47 mF, 630V; C1 of 33 µF, 150V; and zener of 69V, 1W.

Fluid Detector

2009-02-20T23:00:00.000-08:00

Fluid-Detedor Switch Parts List

Quantity	Part	Description
1	R1	470 MΩ, ¼W resistor
1	R2	2 KΩ, ¼W resistor
1	C1	0.001 μF, 25V capacitor (disk)
1	C2	0.05 μF, 25V capacitor (disk)
1	C3	20 μF, 25V electrolytic capacitor
1	D1	1N914
1	T1	2N2222 PNP transistor
1	U1	LM1830 (National Semiconductor)
1	S1	Metal container with polar liquid
1	RL-1	9-12Vdc SPST relay

Fluid Detector With Sound Output Parts List

Quantity	Part	Description
1	R1	1.5 KΩ, ¼W resistor
1	C1	0.001 μF, 25V capacitor (disk)
1	U1	LM1830 (National Semiconductor)
1	S1	MRD 300 phototransistor or thermistor
1	SPKR	8 Ω Speaker

The National LM1830 fluid detector detects the Presence, absence, or level of a polar liquid such as water. The LM1830 is a linear bipolar integrated circuit that operates from a wide range of operating voltages. The fluid detector shown in Fig. 1 determines the presence of a fluid by comparing the resistance of the fluid between the probes and the internal resistance of the IC. An ac signal is used to overcome the plating problems associated with a dc voltage source. Provisions are available for connecting an external resistance in applications where thy fluid impedance is of a different magnitude than an internal resistor in the IC. As the probe resistance increases above a preset value, the internal capacitor produces a signal.

The internal oscillator is coupled to the base of an open collector transistor so that the output signal can drive an LED a speaker, or low-current relay. A 0.001μF timing capacitor between pins 1 and 7 produces a 6 kHz signal on pin 5 of the fluid detector. A filter capacitor on pin 9 enables the fluid detector to operate at a constant output. Removing this filter allows a 50% duty cycle. When used with a speaker, the output will cycle on and off.

Although the LM1830 detector was primarily designed for sensing conductive liquids, it can also be used in the direct-coupled mode. A variable resistance sensing device such as a light-dependent resistor or a thermistor, could be used as the sensing device, as shown in Fig. 2 The fluid detector is a complete, low-cost level-detector system that can be used in many different sensing applications The detector can drive a low-current relay as a local alarm. The relay contacts could be wired to other sensors, and the multiple sensors could activate a central alarm panel.

Parallel Telephone With Secrecy And Call Prevention

2009-02-09T23:30:00.000-08:00

This circuit provides secrecy when two or more telephones are connected in parallel to a telephone line. The circuit also prevents incoming calls to as well as outgoing calls from other telephones connected in parallel, except from the one lifted first.

When someone picks up the handset of the telephone connected in parallel to the original (master) phone for making an outgoing call, no dial tone is heard and the phone appears to be dead. But when a call comes, the ring signal switches the SCRs ‘on’ and conversation can be carried out. As soon as the handset is kept on the hook, the SCR goes off and the telephone can again only receive incoming calls.

When a call comes, conversation can be made only from the telephone which is lifted up first. To carry out conversation from the other telephone, the handset of the telephone that was lifted up first has to be placed on the hook and then the push-to-on switch of the associated circuit of the other telephone has to be pressed after lifting up its handset. Thus the circuit ensures privacy because both the telephones cannot be active at the same time.

Those who are don’t need parallel telephones can rig up the associated circuit of a single telephone to work as an outgoing call preventer. An outgoing call can be made only when one lifts up the handset and presses the push-to-on switch of its associated circuit.

The polarity of the telephone line can be determined by a multimeter. To avoid confusion, a bridge rectifier can be used at the input of the circuit.

