Brightness Control for small Lamps

Parts:

P1________________470K Linear Potentiometer

R1_________________10K 1/4W Resistor
R2_________________47K 1/4W Resistor (See Notes)
R3__________________1K5 1/4W Resistor

C1_________________22nF 63V Polyester Capacitor
C2________________100µF 25V Electrolytic Capacitor

D1,D2___________1N4148 75V 150mA Diodes

IC1_______________7555 or TS555CN CMos Timer IC

Q1_______________BD681 100V 4A NPN Darlington Transistor

LP1_______________1.5V 200mA Bulb (See Notes)

SW1_______________SPST Switch

B1__________________3V (Two 1.5V AA or AAA cells in series, etc.)
Circuit operation:
This device was designed on request, to control the light intensity of four filament lamps (i.e. a ring illuminator) powered by two AA or AAA batteries, for close-up pictures with a digital camera. Obviously it can be used in other ways, at anyone's will.
IC1 generates a 150Hz square wave having a variable duty-cycle. When the cursor of P1 is fully rotated towards D1, the output positive pulses appearing at pin 3 of IC1 are very narrow. Bulb LP1, driven by Q1, is off as the voltage across its leads is too low. When the cursor of P1 is rotated towards R2, the output pulses increase in width, reaching their maximum amplitude when the potentiometer is rotated fully clockwise. In this way the bulb reaches its full brightness.

Notes:
LP1 can be one or more 1.5V bulbs wired in parallel. Maximum total output current allowed is about 1A.
R2 limits the output voltage, measured across LP1 leads, to 1.5V. Its actual value is dependent on the total current drawn by the bulb(s) and should be set at full load in order to obtain about 1.5V across the bulb(s) leads when P1 is rotated fully clockwise.

Variable DC Power Supply

Voltage range: 0.7 - 24V
Current limiting range: 50mA - 2A


Parts:

P1____________500R Linear Potentiometer
P2_____________10K Log. Potentiometer

R1,R2___________2K2 1/2W Resistors
R3____________330R 1/4W Resistor
R4____________150R 1/4W Resistor
R5______________1R 5W Resistor

C1___________3300µF 35V Electrolytic Capacitor (see Notes)
C2______________1µF 63V Polyester Capacitor

D1,D2________1N5402 200V 3A Diodes
D3_____________5mm. Red LED

Q1____________BC182 50V 100mA NPN Transistor
Q2____________BD139 80V 1.5A NPN Transistor
Q3____________BC212 50V 100mA PNP Transistor
Q4 __________2N3055 60V 15A NPN Transistor

T1_____________220V Primary, 36V Center-tapped Secondary
50VA Mains transformer (see Notes)

PL1____________Male Mains plug

SW1____________SPST Mains switch
Device purpose:
A Variable DC Power Supply is one of the most useful tools on the electronics hobbyist's workbench. This circuit is not an absolute novelty, but it is simple, reliable, "rugged" and short-proof, featuring variable voltage up to 24V and variable current limiting up to 2A. Well suited to supply the circuits shown in this website. You can adapt it to your own requirements as explained in the notes below.

Notes:
P1 sets the maximum output current you want to be delivered by the power supply at a given output voltage.
P2 sets the output voltage and must be a logarithmic taper type, in order to obtain a more linear scale voltage indication.
You can choose the Transformer on the grounds of maximum voltage and current output needed. Best choices are: 36, 40 or 48V center-tapped and 50, 75, 80 or 100VA.
Capacitor C1 can be 2200 to 6800µF, 35 to 50V.
Q4 must be mounted on a good heatsink in order to withstand sustained output short-circuit. In some cases the rear panel of the metal box in which you will enclose the circuit can do the job.
The 2N3055 transistor (Q4) can be replaced with the slightly less powerful TIP3055 type.

1992-Colourful cricket, and that rain rule

World Cup
No. 5

Minnows
Zimbabwe

Format
This was the World Cup that thought it was a league. All played all in a qualifying round that went on forever. It was fair but about as exciting as the Nullarbor Plain. The good news was that South Africa joined in for the first time, following the end of apartheid.

