LED Voltage Level Indicator

This circuit is derived from a Siemens Application Note 1974. This circuit uses common components of today.

The circuit is here as it is of high educational value. I have not tested it. You can ‘simulate and test’ or ‘wire it up and try’ and let me know how it worked. The Circuit is also a simple analog to digital converter. You can use optos in place of LEDs.

Battery Level Indicator

T1 and T2 make a differential amplifier. T3, T4 and T5 driving the LEDs are comparators.  When input voltage is increased T1 is turned on which leads to more base current for T3 which Lights LED1. When input voltage is less T2 turns on as it gets a better base current from P3 which turns on LED2 via T4. When both LEDs are off T5 gets biased as no drop across R5 which lights the LED3 thru T5 hopefully.

LED Voltage Level Indicator

What you need to know is a small current Ib thru the base-emitter path in the direction of the emitter arrow will lead to a large Current Ic thru the emitter-collector path in direction of arrow. Ic = B * Ib where B – beta is the DC current gain, it could be 100-400

Fluid or Water Level with Reed Relays 

Beta is different in each transistor you buy and varies with the test conditions and even with temperature and age. The LED1 and LED2 will indicate above or below Limits set by P2 and P1. The Limit Threshold itself is set at P3 i think. LED3 will light when Hi LED and Lo LED both are off.

The applications of this circuit are FM tuning indicator, Stereo Balance Indicator (Wire T2 like T1 then we get two channel inputs) and battery level indicator.

Constant Current Source for LED

This article will explain the way a simple transistor based current source is designed, this will give an idea on how some components can be used in a practical way to make the circuit do some function, the objective is not design but to become familiar with the basic ideas.

Design of a Constant Current Source

In the circuit the LED is used as a reference so to keep it cool a 2.2K is chosen. (20V – 1.6V) / 2.2K = 8.3mA on the high side and when voltage is 10V the current will be 3.8mA min.

Constant Current Source for LED

You should know that the LED forward drop can change with ambient light as it is photo sensitive and will vary with temperature.

The circuit can be improved by using a zener in place of the LED or better still a temperature compensated reference like LM336.

Operating Current of LM336 is 400uA to 10mA, 20V The max. voltage 20V / 3.3K = 6mA. so within limits. Then you can compute the rest, wire it up to see if your design works.

Edison Bulb Life Extender

The Edsion Filament lamp may be less popular, but it is the most cost-effective light even today. It is very affordable, The light is soothing to the eyes and best of all gives a warm feeling.

Edison Bulb Life Extender

Sometimes due to the mains being restored when bulb is on, or a rapid flick of switch, blows it. This may be due to the cold filament resistance which is low, allowing a huge inrush current.

LED Lighting – The Future is Here

The problem can be solved with a soft start based on thyristor or mosfet using PWM etc. The circuit here is a low cost solution which may help a bit in reducing the inrush cold current Protection from long duration high voltages is also required but will make circuit more complex.

Now why would anyone spend for a R-C network for a bulb, you could as well buy a new bulb. But I once got a back sprain while replacing a bulb in a hurry, So the price of the bulb alone is not the issue.

CFL Lamps and Simple Inverter

The two diodes 1A form a AC path with a switching delay of diodes, The R-C network is for soft start. The Varistor-Thermistor has a cold resistance 4.7E which reduces as it warms up. If switch is flicked rapidly or power fluctuates, then varistor cannot help as its response is slow. The 10E resistor ceramic fusible wirewound resistor and C1 does the job of absorbing .

This circuit has not been tested for long times at multiple places, so i am not sure of its performance.

OR gate with two 555

This shows how to OR gate two 555, when one 555 cycles at a low frequency a valve turns on an off, the second 555 stretches the ON duration of the pulse with a diode OR gate.

 Digital Timers Counters and Clocks

OR gate with two 555

The OR output uses sample and hold to get the stable analog data from a sensor after the actuator has gone OFF, this ensures correct reading.

555 is a fundamental Mixed Signal Circuit as it can be made into a VCO using Pin-5. If you see old exar databooks, you can see 555 and PLL and Tone decoders all applications compiled in one base. I feel the Venerable Signetics 555 “Architecture” and Intersil ICL8038 ‘CMOS’ were inspiration behind early communication chip designs, Moving from Bakelite Telephones to Compact Push Button Electronic Phones and more.

