CAR Headlights Timer

 ushbutton activated 

Very simple circuitry 

parts: 

R1 4K7 1/4W Resistor 

R2,R3 1K 1/4W Resistors 

C1 100µF 25V Electrolytic Capacitor (See Notes) 

D1 1N4002 100V 1A Diode 

Q1 BC547 45V 100mA NPN Transistor 

Q2 BC327 45V 800mA PNP Transistor 

P1 SPST Pushbutton 

RL1 Relay with SPDT 10A min. switch 

Coil Voltage 12V. Coil resistance 150-600 Ohms 

Comments: 

This device is a simple timer, allowing to keep on the headlights of your vehicle for about 1min. and 

30sec., e.g. when accessing some dark place, without the necessity of coming back to switch-off the lights. 

Circuit operation: 

Pushing on P1 allows C1 charging to full 12V battery supply. Therefore Q1 is driven hard-on, driving in turn 

Q2 and its Relay load. The headlights are thus activated by means of the Relay contact wired in parallel to 

the vehicle's headlights switch. RL1 remains activated until C1 is almost fully discharged, i.e. when its 

voltage falls below about 0.7V. 

The timing delay of the circuit depends by C1 and R1 values and was set to about 1min. and 30sec. 

In practice, due to electrolytic capacitors wide tolerance value, this delay will vary from about 1min. and 

30sec. to 1min. and 50sec. 

An interesting variation is to use the inside lamp as a command source for the timer. In this way, when the 

door is opened C1 is charged, but it will start to discharge only when the door will be closed, substituting 

pushbutton operation. 

To enable the circuit acting in this way, simply connect the cathode of a 1N4002 diode to R1-C1 junction 

and the anode to the "live" lead of the inside lamp. 

This lead can be singled-out using a voltmeter, as it is the lead where a 12V voltage can be measured in 

respect to the vehicle frame when the lamp is on. 

Notes: 

The Relay contact must be rated at 10A or more. 

Timings obtained trying different tolerance electrolytic capacitors for C1: 

100µF = 1'30" to 1'50"  

47µF = 0'45" to 1'05"  

220µF = 3'15" to 4'15" 

Pulse-Generator & Signal-Tracer

 Dual-purpose test-instrument 

Very simple circuitry, 1.5V Battery-operated 

 

Parts: 

R1 1M 1/4W Resistor 

R2,R4 2K7 1/4W Resistors 

R3 150K 1/4W Resistor 

C1 2n2 630V Ceramic or Polyester Capacitor (See Notes) 

C2,C3 4n7 63V Ceramic or Polyester Capacitors 

D1 1N4148 75V 150mA Diode 

Q1 BC547 45V 100mA NPN Transistor 

Q2 BC557 45V 100mA PNP Transistor 

SW1 SPST miniature Slider Switch (See Notes) 

J1 Stereo switched 3mm. Jack socket (See Notes) 

Probe Metal Probe 3 to 5 cm. long 

Clip Miniature Crocodile Clip 

B1 1.5V Battery (AA or AAA cell etc.) 

Device purpose: 

This simple circuit generates narrow pulses at about 700-800Hz frequency. The pulses, containing 

harmonics up to the MHz region, can be injected into audio or radio-frequency stages of amplifiers, 

receivers and the like for testing purposes. A high-pitched tone can be heard from the speaker of the 

device under test when all is working properly. The clip must be connected to the ground of the device 

under test, touching with the probe the different stages of the circuit, starting from the last stage and 

going up towards the first. When the tone is no longer heard, the defective stage has been found. 

Connecting an earclip or headphone to J1, the circuit will automatically change into a two-stage amplifier 

and any audio signal coming from the device under test and picked-up by the probe will be heard through 

the headphones. The testing of a circuit should be made in the reverse manner, i.e. starting from the first 

stage and going down until the last stage. When nothing is heard, the defective stage has been found. 