Scematic

Pulse Generator

2009-02-09T23:05:00.000-08:00

This circuit is very useful while checking/operating counters, stepping relays, etc. It avoids the procedure of setting a switch for the required number of pulses. By pressing appropriate switches S1 to S9, one can get 1 to 9 negative-going clock pulses, respectively.

Schmitt trigger NAND gate N1 of IC2, resistor R1, and capacitor C1 are wired to produce clock pulses. These pulses are taken out through NAND gate N3 that is controlled by decade counter CD4017 (IC1).

Initially no switch from S1 to S9 is depressed and the LED is glowing. As pins 5 and 6 of NAND gate N2 are pulled up by resistor R3, its output pin 4 goes low. This disables NAND gate N3 to take its output pin 10 to high state, and no pulse is available.

IC1 is a decade counter whose Q outputs normally remain low. When clock pulses are applied, its Q outputs go high successively, i.e. Q0 shifts to Q1, Q1 shifts to Q2, Q3 shifts to Q4, and so on.

If any one of switches S1 through S9, say, S5 (for five pulses), is momentarily depressed, pins 5 and 6 of NAND gate N2 go low, making its output pin 4 high, which fully charges capacitor C2 via diode D. At the same time, this high output of N2 enables NAND gate N3 and clock pulses come out through pin 10. These are the required number of pulses used to check our device.

The clock pulses are fed to clock-enable pin 13 of IC1, which starts counting. As soon as output pin 1 (Q5) of IC1 turns high, input pins 5 and 6 of NAND gate N2 will also become high via switch S5 because high-frequency clock allowed five pulses during momentary pressing. This high input of N2 provides low output at pin 4 to disable NAND gate N3 and finally no pulse will be available to advance counter IC1.

Before the next usage, counter IC1 must be in the standby state, i.e. Q0 output must be in the high state. To do this, a time-delay pulse generator wired around NAND gate N4, resister R4, diode D, capacitor C2, and differentiator circuit comprising C3 and R5 is used.

When output pin 4 of NAND gate N2 is low, it discharges capacitor C2 slowly through resistor R4. When the voltage across capacitor C2 goes below the lower trip point, output pin 11 of NAND gate N4 turns high and a high-going sharp pulse is produced at the junction of capacitor C3 and resistor R5. This sharp pulse resets counter IC1 and its Q0 output (pin 3) goes high. This is represented by the glowing of LED.

Ensure the red LED is glowing before proceeding to get the next pulse. Press any of the switches momentarily and the LED will glow. If the switch is kept pressed, the counter counts continuously and you cannot get the exact number of pulses.

Scematic

DTMF Receiver IC MT8870 Tester

2009-02-06T00:14:00.001-08:00

Today, most telephone equipment use a DTMF receiver IC. One common DTMF receiver IC is the Motorola MT8870 that is widely used in electronic communications circuits. The MT8870 isan 18-pin IC. It is used in telephones and a variety of other applications. When a proper output is not obtained in projects using this IC, engineers or technicians need to test this IC separately. A quick testing of this IC could save a lot of time in research labs and manufacturing industries of communication instruments. Here’s a small and handy tester circuit for the DTMF IC. It can be assembled on a multipurpose PCB with an 18-pin IC base. One can also test the IC on a simple breadboard. For optimum working of telephone equipment, the DTMF receiver must be designed to recognise a valid tone pair greater than 40 ms in duration and to accept successive digit tone-pairs that are greater than 40 ms apart. However, for other applications like remote controls and radio communications, the tone duration may differ due to noise considerations. Therefore, by adding an extra resistor and steering diode the tone duration can be set to different values. The circuit is configured in balanced-line mode. To reject common-mode noise signals, a balanced differential amplifier input is used. The circuit also provides an excellent bridging interface across a properly terminated telephone line. Transient protection may be achieved by splitting the input resistors and inserting zener diodes (ZD1 and ZD2) to achieve voltage clamping. This allows the transient energy to be dissipated in the resistors and diodes, and limits the maximum voltage that may appear at the inputs. Whenever you press any key on your local telephone keypad, the delayed steering (Std) output of the IC goes high on receiving the tone-pair, causing LED5 (connected to pin 15 of IC via resistor R15) to glow. It will be high for a duration depending on the values of capacitor and resistors at pins 16 and 17.