Running Lights with CD4017

The 555 Astable generates a clock for this circuit, an oscillator giving a square wave output at pin 3 which is counted by 4017 to give a running lights effect.

Digital Timers Counters and Clocks

The decade counter-divider CD4017 has 10 outputs, for every low to high transition at the clock input, rising edge, the counter advances one LED. After going one full circle the the first LED lights again and it goes on. You can vary the value of R2 100K Linear potentiometer to make LEDs run fast or slow.

Running Lights with CD4017

The frequency of oscillation of astable 555 is given as f = 1.44 / ((R4 + 2 * (R2 + R3)) * C3)

The 10 outputs have 10 green LEDs. The current thru the LED is limited by R1, the current can be calculated like this (9V – 1.6V) / 1K = 7.4mA this is within 20mA which is the danger limit of the CMOS output. You want it to be bright use transistors for every output.

The cap C1 is a filter and C2 is to prevent noise at pin 5 influencing the output as it is a control voltage point.
You can cascade or chain many more counters with the CO or carry out pin 12 of 4017. The pin 15 reset is kept at low for counting, on high it will reset the counter but is not used in this circuit.

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Perpetual Candle – White LED Lamp on Ni-Cd

This is a easy to build LED lamp circuit for Learning and building skills. This is the first draft schematic V 1.0. It will need improvements for Higher Power Lighting.

Perpetual Candle Project

I will give a short summary, The LM317 here configured for around 6.4V DC. The Q3 BC547 limits the current, you can select R3 to suit, make it 1/2W. The Ni-Cd battery pack 1.2 * 4 will not get Over-Current or Over-Voltage due to this circuit.

The IRF540 Mosfet or any other equivalent you have around, along with Q2 BC547 forms a current source for the parallel 12 LED array. Ultra-bright White LED at 20mA each or use a 1W ready LED Chip. R4/R6 can be selected for the Max LED current. The voltage of LED is around 3.1 and 20mA * 12 = 240mA is the max current. You can Tweak the design for even 5A or more but then you will need a DC/DC High frequency converter in place of LM317. The Current source also needs to switch to improve efficiency. A PWM on the mosfet gives brightness control. The entire solution (switching) can be found in many chips with semiconductor vendors today.

One Single High Current LED may work well. 12 Matched LEDs also can be used. In LED Array some are dim, put min. resistor (3.9 ohm) for all 12 for current sharing. The resistor addition will impar the ability of Candle to work at lower battery voltages. Also resistor is less green, It wastes power, so use PWM and Single Die high current LED. Ni-Cd system may last over 5 Years if Candle is allways on Mains. Sealed Lead Acid system may go upto 2-3 years life but will have more punch. A SuperCap system may last more than 12 Years, i feel.

Source in Cadsoft Eagle format – candle.zip

Flashing Mains Neon Lamp

Here is a Neon Flasher circuit (untested) for a user request at Circuits FAQ. This can be built into a switchboard or a gadget for indicating Live Power.

D1-C1 form a simple half-wave rectifier, The Cap charges to peak voltage and can store charge for a long time if there is no bleeder. So while building it take extra care. This forms a DC supply across C1. C1 is a Plastic High-voltage cap, IN4007 has a 1KV rating, so it is ok for 230V rectifier.

R1 Charges C2 and when C2 reaches 60-80V depending on Neon, the neon breaksdown. C2 Discharges, Neon Recovers, The C2 starts charging again and so on and on. It Oscillates, probably in a Ramp Waveform. But do not use your Scope on this, you will regret it a lot. This is a live circuit and needs a special probe.

“Oh, i will put the probe it in 10M mode” will not do. The ground clip of the probe goes to Electrical Earth which is ‘connected’ to Neutral in the mains wiring. So you put the earth crocodile clip on the live point. There will be flashes and fireworks. So you need to isolate both terminals of scope. Please use your costly equipment with great care.

For the 1 Meg use two 470K in Series for 230V AC, that is safer. The circuit is live, so take precautions. The 0.47 Micro Farad can be increased if you want a slow flash. If the Mains 50/60 Hz Flicker is too much, the 1 uF can be made 2 uF, or use 4 – 1N4007 as a bridge rectifier.

From Schematics of delabs

User Feedback –

R1 of 4.7M and C2 of 0.47uF Works well at 230V AC. Try your own Combination. Less than 1M may damage Neon.