Circuit operation: 

Q1 & Q2 form a complementary astable multivibrator, whose operating frequency is set mainly by R3, C2 & 

C3 values. Output pulses are taken at Q2 Collector and applied to the probe by means of decoupling 

capacitor C1. D1 provides a symmetrical shape for the output waveform. 

If an earclip or headphone jack is plugged into J1, the connection from Q2 Collector and C1-C2 is broken 

by the switch incorporated into J1: in this case the circuit becomes a two-stage amplifier. 

  

Notes: 

If you intend to use the circuit to test valve operated devices C1 must be a 630V type. Working with low 

voltage supply transistor devices the voltage of C1 can be lowered to 63 or 100V. 

If instead of a short probe, you intend to connect the circuit to the device under test by means of a piece of 

wire longer than a few centimeters, a small ceramic capacitor (470 to 1000pF) should be added in parallel 

to D1 to prevent unwanted RF oscillation. 

Current drawing when in Pulse-Generator mode is about 60µA and 1.2mA when in Signal-Tracer mode 

operation. Therefore SW1 can be omitted, provided that the earclip or headphones are unplugged when the 

circuit is unused. 

J1 is a stereo switched jack socket wired to obtain a series connection of the two earpieces forming a 

stereo headphone. In this manner the circuit is loaded with a higher impedance and sensitivity will be 

improved. 

Therefore, the higher the load impedance the more sensitive the Signal-Tracer. In any case, common 32 

Ohm impedance mini-headphones suitable for walkman sets will work fine. 

A crystal (high impedance) earpiece is a good solution, provided you substitute J1 with a mono switched 

jack socket. 

The entire circuit can be easily fitted into a pen-like enclosure, with the probe protruding like a nib. 

Ultrasonic Dog Whistle

 work simply because dogs can't hear it. Therefore, I decided to construct a new circuit (based on the 

venerable 555, of course) with a variable pitch and a relatively loud 82 dB miniature piezo beeper. The 

circuit is very simple and can be easily assembled in half an hour. Most of the components are not really 

critical, but you should keep in mind that other values will probably change the operating frequency. 

Potentiometer determines the pitch: higher resistance means lower frequency. Since different dogs react to 

different frequencies, you'll probably have to experiment a bit to get the most out of this tiny circuit. The 

circuit is shown below: 

Circuit diagram 

Despite the simplicity of the circuit, there is one little thing. The 10nF (.01) capacitor is critical as it, too, 

determines the frequency. Most ceramic caps are highly unstable and 20% tolerance is not unusual at all. 

Higher capacitance means lower frequency and vice-versa. For proper alignment and adjustment, an 

oscilloscope would be necessary. Since I don't have one, I used Winscope. Although it's limited to only 22 

kHz, that's just enough to see how this circuit works. There is no need to etch a PCB for this project, perf 

board will do. Test the circuit to see how it responds at different frequencies. A 4k7 potentiometer in 

conjunction with a 10nF (or slightly bigger) capacitor gives some 11 to 22kHz, which should do just fine. 

Install the circuit in a small plastic box and if you want to, you can add a LED pilot light. Power 

consumption is very small and a 9V battery should last a long time. Possible further experimentation: I'm 

working on an amplified version of the whistle to get a louder beep. All attempts so far haven't been 

successful as high frequency performance tends to drop dramatically with the 555. Perhaps I could use a 

frequency doubler circuit - I just don't know and I've run out of ideas. One other slightly more advanced 

project could be a simple "anti-bark" device with a sound-triggered (clap) switch that sets off the ultrasonic 

buzzer as soon as your dog starts to bark

এটা সকলেই জানেন যে অনেক প্রাণীই উচ্চ-ফ্রিকোয়েন্সি শব্দের প্রতি বিশেষভাবে সংবেদনশীল যা মানুষ

শুনতে পারে না। এই নীতির উপর ভিত্তি করে অনেক বাণিজ্যিক কীটপতঙ্গ নিবারক পাওয়া যায়, যার বেশিরভাগই

