Radio Transmitter

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Saturday, June 13, 2009


Commercially available hearing aids are quite costly. Here is an inexpensive hearing aid circuit that uses just four transistors and a few passive components. On moving power switch S to ‘on’ position, the condenser microphone detects the sound signal, which is amplified by transistors T1 and T2. Now the amplified signal passes through coupling capacitor C3 to the base of transistor T3. The signal is further amplified by pnp transistor T4 to drive a low impedance earphone. bg8j9xp2ym

Capacitors C4 and C5 are the power supply decoupling capacitors. The circuit can be easily assembled on a small, general-purpose PCB or a Vero board. It operates off a 3V DC supply. For this, you may use two small 1.5V cells. Keep switch S to ‘off’ state when the circuit is not in use. To increase the sensitivity of the condenser microphone, house it inside a small tube. This circuit costs around Rs 65.

Friday, June 12, 2009

Magnetic Levitation

How Levitation Works

If you hold two permanent magnets close together, you see that one of them will jump strongly toward (or away) from the other. In 1842, Samuel Earnshaw expressed the perversity of inanimate magnetic objects in his theorem. It explains this frustrating behavior will always prevent you from suspending one permanent magnet above or below another, no matter how one arranges the two magnets. However, an active control circuit can get around this problem by rapidly adjusting the magnet's strength.

Click this image to see closeup of antigravity relay (38K) The general principle is straight forward: An electromagnet pulls a ball upward while a light beam measures the exact position of the ball's top edge. The magnet's lifting force is adjusted according to position.

As less light is detected, the circuit reduces the electromagnet's current. With less current, the lifting effect is weaker and the ball can move downward until the light beam is less blocked. Voila! The ball stays centered on the detector! It is a small distance across the photodetector, perhaps a millimeter or two, but this is sufficient to measure small changes in position. Of course, if the ball is removed the coil runs at full power. And conversely, if the light beam is blocked the coil is turned completely off.

Click here to see image of breadboard circuit (54K) This device uses two photodetectors: the "signal" detector looks for an interruption in the light beam, and the "reference" detector measures the background light. The circuit subtracts one signal from the other to determine the ball's position. The use of two detectors is my small contribution to advance the art of levitation. This design automatically compensates for changes in ambient light, and eliminates a manually

Magnetic Levitation Parts List


Resistors listed in order by value are 1/4-watt, 5% unless otherwise indicated.
  • 300 ohms R11
  • 500 ohms R2
  • 1,000 ohms R1, R12, R13, R14
  • 1,500 ohms R10
  • 10,000 ohms R4
  • 11,000 ohms R6
  • 22,000 ohms R8
  • 56,000 ohms R3
  • 100,000 ohms R5
  • 150,000 ohms R7
  • 370,000 ohms R9
  • 50K linear taper VR1 (and VR2 opt.)
  • C1,C2 47 uF electrolytic
  • C3 0.1 uF ceramic or tantalum (must not be electrolytic)
  • Q1,Q2 OP505A infrared photo detector, or equivalent
  • Q3 2N3055 NPN power transistor
  • LED1,2,4 Red light-emitting diode
  • LED 3 Infrared LED emitter
  • IC1-4 LM741 op amp, Radio Shack 276-007
  • D1 1N4001 (or 1N4004) silicon diode, 50v (or more) peak inverse voltage

  • +/- 15 vdc power supply, 1 amp
  • 9 vdc power supply, 1 amp
  • Breadboard wiring pad (or printed circuit board by Amadeus)
  • 18-ga stranded wire for power
  • Solid hook-up wire
  • 24-ga (or thicker) magnet wire for lifting coil
  • 6-terminal barrier strip (2 ea.)
  • Wood for base and frame
  • Alternatives for the LM741 Op-Amp

I chose the LM741 op-amp out of nostalgia and convenience. It was an extremely successful and common op-amp about twenty years ago.

There are lots of modern choices for dual- and quad-package op-amps. By using a package with multiple op-amps, you can reduce the number of parts and lower the cost. For example, you could use a single quad-package op-amp instead of four separate 741s. This would allow a very small printed circuit board to contain all the electronics!

Electronic Stethoscope

Stethoscopes are not only useful for doctors, but home mechanics, exterminators, spying and any number of other uses. Standard stethoscopes provide no amplification which limits their use. This circuit uses op-amps to greatly amplify a standard stethoscope, and includes a low pass filter to remove background noise.