The Status of LEDs on Pressing Keys on the Telephone Keypad
Key No. LED4 LED3 LED2 LED1
(MSB) (LSB)
1 Off Off Off On
2 Off Off On Off
3 Off Off On On
4 Off On Off Off
5 Off On Off On
6 Off On On Off
7 Off On On On
8 On Off Off Off
9 On Off Off On
0 On Off On Off
A On On Off On
B On On On Off
C On On On On
D Off Off Off Off

Note. 1. LED5 blinks momentarily whenever any key is pressed.
2. On = 1, while Off = 0

The optional circuit shown within dotted line is used for guard time adjustment. The LEDs connected via resistors R11 to R14 at pins 11 through 14, respectively, indicate the output of the IC. The tone-pair DTMF (dual-tone multi-frequency) generated by pressing the telephone button is converted into binary values internally in the IC. The binary values are indicated by glowing of LEDs at the output pins of the IC. LED1 represents the lowest significant bit (LSB) and LED4 represents the most significant bit (MSB). So, when you dial a number, say, 5, LED1 and LED3 will glow, which is equal to 0101. Similarly, for everyother number dialled on your telephone, the corresponding LEDs will glow. Thus, a non-defective IC should indicate proper binary values corresponding to the decimal number pressed on your telephone keypad. To test the DTMF IC 8870/KT3170, proceed as follows: 1. Connect local telephone and the circuit in parallel to the same telephone line. 2. Switch on S1. (Switch on auxiliary switch S2 only if keys A, B, C, and D are to be used.) 4. Now push key ‘*’ to generate DTMF tone. 5. Push any decimal key from the telephone keypad. 6. Observe the equivalent binary as shown in the table. 7. If the binary number implied by glowing of LED1 to LED4 is equivalent to the pressed key number (decimal/A, B, C, or D), the DTMF IC 8870 is correct. Keys A, B, C, and D on the telephone keypad are used for special signalling and are not available on standard pushbutton telephone keypads. Pin 5 of the IC is pulled down to ground through resistor R8. Switch on auxiliary switch S2. Now the high logic at pin 5 enables the detection of tones representing characters A, B, C, and D.

Three-Colour Display Using Bicolour LEDs

2009-02-05T23:50:00.000-08:00

The circuit presented here uses bicolour LEDs to generate a display in three colours, namely, red, green, and yellowish green.

Transistors T1 through T20 form a grid to which common-cathode bicolour LEDs (LED1 through LED10) are connected. Transistors T1 through T10 have their collector terminals connected to he emitter of transistor T21. Similarly, transistors T11 through T20 have their collector terminals connected to the emitter of transistor T22. The bases of each pair of transistors (i.e.T1 and T11, T2 and T12,…, T10 and T20) are tied to outputs Q0, Q1,…, Q9, respectively, of IC1 (CD4017) through 10-kilo-ohm resistors as shown in the figure. Positive supply to collectors of transistors T1 through T10 is controlled by transistor T21. Similarly, positive supply to collectors of transistors T11 through T20 is controlled by transistor T22.

IC1 and IC2 are decade counters. Clock pulse to IC1 is provided by the oscillator circuit comprising NOR gates N1 and N2. The outputs of IC1 advance sequentially with each clock. (Any other source of squarewave pulses also serves the purpose.) IC2 is used to select the mode of display. Clock input pin 14 of IC2 is connected to Q9 output of IC1. Thus IC2 receives one pulse after every ten pulses received by IC1.