30 থেকে 50 kHz এর মধ্যে কাজ করে। তবে, আমার লক্ষ্য ছিল একটি সামান্য ভিন্ন এবং কিছুটা বেশি শক্তিশালী

অডিও ফ্রিকোয়েন্সি/আল্ট্রাসনিক শব্দ জেনারেটর ডিজাইন করা যা কুকুরদের প্রশিক্ষণের জন্য ব্যবহার করা যেতে পারে। সম্ভাবনাগুলি কল্পনা করুন -

আপনি রাতের মাঝখানে আবার ঘেউ ঘেউ করার আগে আপনার পোষা প্রাণীকে দুবার ভাবতে বাধ্য করতে পারেন অথবা এমনকি শত্রু

কুকুরদের দমন করতে পারেন (এবং আমার ধারণা চোররা এটি পছন্দ করবে!)। আমি যা পড়েছি তা থেকে, কুকুর এবং একই আকারের অন্যান্য স্তন্যপায়ী প্রাণী

পোকামাকড়ের চেয়ে অনেক আলাদা আচরণ করে। তারা 15 থেকে 25 kHz এর মধ্যে ফ্রিকোয়েন্সিতে সবচেয়ে ভালো সাড়া দেয়

এবং বয়স্করা উচ্চ স্বরের প্রতি কম সংবেদনশীল। এর মানে হল যে একটি সাধারণ কীটপতঙ্গ নিবারক

কাজ করবে না কারণ কুকুর এটি শুনতে পায় না। অতএব, আমি একটি নতুন সার্কিট তৈরি করার সিদ্ধান্ত নিলাম (অবশ্যই

পূজনীয় 555 এর উপর ভিত্তি করে) যার একটি পরিবর্তনশীল পিচ এবং তুলনামূলকভাবে জোরে 82 dB ক্ষুদ্র পাইজো বিপার থাকবে।

সার্কিটটি খুবই সহজ এবং আধ ঘন্টার মধ্যে সহজেই একত্রিত করা যাবে। বেশিরভাগ উপাদানই আসলে

গুরুত্বপূর্ণ নয়, তবে আপনার মনে রাখা উচিত যে অন্যান্য মান সম্ভবত অপারেটিং ফ্রিকোয়েন্সি পরিবর্তন করবে।

পটেনশিওমিটার পিচ নির্ধারণ করে: উচ্চতর প্রতিরোধের অর্থ কম ফ্রিকোয়েন্সি। যেহেতু বিভিন্ন কুকুর

বিভিন্ন ফ্রিকোয়েন্সিতে প্রতিক্রিয়া দেখায়, তাই এই ক্ষুদ্র সার্কিট থেকে সর্বাধিক সুবিধা পেতে আপনাকে সম্ভবত কিছুটা পরীক্ষা করতে হবে।

সার্কিটটি নীচে দেখানো হয়েছে:

সার্কিটের সরলতা সত্ত্বেও, একটি ছোট জিনিস রয়েছে। 10nF (.01) ক্যাপাসিটরটিও গুরুত্বপূর্ণ কারণ এটি

ফ্রিকোয়েন্সি নির্ধারণ করে। বেশিরভাগ সিরামিক ক্যাপ অত্যন্ত অস্থির এবং 20% সহনশীলতা মোটেও অস্বাভাবিক নয়।

উচ্চতর ক্যাপাসিট্যান্স মানে কম ফ্রিকোয়েন্সি এবং তদ্বিপরীত। সঠিক সারিবদ্ধকরণ এবং সমন্বয়ের জন্য, একটি

অসিলোস্কোপ প্রয়োজন হবে। যেহেতু আমার কাছে একটি নেই, আমি Winscope ব্যবহার করেছি। যদিও এটি মাত্র 22

kHz-এর মধ্যে সীমাবদ্ধ, এই সার্কিটটি কীভাবে কাজ করে তা দেখার জন্য এটি যথেষ্ট। এই প্রকল্পের জন্য PCB খোদাই করার প্রয়োজন নেই, পারফ