Innovations
Four big ones. The players wore coloured clothing, with names on the back. There were floodlights for most of the 36 games. The white ball - in fact two of them, one at each end (so they didn't get too grubby), which meant they swung prodigiously. The fielding-circle rules were refined, allowing only two men outside the ring in the first 15 overs; after the first 15 it was as before: a minimum of four inside the circle. Result: the birth of the pinch-hitter. Ian Botham did the job for England, and Mark Greatbatch was deployed by New Zealand.
Early running
Australia, the holders and hosts, were such hot favourites that the pressure got to them. They lost the opening game, in New Zealand (Martin Crowe 100 not out), and then faced England in Sydney. Botham sniffed the chance to trample Australia into the dirt for one last time and took 4 for 31 and then made 53 not out as England won by eight wickets. Pakistan started dreadfully, losing to West Indies by 10 wickets, and would have gone out if rain had not saved them in Adelaide after England bowled them out for 74. England and New Zealand were the best teams for a long time, but both had peaked too soon. Imran Khan famously told his team: "Listen, just be as if you were a cornered tiger," and they moved into top gear.

The semis
What's Afrikaans for "We wuz robbed"? South Africa, playing England, needed 22 off 13 balls when it rained in Sydney. By the time it stopped, they needed 21 off one ball. New Zealand's brave run came to an end as Pakistan successfully chased 263, with the unknown Inzamam-ul-Haq thumping 60 off 37 balls.

The final
Pakistan were on fire at the MCG, and England were not. Derek Pringle (3 for 22) removed the openers, but Imran Khan and Javed Miandad made 72 and 58 as Pakistan recovered to 249 for 6. England were soon 69 for 4 (Botham 0), and when Neil Fairbrother (62) and Allan Lamb (31) launched a recovery, Wasim Akram snuffed it out, bowling Lamb and Chris Lewis with consecutive beauties. Pakistan won by 22 runs.

Controversies
There was only one of note: the rain rule, drawn up by a panel of experts, including Richie Benaud, for matches affected by bad weather. The idea behind the rule was to avoid the old system - work out the runs-per-over of the first innings and then deduct that for each over lost by the side batting second - which heavily disadvantaged the side batting first. Under the rain rule, the reduction in the target was to be proportionate to the lowest scoring overs of the side batting first, a method that took into account the benefits of chasing, as opposed to setting, a target. The rule raised eyebrows during the washout between England and Pakistan in Adelaide and was utterly discredited when South Africa's chances of qualifying for the final were shattered by 12 minutes of rain, which changed an achievable equation to an impossible one.

Cell-Phone calling Actuator

Parts:

R1,R3,R4,R6______1M 1/4W Resistors
R2_______________3K9 1/4W Resistor
R5,R8____________1K 1/4W Resistors (Optional: see Text)
R7______________10K 1/4W Resistor

C1_____________100nF 63V Polyester or Ceramic Capacitor
C2_______________1µF 63V Polyester or Electrolytic Capacitor
C3______________10µF 25V Electrolytic Capacitor
C4______________10nF 63V Polyester or Ceramic Capacitor
C5_____________470µF 25V Electrolytic Capacitor

D1,D4_________1N4148 75V 150mA Diodes
D2,D3___________LEDs 3 or 5mm. (Optional: see Text)

Q1_____________BC547 45V 100mA NPN Transistor
Q2_____________BC557 45V 100mA PNP Transistor
Q3_____________BC337 45V 800mA NPN Transistor

IC1_____________4069 Hex Inverter IC
IC2_____________7555 or TS555CN CMos Timer IC

L1______________10mH miniature inductor

RL1____________Relay with SPDT or DPDT switch
Coil Voltage 12V. Coil resistance 200-300 Ohm

J1_____________Two ways output socket
Comments:
This design is a development of the well known Cellular Phone calling Detector circuit. Many correspondents required a circuit of this kind but capable of driving a relay and supplied at 12V.
The final circuit adds to the original pulse detector coil and transistor amplifier a further amplifier and squarer, a pulse to dc converter, a timer and the relay driver.
The timer was necessary to avoid false triggering: in this way the relay will be energized only after the cell-phone is ringing since at least 10 seconds.

Circuit operation
Q1 amplifies the signal generated by the cell-phone during an incoming call and detected by L1. IC1A wired as an analog amplifier drives three inverters in series (IC1B, IC1C and IC1D) acting as square wave converters. IC1E and related components form the pulse to dc converter: when a train of pulses appears at IC1D output, a 12V steady positive voltage is present at the output of IC1E.
An optional LED (D2) can be useful to signal that a call is incoming, mainly when the cell-phone is muted.