When the circuit is switched on, Q0 output of IC2 is active high. Thus transistor T21 gets forward biased via diode D3 and it conducts to extend positive supply to transistors T1 through T10. Transistors T1 through T10 are forward biased sequentially by Q0 through Q9 outputs of IC1, i.e. at a time only one of these ten transistors is forward biased (on). Thus only red LED parts of bicolour LEDs light up sequentially. (Transistor T22 is not conducting at this moment.)

When red LED part of LED10 glows, IC2 receives a clock pulse and its Q1 output goes high. Transistor T21 still conducts, as it is forward biased through diode D6, and next again via diode D5. Thus red LEDs complete two more glowing sequences.

After completion of the third glowing sequence of red LEDs, when Q3 output of IC2 goes high, transistor T21 stops conducting and T22 starts conducting with the next three sequences of green LEDs of bicolour LEDs (LED1 through LED10) glowing sequentially.

After completion of three sequences of green LEDs, output Q6 of IC2 goes high. Now both transistors T21 and T22 conduct due to diodes D1 and D2. Thus both red and green LEDs in bicolour LEDs (LED1 through LED10) glow sequentially. The effect of red and green LEDs glowing to gether is a distinct yellowish orange colour. This sequence repeats four times.

Thereafter, the whole sequence repeats, starting with red LEDs. Thus the bicolour-LED display shows three colours—red, green, and yellowish green—one after the other.

The speed of display can be controlled by preset VR1. One can omit automatic selection of different colours by omitting IC2 and replacing connections to pins 3, 5, and 7 of IC2 with SPDT switches. (Thus diodes D3-D12 are also omitted.)

Scematic

Clap Switch

2009-01-05T21:48:00.000-08:00

Here’s a clap switch free from false triggering. To turn on/off any appliance, you just have to clap twice. The cir-cuit changes its output state only when you clap twice within the set time period. Here, you’ve to clap within 3 seconds.
The clap sound sensed by condenser microphone is amplified by transistor T1. The amplified signal provides negative pulse to pin 2 of IC1 and IC2, triggering both the ICs. IC1, commonly used as a timer, is wired here as a monostable multivibrator. Trigging of IC1 causes pin 3 to go high and it remains high for a certain time period depending on the selected values of R7 and C3. This ‘on’ time (T) of IC1 can be calculated using the following relationship:
T=1.1R7.C3
seconds
where R7 is in ohms and C3 in microfarads.
On first clap, output pin 3 of IC1 goes high and remains in this standby position for the preset time. Also, LED1 glows for this period. The output of IC1 provides supply voltage to IC2 at its pins 8 and 4. Now IC2 is ready to receive the triggering signal. Resistor R10 and capacitor C7 connected to pin 4 of IC2 prevent false triggering when IC1 provides the supply voltage to IC2 at first clap.
On second clap, a negative pulse triggers IC2 and its output pin 3 goes high for a time period depending on R9 and C5. This provides a positive pulse at clock pin 14 of decade counter IC 4017 (IC3). Decade counter IC3 is wired here as a bistable.
Each pulse applied at clock pin 14 changes the output state at pin 2 (Q1) of IC3 because Q2 is connected to reset pin 15. The high output at pin 2 drives transistor T2 and also energises relay RL1. LED2 indicates activation of relay RL1 and on/off status of the appliance. A free-wheeling diode (D1) prevents damage of T2 when relay de-energises.