বোর্ডটি করবে। বিভিন্ন ফ্রিকোয়েন্সিতে এটি কীভাবে প্রতিক্রিয়া দেখায় তা দেখার জন্য সার্কিটটি পরীক্ষা করুন। 10nF (অথবা সামান্য বড়) ক্যাপাসিটরের সাথে

সংযোগে একটি 4k7 পোটেনশিওমিটার প্রায় 11 থেকে 22kHz দেয়, যা ঠিকঠাক কাজ করবে।

একটি ছোট প্লাস্টিকের বাক্সে সার্কিটটি ইনস্টল করুন এবং আপনি যদি চান, আপনি একটি LED পাইলট লাইট যোগ করতে পারেন।

বিদ্যুতের ব্যবহার খুব কম এবং একটি 9V ব্যাটারি দীর্ঘ সময় ধরে চলবে। সম্ভাব্য আরও পরীক্ষা: আমি

জোরে বীপ পেতে হুইসেলের একটি পরিবর্ধিত সংস্করণে কাজ করছি। এখন পর্যন্ত সব প্রচেষ্টা সফল হয়নি কারণ ৫৫৫ এর সাথে উচ্চ ফ্রিকোয়েন্সি কর্মক্ষমতা নাটকীয়ভাবে হ্রাস পায়। সম্ভবত আমি একটি

ফ্রিকোয়েন্সি ডাবলার সার্কিট ব্যবহার করতে পারি - আমি জানি না এবং আমার কোনও ধারণা নেই। আরেকটি সামান্য উন্নত

প্রকল্প হতে পারে একটি সাধারণ "অ্যান্টি-বার্ক" ডিভাইস যার একটি শব্দ-ট্রিগার (তালি) সুইচ রয়েছে যা আপনার কুকুর ঘেউ ঘেউ শুরু করার সাথে সাথে আল্ট্রাসনিক

বাজারটি বন্ধ করে দেয়।

 A circuit diagram that can be used for the generation of CW Morse code is shown here.

This circuit can be   very useful those who would like practice Ham Radio.

The circuit is nothing but an astable multivibrator   based on NE 555.

The frequency of oscillations of the circuit depends on the components R1,R2 & C1.

The   circuit can be powered from a 9V PP3 battery.

Notes. 

• The POT R2 can be used for frequency adjustments.  

• POT R3 can be used for volume adjustments.  

• The switch S1 can  be a Morse code key.  

বর্ণনা।

 CW মোর্স কোড তৈরির জন্য ব্যবহার করা যেতে পারে এমন একটি সার্কিট ডায়াগ্রাম এখানে দেখানো হয়েছে। যারা হ্যাম রেডিও অনুশীলন করতে চান তাদের জন্য এই সার্কিটটি খুবই কার্যকর হতে পারে। সার্কিটটি NE 555 এর উপর ভিত্তি করে একটি আশ্চর্যজনক মাল্টিভাইব্রেটর ছাড়া আর কিছুই নয়। সার্কিটের দোলনের ফ্রিকোয়েন্সি R1, R2 এবং C1 উপাদানগুলির উপর নির্ভর করে। সার্কিটটি 9V PP3 ব্যাটারি থেকে চালিত হতে পারে।

বিঃদ্রঃ। • ফ্রিকোয়েন্সি সমন্বয়ের জন্য POT R2 ব্যবহার করা যেতে পারে। • ভলিউম সমন্বয়ের জন্য POT R3 ব্যবহার করা যেতে পারে।

LASER Transmitter/Receiver

 This set of two circuits from the basis for a very simple light wave transmitter. A LASER beam is modulated 

and then aimed at a receiver that demodulates the signal and then presents the information (voice, data, 

etc..). The whole thing is very easy to build and requires no specialized parts execpt for the LASER itself. 

LASERs are available from MWK Industries.