Q2, IC2 and related components form a 10-seconds timer followed by the relay driver (IC1F and Q3).
When the output of IC1E is low, the output of IC2 is high: therefore the output of the inverter IC1F is low and Q3 is cut off.
When the output of IC1E is high, C3 starts charging through R6 and after about 10 seconds IC2 will be triggered and its output voltage will fall to zero, forcing the output of IC1F to go high: this causes the transistor to conduct and the relay will be energized.
The LED D3 is optional and can be useful to signal when the relay is on.

Notes:
A commercial 10mH miniature inductor, usually sold in the form of a tiny rectangular plastic box, was found useful as a detector coil in place of the self-made coil. Contrary to the Cellular Phone calling Detector circuit, a high sensitivity is not required here in order to avoid false triggering of the relay.
Place the cell-phone in close contact with L1.

Temperature-controlled Fan

Parts:

P1_____________22K Linear Potentiometer (See Notes)

R1_____________15K @ 20°C n.t.c. Thermistor (See Notes)
R2____________100K 1/4W Resistor
R3,R6__________10K 1/4W Resistors
R4,R5__________22K 1/4W Resistors
R7____________100R 1/4W Resistor
R8____________470R 1/4W Resistor
R9_____________33K 4W Resistor

C1_____________10nF 63V Polyester Capacitor

D1________BZX79C18 18V 500mW Zener Diode
D2_________TIC106D 400V 5A SCR
D3-D6_______1N4007 1000V 1A Diodes

Q1,Q2________BC327 45V 800mA PNP Transistors
Q3___________BC337 45V 800mA NPN Transistor

SK1__________Female Mains socket

PL1__________Male Mains plug & cable
Device purpose:
This circuit adopt a rather old design technique as its purpose is to vary the speed of a fan related to temperature with a minimum parts counting and avoiding the use of special-purpose ICs, often difficult to obtain.

Circuit operation:
R3-R4 and P1-R1 are wired as a Wheatstone bridge in which R3-R4 generate a fixed two-thirds-supply "reference" voltage, P1-R1 generate a temperature-sensitive "variable" voltage, and Q1 is used as a bridge balance detector.
P1 is adjusted so that the "reference" and "variable" voltages are equal at a temperature just below the required trigger value, and under this condition Q1 Base and Emitter are at equal voltages and Q1 is cut off. When the R1 temperature goes above this "balance" value the P1-R1 voltage falls below the "reference" value, so Q1 becomes forward biased, pulse-charging C1.
This occurs because the whole circuit is supplied by a 100Hz half-wave voltage obtained from mains supply by means of D3-D6 diode bridge without a smoothing capacitor and fixed to 18V by R9 and Zener diode D1. Therefore the 18V supply of the circuit is not true DC but has a rather trapezoidal shape. C1 provides a variable phase-delay pulse-train related to temperature and synchronous with the mains supply "zero voltage" point of each half cycle, thus producing minimal switching RFI from the SCR. Q2 and Q3 form a trigger device, generating a short pulse suitable to drive the SCR.

Notes:
The circuit is designed for 230Vac operation. If your ac mains is rated at about 115V, you can change R9 value to 15K 2W. No other changes are required.
Circuit operation can be reversed, i.e. the fan increases its speed as temperature decreases, by simply transposing R1 and P1 positions. This mode of operation is useful in controlling a hot air flux, e.g. using heaters.
Thermistor value is not critical: I tried also 10K and 22K with good results.
In this circuit, if R1 and Q1 are not mounted in the same environment, the precise trigger points are subject to slight variation with changes in Q1 temperature, due to the temperature dependence of its Base-Emitter junction characteristics. This circuit is thus not suitable for use in precision applications, unless Q1 and R1 operate at equal temperatures.
The temperature / speed-increase ratio can be varied changing C1 value. The lower the C1 value the steeper the temperature / speed-increase ratio curve and vice-versa.
Warning! The circuit is connected to 230Vac mains, then some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when plugged and enclose it in a plastic box.

Plant Watering Watcher

Parts:

R1,R4________470K 1/4W Resistors
R2____________47K 1/2W Trimmer Cermet or Carbon
R3___________100K 1/4W Resistor
R5_____________3K3 1/4W Resistor
R6____________15K 1/4W Resistor
R7___________100R 1/4W Resistor

C1_____________1nF 63V Polyester Capacitor
C2___________330nF 63V Polyester Capacitor
C3,C4_________10µF 25V Electrolytic Capacitors

D1__________1N4148 75V 150mA Diode
D2_____________5mm. Red LED

IC1___________4093 Quad 2 input Schmitt NAND Gate IC

Q1___________BC557 45V 100mA PNP Transistor

P1,P2_______Probes (See Notes)

B1______________3V Battery (2xAA, N or AAA 1.5V Cells in series)
Device purpose:
This circuit is intended to signal when a plant needs water. A LED flashes at a low rate when the ground in the flower-pot is too dry, turning off when the moisture level is increasing. Adjusting R2 will allow the user to adapt the sensitivity of the circuit for different grounds, pots and probe types.