Infra Red Remote Control

2009-01-05T21:11:00.000-08:00

This infrared remote control timer can be used to turn an appliance on/off for a period of 0.11 second to 110.0 seconds.
The circuit comprises two sections, namely, the transmitter section and the receiver section.
Fig. 1 shows the IR transmitter section. The astable multivibrator NE555 (IC1) is used to generate a 10kHz modulated IR signal. The output of IC1 is connected to the base of pnp transistor T1 via resistor R2. Two infrared LEDs (IR1 and IR2) are connected in series between the collector (via resistor R3) and ground.
When switch S1 is pressed, the IR LEDs transmit the modulated IR signal of 10-11 kHz. This frequency can be changed with the help of VR1 potmeter.
In the receiver section shown in Fig. 2, two photodiodes (IR3 and IR4) receive the IR signal transmitted by the IR transmitter. Transistors T2 and T3 amplify the weak signal. The amplified signal is filtered by capacitors C6 and C7. The amplified and filtered signal is now fed to the inverting input pin 2 of op-amp IC2 (IC 741). The output of IC2 is further connected to trigger pin 2 of timer NE555 (IC3) that is used as a monostable multivibrator whose frequency may be varied with the help of potmeter VR3.
When switch S1 of the transmitter is pressed, the modulated IR rays are generated, which are received by photodiodes in the receiver section and amplified by the amplifier circuit. The output of op-amp goes low to trigger the monostable. Then high output at pin 3 of IC3 activates the twochangeover relay RL via transistor T3 (BC548) for a preset time.
The on/off time can be set in the timer with the help of VR3 and C10. Switch S2 is used to reset the monostable. If you want to turn the appliance on for a preset time, connect the appliance via relay RL(a). On the other hand, if you want to turn the appliance off for a preset time, connect the appliance via relay RL(b). The timer can be reset by pressing reset switch S2.
The circuit works up to 3 metres without using any focusing lens. However, you can increase the operating range by using focusing lens.

Computerised Universal Timer

2008-12-29T01:05:00.000-08:00

This simple and flexible timer is more accurate than the real-time clock of the computer used for the purpose. It can be used in laboratories, dark rooms, kitchens, and for competitions in educational institutes. The program written in Q-Basic is self-explanatory.
Generally, a universal timer provides the facility for switching on an electrical/electronic device after elapse of a certain time period, say, 5 minutes. The software does the same job here.
When the program is executed, it displays 0:0:0 on the monitor, indicating 0 hour, 0 minute, and 0 second. The display time 0:0:0 is increased by 10 seconds each time function key F1 of the computer keyboard is depressed. So by depressing function key
F1 the required time is set for which the electrical or electronic device is to be switched on. However, in debate competitions the time allowed for a candidate to speak is filled the way it is discussed above. The program may be changed as indicated by REM statements and the single quote (‘) in the beginning of a program line may be accordingly removed in the program.
Now, after setting the time in the manner as discussed above, function key F2 of the computer keyboard is depressed to switch on any device. Simultaneously, the countdown of the time in the display box starts. The device will remain on until the display box shows 0:0:0 and then it will get switched off. The figure shows the relay
interface circuit connected between D0 line (pin 2) and ground line (pin 25) of 378H output port of LPT1 printer port of the computer. By D.K. Kaushik.