Parts: 

C1, C2 0.1uf Ceramic Disc Capacitor 

C3 100uf 25V Electrolytic Capacitor 

R1 100K Ohm 1/4W Resistor 

R2 1M Ohm 1/4W Resistor 

R3 10K Pot 

Q1 NPN Phototransistor 

U1 741 Op Amp 

U2 LM386 Audio Amp 

SPKR1 8 Ohm Speaker 

T1 8 Ohm:2K Audio Transformer 

MISC Wire, Board, Knob For R3, LASER Tube and Power Supply 

Notes: 

1. In the transmitter schematic, no ballast resistor is shown because most small LASER power supplies 

already have one built in. Yours may differ, and a resistor may be needed.

2. The receiver should be kept away from bright lights. You may want to put a piece of wax paper in front 

of Q1 to keep the LASER from swamping it. 

3. In order to get any decent amount of modulation, you may need to drive T1 with more then a watt. 

4. The circuit can be made to transmit computer data with the use of two modem chips. 

Magnetic-Radiation Remote-Control

 Short-range 35KHz operation, single-channel unit 

Simple circuitry, no outer antennas required

 

Transmitter circuit diagram: 

Transmitter parts: 

R1 68K 1/4W Resistor 

C1 4n7 630V Ceramic or Polyester Capacitor 

C2 60-80pF 63V Ceramic Trimmer 

C3 100µF 25V Electrolytic Capacitor 

Q1 BC337 45V 800mA NPN Transistor 

Q2 BD139 80V 1.5A NPN Transistor 

L1 500 turns on a 10mm. diameter, 10cm. long ferrite rod. 

Enameled wire diameter: 0.2mm. 

The tap is made after 200 turns, ground side 

P1 SPST Pushbutton 

B1 6-9V Battery (4 to 6 AA 1.5V Cells in series, see Notes)

Receiver parts: 

R1,R3 1M 1/4W Resistors 

R2,R4 47K 1/4W Resistors 

R5 330K 1/4W Resistor 

R6,R7 68K 1/4W Resistors 

R8 180R 1/4W Resistor 

R9 100R 1/4W Resistor 

C1 470pF 63V Ceramic Capacitor (See Notes) 

C2 10nF 63V Polyester or Ceramic Capacitor 

C3 100µF 25V Electrolytic Capacitor 

C4,C5 100nF 63V Polyester or Ceramic Capacitors 

C6 1µF 63V Polyester, Ceramic or Electrolytic Capacitor 

D1 5 or 3mm. Red LED 

Q1,Q2,Q3 BC549C 25V 100mA NPN High-gain Low-noise Transistors 

Q4 BD328 30V 800mA PNP Transistor 

L1 700 turns on a 10mm. diameter, 10cm. long ferrite rod. 

Enameled wire diameter: 0.2mm. 

The tap is made after 350 turns, i.e. at the center of the winding 

BZ1 Piezo sounder (incorporating 3KHz oscillator, optional, see Notes) 

RL1 5V DIL Reed-Relay SPDT or DPDT (Optional, see Notes) 

B1 3V Battery (2 x 1.5V AA, AAA or AAAA Cells in series or 1 x 3V Lithium Cell) 

Device purpose: 

This unit can be useful as a short-range, single-channel remote-control. When the pushbutton in the 

transmitter circuit is briefly activated, the LED D1 in the receiver illuminates and an optional beeper or 

relay can be operated. 

Circuit operation is based on a non-modulated 35KHz frequency carrier transmitter, and on a high-gain 

two-stage 35KHz amplifier receiver, followed by a frequency-voltage converter and DC load driver. 

Outstanding features for this design are as follows: 

No outer antenna is required on both transmitter and receiver sections, due to the very low frequency 

operation. The antennas are 10mm. diameter, 10cm. long ferrite rods supporting the coils. 

Unlike Infra-red remote-controls, this unit operates through the walls etc. 

No radio-frequency interference in spite of simple circuitry. 

The receiver operates at ultra-low voltage supply (3V) and standing current (100µA): in this manner it can 

be left in stand-by mode for years before a battery replacement is needed.  