Part Total Qty. Description

R1 ---------------1 ----------- 10K 1/4W Resistor
R2 ---------------1 ----------- 2.2K 1/4W Resistor
R4 ---------------1------------ 47K 1/4W Resistor
R5, R6, R7 -------3------------ 33K 1/4W Resistor
R8 ---------------1 ----------- 56K 1/4W Resistor
R10 --------------1 ----------- 4.7K 1/4W Resistor
R11 --------------1 ------------ 2.2K to 10K Audio Taper Pot
R12 --------------1------------ 330K 1/4W Resistor
R13, R15, R16---- 3------------ 1K 1/4W Resistor
R14 --------------1 ----------- 3.9 Ohm 1/4W Resistor
C1, C8 -----------2 ---------- 470uF 16V Electrolytic Capacitor
C2 ---------------1-----------4.7uF 16V Electrolytic Capacitor
C3, C4 -----------2----------- 0.047uF 50V Metalized Plastic Film Capacitor
C5 ---------------1----------- 0.1uF 50V Ceramic Disc Capacitor
C6, C7 -----------2----------- 1000uF 16V Electrolytic Capacitor
U1 ---------------1 ----------- TL072 Low Noise Dual Op-Amp
U4 ---------------1----------- 741 Op-Amp
U5 ---------------1----------- LM386 Audio Power Amp
MIC --------------1 -----------Two Wire Electret Microphone
J1 ----------------1 -----------1/8" Stereo Headphone Jack
Batt1, Batt2 ------2----------- 9V Alkaline Battery
LED --------------1 ----------- Red/Green Dual Colour Two Wire LED
SW ---------------1 ----------- DPST Switch
MISC -------------1 ----------- Stethoscope head or jar lid, rubber sleeve for microphone, board, wire, battery clips, knob for R11

  • MIC is an assembly made out of a stethoscope head and electret mic. Cut the head off the stethoscope and use a small piece of rubber tube to join the nipple on the head to the mic.
  • Be careful with the volume, as excess noise levels may damage your ears.
  • R11 is the volume control.
  • The circuit marked as optional is not required for the main circuit to function. The optional circuit blinks an LED to the heartbeat as it is heard by the microphone. Even if the optional circuit is not included, sound will still be heard via the headphone jack.

Muscular Bio-Stimulator

Particularly suitable for cellulite treatment
3V battery supply, portable set

Device purpose:

This is a small, portable set, designed for those aiming at look improvement. The Bio-Stimulator provides muscles' stimulation and invigoration but, mainly, it could be an aid in removing cellulite.

Tape the electrodes to the skin at both ends of the chosen muscle and rotate P1 knob slowly until a light itch sensation is perceived. Each session should last about 30 - 40 minutes.


  • P1______________4K7 Linear Potentiometer
  • R1____________180K 1/4W Resistor
  • R2______________1K8 1/4W Resistor (see Notes)
  • R3______________2K2 1/4W Resistor
  • R4____________100R 1/4W Resistor
  • C1____________100nF 63V Polyester Capacitor
  • C2____________100µF 25V Electrolytic Capacitor
  • D1______________LED Red 5mm.
  • D2___________1N4007 1000V 1A Diode
  • Q1,Q2_________BC327 45V 800mA PNP Transistors
  • IC1____________7555 or TS555CN CMos Timer IC
  • T1_____________220V Primary, 12V Secondary 1.2VA Mains transformer (see Notes)
  • SW1____________SPST Switch (Ganged with P1)
  • B1_____________3V Battery (two 1.5V AA or AAA cells in series etc.)


The use of this device is forbidden to Pace-Maker bearers and pregnant women.
Do not place the electrodes on cuts, wounds, injuries or varices.
Obviously we can't claim or prove any therapeutic effectiveness for this device.

Circuit operation:

IC1 generates 150µSec. pulses at about 80Hz frequency. Q1 acts as a buffer and Q2 inverts the polarity of the pulses and drives the Transformer. The amplitude of the output pulses is set by P1 and approximately displayed by the brightness of LED D1. D2 protects Q2 against high voltage peaks generated by T1 inductance during switching.

  • T1 is a small mains transformer 220 to 12V @ 100 or 150mA. It must be reverse connected i.e. the 12V secondary winding across Q2 Collector and negative ground, and the 220V primary winding to output electrodes.
  • Output voltage is about 60V positive and 150V negative but output current is so small that there is no electric-shock danger.
  • In any case P1 should be operated by the "patient", starting with the knob fully counter-clockwise, then rotating it slowly clockwise until the LED starts to illuminate. Stop rotating the knob when a light itch sensation is perceived.
  • Best knob position is usually near the center of its range.
  • In some cases a greater pulse duration can be more effective in cellulite treatment. Try changing R2 to 5K6 or 10K maximum: stronger pulses will be easily perceived and the LED will shine more brightly.
  • Electrodes can be obtained by small metal plates connected to the output of the circuit via usual electric wire and can be taped to the skin. In some cases, moistening them with little water has proven useful.
  • SW1 should be ganged to P1 to avoid abrupt voltage peaks on the "patient's" body at switch-on, but a stand alone SPST switch will work quite well, provided you remember to set P1 knob fully counter-clockwise at switch-on.
  • Current drawing of this circuit is about 1mA @ 3V DC.
  • Some commercial sets have four, six or eight output electrodes. To obtain this you can retain the part of the circuit comprising IC1, R1, R2, C1, C2, SW1 and B1. Other parts in the diagram (i.e. P1, R3, R4, D1, D2, Q2 & T1) can be doubled, trebled or quadrupled. Added potentiometers and R3 series resistors must be wired in parallel and all connected across Emitter of Q1 and positive supply.
  • Commercial sets have frequently a built-in 30 minutes timer. For this purpose you can use the Timed Beeper the Bedside Lamp Timer or the Jogging Timer circuits available on this Website, adjusting the timing components to suit your needs.