Computer Program In Q-Basic
CLS : SCREEN 1: COLOR 3, 10
N1 = 0: N2 = 0: N3 = 0
LOCATE 4, 10: PRINT “ PROGRAM DEVELOPED BY DR.D.K.KAUSHIK”
LINE (90, 60)-(200, 60), 2: LINE -(200, 75), 2
LINE -(90, 75), 2: LINE -(90, 60), 2
LOCATE 9, 14: PRINT N3; “:”; N2; “:”; N1
LOCATE 13, 9: PRINT “PRESS F1 KEY FOR TIME ADJUSTMENT”
LOCATE 15, 9: PRINT “PRESS F2 KEY FOR START”
LOCATE 17, 9: PRINT “PRESS F3 KEY TO STOP”
KEY(2) ON: ON KEY(2) GOSUB START
KEY(3) ON: ON KEY(3) GOSUB LAST1
PORT% = &H378
DELAY:
KEY(1) ON: ON KEY(1) GOSUB SET
GOTO DELAY
SET:
SOUND 650, 3
N1 = N1 + 10
IF N1 < 60 THEN LOCATE 9, 14: PRINT N3; “:”; N2; “:”; N1: RETURN
N1 = 0
N2 = N2 + 1
IF N2 < 60 THEN LOCATE 9, 14: PRINT N3; “:”; N2; “:”; N1: RETURN
N2 = 0: N3 = N3 + 1
LOCATE 9, 14: PRINT N3; “:”; N2; “:”; N1
RETURN
START:
KEY(1) OFF: KEY(2) OFF
Z1 = VAL(RIGHT$(TIME$, 2))
OUT PORT%, 1
REM FOR DEBATE COMPETITION OUT PORT%,1 SHOULD BE REPLACED
REM BY OUT PORT%,0
START1:
Z2 = VAL(RIGHT$(TIME$, 2))
IF Z2 = Z1 THEN GOTO START1 ELSE GOTO START2
START2:
IF N1 = 0 AND N2 = 0 AND N3 = 0 THEN LOCATE 9, 14
PRINT N3; “:”; N2; “:”; N1: GOTO LAST
N1 = N1 - 1
IF N1 < 0 THEN N1 = 59: N2 = N2 - 1
IF N2 < 0 THEN N2 = 59: N3 = N3 - 1
IF N3 < 0 THEN N3 = 0
LOCATE 9, 14: PRINT N3; “:”; N2; “:”; N1
Z1 = Z1 + 1
IF Z1 = 60 THEN Z1 = 0
IF N1 = 0 AND N2 = 0 AND N3 = 0 THEN GOTO LAST ELSE GOTO START1
LAST:
OUT PORT%, 0
REM FOR DEBATE COMPETITION OUT%,0 SHOULE BE REPLACED
BY OUT PORT%,1
FOR T = 1 TO 3
SOUND 550, 17
NEXT T
‘ FOR J=1 TO 10
‘FOR J=1 TO 50000 : NEXT J : NEXT I
‘OUT PORT%,0
END
LAST1:
OUT PORT%, 0
END

Tan Meter

2008-12-29T00:48:00.000-08:00

Six timing positions suited to different skin types

Timing affected by sunlight intensity

Parts:

R1           47K   1/4W Resistor

R2           1M   1/4W Resistor

R3,R5      120K   1/4W Resistors

R4           Photo resistor (any type)

C1,C3      10µF   25V Electrolytic Capacitors

C2           220nF   63V Polyester Capacitor

D1,D2     1N4148   75V 150mA Diodes

IC1          4060  14 stage ripple counter and oscillator IC

IC2          4017  Decade counter with 10 decoded outputs IC

Q1          BC337   45V 800mA NPN Transistor

SW1       2 poles 6 ways Rotary Switch (see notes)

SW2       SPST  Slider Switch

BZ1        Piezo sounder (incorporating 3KHz oscillator)

B1         3V Battery (two 1.5V AA or AAA cells in series etc.)

Device purpose:

This timer was designed for people wanting to get tanned but at the same time wishing to avoid an excessive exposure to sunlight.
A Rotary Switch sets the timer according to six classified Photo-types (see table).
A Photo resistor extends the preset time value according to sunlight brightness (see table).
When preset time ends, the beeper emits an intermittent signal and, to stop it, a complete switch-off of the circuit via SW2 is necessary.

Photo-type	Features	Exposure time
I & children	Light-eyed, red-haired, light complexion, freckly	20 to 33 minutes
II	Light-eyed, fair-haired, light complexion	28 to 47 minutes
III	Light or brown-eyed, fair or brown-haired, light or slightly dark complexion	40 to 67 minutes
IV	Dark-eyed, brown-haired, dark complexion	52 to 87 minutes
V	Dark-eyed, dark-haired, olive complexion	88 to 147 minutes
VI	The darkest of all	136 to 227 minutes
Note that pregnant women belong to Photo-type I

Notes:

Needing only one time set suitable for your own skin type, the rotary switch can be replaced by hard-wired links.
A DIP-Switch can be used in place of the rotary type. Please pay attention to use only one switch at a time when the device is off, or the ICs could be damaged.