Snags are: the short-range operation (about a medium-sized apartment), the high number of windings for 

the coils and the high current drawn by the transmitter. 

Luckily, this latter snag is compensated by the fact that only a short pulse from the transmitter is needed 

 to operate the receiver. Therefore, if the transmitter is not operated continuously, its battery should last 

long. 

    Notes: 

 Q2 in the transmitter should have a small heatsink.   

A good compromise is to use a 6V supply for the transmitter (four 1.5V AA cells in series). 

In this case   current drawing is 300mA.   

Needing a shorter range operation, Q2 in the transmitter can be omitted. 

Therefore, the emitter of Q1 will   be connected to the tap of L1 coil. 

In this case the circuit could be powered by a 9V PP3 alkaline battery,   drawing about 100mA current.   

The receiver must be tuned to the transmitter frequency. 

Starting with a 470pF value for C1, you should   

try to modify its value by means of small capacitors wired in parallel to it, 

in order to obtain the highest AC   voltage output at Q2 or Q1 collector (best measured with an oscilloscope).

 C1 value might vary from about   400 to 800pF.  

 Do this setup with transmitter placed 4-5 meters away from receiver.

 During setup it is wise to temporarily   

connect the transmitter to a 6 or 9V regulated power supply, in order to save batteries.  

 A small DIL 5V reed-relay was used in spite of the 3V supply of the receiver.

 Several devices of this type   

were tested and it was found that they switch-on with a coil voltage value comprised in the 1.9 - 2.1V   range.

 The coil resistance values varied from 140 to 250 Ohm.

IR Remote Control Extender Circuit

Description: 

This is an improved IR remote control extender circuit. It has high noise immunity, is resistant to ambient 

and reflected light and has an increased range from remote control to the extender circuit of about 7 

meters. It should work with any domestic apparatus that use 36-38kHz for the IR carrier frequency. Please 

note that this is NOT compatible with some satellite receivers that use 115KHz as a carrier frequency. 

Circuit diagram 

Notes: 

The main difference between this version and the previous circuit, is that this design uses a commercially 

available Infra Red module. This module, part number IR1 is available from Harrison Electronics in the UK. 

The IR module contains a built in photo diode, amplifier circuit and buffer and decoder. It is centerd on the 

common 38kHz carrier frequency that most IR controls use. The module removes most of the carrier 

allowing decoded pulses to pass to the appliance. Domestic TV's and VCR's use extra filtering is used to 

completely remove the carrier. The IR1 is packaged in a small aluminium case, the connections viewed 

from underneath are shown below: 

The Output Adjustable Flyback Converter

 Description

 A high voltage step-up DC power supply using adjustable flyback conversion. 

Specification

 Vin = 220Vac +-10% @ 50/60Hz 

Vout =0~600Vdc @ 0.25A 

Switching Frequency: 70~100KHz 

Design Guidelines

 DCM mode, output power is 200W 

The input RMS current in worse condition with discontinuous current mode may be calculated as: 

If the optimum operating duty cycle is set at D=0.35, then input peak current can be found as: 

  

Therefore the voltage sensing limit voltage level from the FAN7554 data sheet is 1.5V 

Supply Voltage Indicator

Description

 A novel supply voltage monitor which uses a LED to show the status of a power supply. 

Notes

 This simple and slightly odd circuit can clearly show the level of the supply voltage (in a larger device): as 

long as the indicator has good 12 volts at its input, LED1 gives steady, uninterrupted (for the naked eye) 

yellow light. If the input voltage falls below 11 V, LED1 will start to blink and the blinking will just get 

slower and slower if the voltage drops further - giving very clear and intuitive representation of the 

supply's status. The blinking will stop and LED1 will finally go out at a little below 9 volts. 

On the other hand, if the input voltage rises to 13 V, LED2 will start to glow, getting at almost full power at 

14 V. 

The characteristic voltages can be adjusted primarily by adjusting the values of R1 and R4.  