Improvements:
This little gadget encountered a long lasting success amongst electronics enthusiasts since its first appearance on this website in 1999. Nevertheless, in the correspondence exchanged during all these years with many amateurs, some suggestions and also criticism prompted me to revise thoroughly the circuit, making some improvements requiring the addition of four resistors, two capacitors and one transistor.
This resulted in a more stable and easy to setup device, featuring a more visible flashing indicator with no resort to ultra bright LED devices.
Extensive tests were also carried out with different flower-pots and probes. Although, as can be easily imagined, differences from various pots and probe types proved to be exceedingly high, typical resistance values across two 60mm long probes driven fully into the pot's ground about 50mm apart measured around 500 to 1000 Ohm with a high water content and about 3000 - 5000 Ohm when the ground was dry.

Circuit operation:
IC1A and related components R1 and C1 form a 2KHz square wave oscillator feeding one gate input of IC1B through the voltage divider R2/R3 made variable by adjusting the Trimmer R2. If the resistance across the probes is low (as when there is a sufficient quantity of water into the pot) C2 diverts the square wave to ground, IC1B is blocked and its output will go steady hight. IC1C inverts the high status to low, thus keeping IC1D blocked: the LED is off.
When the ground in the flower-pot is becoming too dry the resistance across the probes will increase and C2 will be no longer able to divert the square wave to ground. Therefore, IC1B output begins to transfer the 2kHz signal to IC1C which, in turn, passes it to the oscillator built around IC1D.
No longer disabled by a low level on its input, the IC1D oscillator slowly pulses Q1 base low causing the LED to flash, signalling the necessity to water the plant.
The short low pulse driving the base of Q1 is actually a burst of 2kHz pulses and therefore the LED flickers about 2,000 times per second - appearing to the human eye as if the LED was steadily on for the entire duration of the pulse.

Notes:
A square wave is used to avoid problems of probes oxidization.
Probes are made with two pieces of bare, stiff lighting cable of 1mm diameter and should be about 60mm long.
The probes should be driven fully in the pot's ground about 30 - 50mm apart. Please note that all parameters regarding probes material, dimensions and spacing are not critical.
Current consumption: LED off = 150µA; LED on = 3mA for 0.1 sec. every about 2 sec. allowing the battery to last for years.
The quiescent current consumption is so low that the use of a power on/off switch was considered unnecessary. In any case, to switch the circuit completely off, you can short the probes.

Fridge door Alarm

Parts:

R1____________10K 1/4W Resistor
R2___________Photo resistor (any type)
R3,R4________100K 1/4W Resistors

C1____________10nF 63V Polyester Capacitor
C2___________100µF 25V Electrolytic Capacitor

D1,D2_______1N4148 75V 150mA Diodes

IC1___________4060 14 stage ripple counter and oscillator IC

Q1___________BC337 45V 800mA NPN Transistor

BZ1__________Piezo sounder (incorporating 3KHz oscillator)

SW1__________Miniature SPST slide Switch

B1___________3V Battery (2 AA 1.5V Cells in series)
Circuit operation:
This circuit, enclosed into a small box, is placed in the fridge near the lamp (if any) or the opening. With the door closed the interior of the fridge is in the dark, the photo resistor R2 presents a high resistance (>200K) thus clamping IC1 by holding pin 12 high. When a beam of light enters from the opening, or the fridge lamp illuminates, the photo resistor lowers its resistance (<2K), pin 12 goes low, IC1 starts counting and, after a preset delay (20 seconds in this case) the piezo sounder beeps for 20 sec. then stops for the same lapse of time and the cycle repeats until the fridge door closes. D2 connected to pin 6 of IC1 allows the piezo sounder beeping 3 times per second.

Notes:
Connecting D1 to pin 2 of IC1 will halve the delay time.
Delay time can be varied changing C1 and/or R3 values.
Any photo resistor type should work.
Quiescent current drawing is negligible, so SW1 can be omitted.
Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.
Do not put this device in the freezer.