Automated Crib Lights

Device purpose:

This circuit is intended to drive the various lights decorating the crib prepared during Christmas season at many homes in Latin Countries, especially for children delight, in order to obtain realistic light-effects.


  • Alternating day and night with lights gradually dimming from full-on to full-off and the opposite.
  • Slow turn on of model-houses interior as night approaches, and slow turn off as sun rises, with presettable intensity, thus imitating candles' light for a more realistic effect.
  • Flickering ever-running circuit driving bulbs for fires, firesides, lanterns effects etc.
  • Total cycle duration: 2 minutes. Day duration: 1 minute, 15 seconds. Night duration: 45 seconds. (All values are approximate).


  • R1___________150K 1/4W Resistor
  • R2,R9,R14_____22K 1/4W Resistors
  • R3,R11_______220K 1/4W Resistors
  • R4,R12________10K 1/4W Resistors
  • R5___________100K 1/2W Trimmer Cermet
  • R6,R7,R13,R15__1R 1/4W Resistors
  • R8____________33K 1/4W Resistor
  • R10__________270K 1/4W Resistor
  • R16___________47R 1/4W Resistor
  • C1,C4________100nF 63V Polyester Capacitors
  • C2,C6_________10µF 25V Electrolytic Capacitors
  • C3,C5________100µF 25V Electrolytic Capacitors
  • D1-D3_______1N4148 75V 150mA Diodes
  • IC1___________4060 14 stage ripple counter and oscillator IC
  • IC2__________LM324 Low power Quad Op-Amp IC
  • IC3__________78L12 12V 100mA Voltage regulator IC
  • Q1,Q3,Q5_____BC238 25V 100mA NPN Transistors
  • Q2,Q4,Q6_____BD681 100V 4A NPN Darlington Transistors
  • J1___________Miniature input socket,
  • suited for commercial plug-in variable voltage power supplies
  • J2-J5________Two ways output sockets
Load requirements:

  • Input J1 is connected to a commercial wall plug-in power supply transformer adapter with variable output settled to 12-15Vdc, and a required minimum output capability of 600mA @ 12V. Using a good number of bulbs the output capability must reach about 1.5A.
  • Output J2 is connected to a permanently-on 12V 1W blue bulb(s) for night effect.
  • Output J3 is connected to several 12V 2.2W bulbs in parallel for sunlight effect. Max. output current: 1.2A (i.e. 6-7 bulbs).
  • Output J4 is connected to several 12V 1W or 1/2W micro-bulbs in parallel for house-interiors lights. Max. output current: 600mA (i.e. 7-8 1W bulbs, doubling in number if 1/2W).
  • Output J5 is connected to one or several 12V 1W or 1/2W micro-bulbs in parallel for fires, firesides, lanterns effects etc. Max. output current: 600mA (bulbs total number same as above).
  • All outputs are current limited, and short-proof for a reasonable lapse of time.
Circuit operation:

IC1 oscillates at a frequency calculated to obtain a pin 2 level change around every minute. IC2A is then enabled to slowly charge and discharge C5 through R10 during a 2 minutes period. IC1 pin 9 drives D2, R3 & C4, generating a sawtooth for IC2C & IC2D comparators. IC2D comparing the voltage at pin 13 with the sawtooth, generates a squarewave with variable mark-space ratio driving the output darlington Q2 for daylight bulbs. IC2B changes its output at a threshold voltage settled by R8 & R9, activating IC2C & Q4 that act like IC2D & Q2 driving model-houses bulbs as evening approaches and turning them off at dawn. R11 & C6 provide slow turn on and off and R5 sets the basic brightness of these bulbs. IC1 pin 7 drives the output darlington Q6 for flickering fires bulbs and R16 prevents them to turn off completely for a more realistic effect. Q1, Q3, Q5 and associated Base resistors provide current limiting.


  • Total period length can be varied changing C1 and/or R1 values.
  • Day-night ratio can be varied changing R10 value slightly.
  • Threshold voltage of turn on and off of model-houses lights can be varied changing slightly R8 and/or R9 values.
  • Turn on and off speed of model-houses lights can be varied changing R11 value.
  • Current limiting can be varied changing Q2, Q4 & Q6 Emitter resistors.
  • Heatsinks for Q2, Q4 & Q6 are needed if current limits are increased.