The base-emitter diode of T2 basically just stands in for a zener diode. The emitter-collector path of T1 is 

inversely polarized and if the input voltage is high enough - T1 will cause oscillations and the frequency will 

be proportional to the input voltage. The relaxation oscillator ceases cycling when the input voltage gets so 

low that it no longer can cause breakdown along the emitter-collector path.  

Not all small NPN transistors show this kind of behavior when inversely polarized in a similar manner, but 

many do. BC337-40 can start oscillations at a relatively low voltage, other types generally require a volt or 

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two more. If experimenting, be careful not to punch a hole through the device under test: they oscillate at 

9-12 V or not at all. 

 

Soft Start PSU

 Description

 Two soft start power supplies. The output voltage slowly increases to the desired output. 

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Notes

 The output voltage rises slowly and reaches 15 V in 5 seconds. 

In order to perform this soft-start function, the LM317 voltage regulator IC requires an external universal 

PNP transistor and the L200 uses its internal comparator (pin 2).  

After switch-on, the rising voltage on the positive side of the charging electrolytic capacitor slowly turns the 

(initially conducting) transistor off, thus raising the voltage (relative to ground) on the adjustment pin of 

the LM317. In the L200 circuit, the corresponding electrolytic capacitor's rising voltage gradually relaxes 

the current-regulation loop inside the L200.

Small Variable power Supply

 Description

 Features: 1.3-12.2 V, 1 A, over-current protection. This is a simple but reliable device based one of the 

oldest integrated voltage regulators of them all - the LM723. 

Notes

 R2 sets the output voltage. The maximum current is determined by the value of R3: the over-current 

protection circuitry inside the LM723 senses the voltage across R3 and starts shutting the output stage off 

as soon as this voltage approaches 0.65 V. This way the current through R3 can never exceed 0.65/R3, 

even if the output is shorted. 

C3 and C4, both ceramic, must be placed as close as possible to the integrated circuit, because the LM723 

can be prone to unwanted oscillations. It is not an overkill to solder them directly (and very carefully) to 

the pins of the IC. All other connections should also be kept short. 

The LM723 works with input DC voltages from 9.5 to 40 V and the IC itself can source some 150 mA if the 

output voltage is not more than 6-7 V below the input. When an external pass transistor is used (in the 

usual emitter-follower mode), the base-emitter junction of T1 represents a significant resistance and the 

integrated circuit's output stage is relatively lightly loaded. All the current drawn by the load passes 

through T1 and it dissipates an amount of power that is directly proportional to the current and the 

difference between the input and the output DC voltage. 

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Finished project

 The plastic box is only 160x140x60 mm, yet everything found its place in it somehow. Both meters are 

second-hand items, but properly shunted, with new face plates and freshly calibrated dials. 

Specification

 Output (approximate values): 

Vmin = (R4 + R5) / (R5*1.3) 

Vmax = (7.15 / R5) * (R4 + R5) 

Imax = 0.65/R3 

Max. Power on R3: 0.42/R3 

Min. Input DC Voltage (pin 12 to pin 7): Vmax + 5 

Parts List

 B1 40V/2.5A 

C1 2200uF (3300uF even better) 

C2 4.7uF 

C3 100nF 

C4 1nF 

C5 330nF 

C6 100uF 

D1 Green LED 

D2 1N4003 

F1 0.2A F 

F2 2A M 

IC1 LM723 (in a DIL14 plastic package) 

R1 1k 

R2 Pot. 5k 

R3 0.56R/2W 

Regulated DC power supply

 Description 

This is a Regulated DC power supply with short circuit protection and with current limiter. 

Notes 

This PSU has been especially designed for current-hungry ham radio transceivers. It delivers safely around 

20Amps at 13.8V. For lower currents, a separate current limiting output, capable of 15ma up to a total of 

20A has been added. 

The power transformer should be capable to deliver at least 25A at 17.5 to 20V. The lower the voltage, the 

lower power dissipation. 

The rectified current will be "ironed" by C1, whose capacity should not be less than 40.000uF, (a golden 

rule of around 2000uF/A), but we recommend 50.000uF. This capacity can be built up by several smaller 

capacitors in parallel. 

The base of this design is a simple 12V regulator (7812). The output voltage can be brought to desired 

value (here 13.8V) by two external resistors (R5 and R6) using this formula: 

U= 12(1+R5/R6) 

The low currents (here 15mA) will keep the 7812 in its regular function. As soon as the current rises over 

15ma, the voltage drop on R4 will "open" the Q3, actually handling the high output current. This is a PNP 

539  

transistor (Ic > 25) and current amplification factor of at least 20. The one that has been tested and 

proven here is the 2N5683. 

The current limiting resistance RL, for the maximum output of 20 Amps should be 0.03 Ohms, rated at 

least 15W. You can use the resistance wire or switch several resistors in parallel, totaling the 

resistance/power values. Values for other currents can be calculated by the rule: 

RL=0.7/Imax 

The RL and Q2 (3A PNP such as BD330) form a short circuit automatic fuse. As soon as the maximum 

current reaches 20Amps, the voltage drop over the resistor RL will open Q2, and thus limit the B-E Current 

of Q3. Parallel to Q2 is Q1, which lights the LED 1 whenever the current limiting circuit is active. When the 

fuse is active, the Q2 bridges the R3, so the full current would flow through the IC1, and damage it. 

Therefore the R4 is inserted, as to limit the IC1 current to 15mA. This makes it possible to run the IC1 

without any cooling aid.  

The LED 2 will light up every time the PSU is switched on. 

There is an adjustable current limiter in parallel to the fixed output, thus providing adjustable current 

source for smaller currents. 

This circuit is very simple too. You will notice that there is no current sensing resistor. But it is really there, 

in a form of the Rds-on resistance of the N-channel FET, which actually handles the load cutoff from the 

source. The function of the FET is shown in the diagram 2. When the current Id is rising, the tension Uds 

over the resistance Rds rises very slowly in the beginning, but very fast after the knick. This means, that 

before the knick the FET behaves as a resistor but after it, works as constant current source. 

The D2, R3 and B-E connection of the Q4 will sense the Uds voltage of the FET1. When the voltage rises 

enough, the Q4 will shortcut the FET1 gate to mass, and cut the current flow through the FET 1 off. 

However, to enable the FET1 to open, there is certain gate voltage necessary, which in this case is brought 

up by the voltage divider consisting of R8, Z1, P1 and R9. So the maximum Gate voltage will be the one of 

the Z1, and the minimal will be around 3V6. The Z1 voltage (Uz1) will thus determine the max current 

flowing through the FET 1. The diagram 2 will show that for 5 Amps the Uz1 should be 5V6, and for 

20Amps around 9V6. 

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The Capacitor C4 will determine the “velocity” or the reaction time of the limiter. 100 uF will make the 

reaction time to be around 100ms, and 1n will make it 1us. Within the designed limits, the P1 will limit the 

current output in the range of 15mA to 20A.  

You can use both output simultaneously, but the total output current will be limited by the value of the RL. 

This PSU can be built also for higher outputs, as long as the transformer will handle the current 

requirements, and you provide sufficient cooling for the Q3.

Throttle -simple

এই সার্কিট দ্বারা মোটরের গতি

৫% থেকে ৯৫% পর্যন্ত সামঞ্জস্য করা যেতে পারে। সার্কিটটি একটি FET ব্যবহার করে এবং কোনও হিটসিঙ্কের প্রয়োজন হয় না। 

এই সার্কিটটি টকিং ইলেকট্রনিক্স থেকে সম্পূর্ণরূপে একত্রিত মডিউল হিসাবে পাওয়া যায়।

The speed of a motor can be adjusted by this circuit, from 

5% to 95%. The circuit uses a FET and no heatsink is 

needed. This circuit is available from Talking Electronics 

as a fully assembled module.