LED-Circuits (Page 3)

This Circuit  is a DPM or digital panel meter. It has a analog bar graph display and a 3-1/2 digit digital display. ICL7107 is used in the 200mV configuration.

U4A opamp LF353 amplifies the 200mV Full scale input to the level required for the LM3914 display circuit. D13-D14 are clamping protection diodes. Adjust P1 trimpot for a reading of 1000 counts when a 100.0 mV signal is fed at Vin. Adjust R8 trimpot to get the 5th led  to just turn on at 100mV input.

Analog and Digital Voltmeter using ICL7107

A combination of digital and analog display is helpful for quick decision making. Analog indicates even from a distance the process dimension.

Human Brain understands analog better. The digital is required to note down and record values for determining a setpoint or performance of a system.

Sometimes a analog recorder with a ink-pen plot against time is a very good way of process analysis. Many systems are better studied using graphs not tables and lists of numbers. A Computer based data-logger gives greater power to this methods. You can measure and plot graphs of various types and at different points and for much longer periods.

This is a part of my Build a DMM or Digital Multi Meter

Here is a easy to read ‘Analog’ Millivoltmeter. Just like the Moving Coil Voltmeter, but does not have that resolution. This gives a easy indication of process progress or parameter magnitude from a large distance. A bargraph is easy on decision making too, compared to a digital readout.

How this Works ? – The analog input in mV – millivolts is fed to R18, RC reduces Noise and the Zener Clamps protect. The LF353 FET Opamp offers High Impedance as a Non-Inverting Amplifier, which nullifies measurement burden, Remember the Moving coil voltmeter loads the measured circuit, causing sizable errors. Then they invented the Vacuum Tube Voltmeter to solve this issue. An FET voltmeter is near ideal, they ought to have inventing this first.

The Zeners and C3 Plastic cap should not leak, even if they do it should be in Pico Amps. Get quality stuff and do a neat job putting them together. To master this leakage and other aspects, try building an Electrometer with CA3140. Another Measurement challenge is uV Microvolt measurements, you will be faced with new glitches in connectors and PCB due to thermocouple effects and contact resistance. A soldered joint near a hot resistor will set up enough thermal gradients and create many thermocouples all over the board. Try to measure 1 Microohm with a 10 Amp pulse or 1 A DC. You will learn many things. Connectors have a craze for the Precious Metal, they act funny if they do not have enough Gold on em.

Once i observed, very low voltages or circuits with nominal voltages but very low currents, cannot break a near invisible layer between the plates of a good connector. A sub-micron coat of corrosion, dust or even some organic deposit, was forming a dielectric layer which was impervious to uV and pA. A good cleaning with a volatile organic solvent solved the problem but messed up other plastics nearby.

Millivolt Meter using a LM3914 LED Dot Display

Millivolt Meter using a LM3914 LED Dot Display. – This circuit is a part of my Build a DMM or Digital Multi Meter

This is a LED Analog Meter, This can be used as a Resistance Meter and Low Impedance Voltmeter for Battery Levels. To measure battery voltage, the R5-R12-R17 etc. part of the Reference Resistor Divider Network can be modified to suit. Shown here is for 4 LEDs, Use Three LM324 for 12 or More LEDs and Cascade as shown.

Resistance Measurement Analog LED Meter

This cannot Measure Voltage levels from High Impedance Sources, will work for Battery Voltage Tests. To make it into a Continuity tester. R27 must be a short and R23 5 Ohms. The Black probe should have a Built in Resistance of 2 Ohms. If you want it to be a dedicated voltmeter, remove R3, The Probe has to be a 10X Attenuator with 10M Ohm and The Resistor Divider Steps in 100mV per Step. The R27, R23 etc. is 20K. A Leakage Tester a Mains Voltage Monitor are other possibilities. Use LM3914 for a easier solution. A nice book for your Design Library – Measuring Circuits By Rudolf F. Graf

The proximity switch can work for a wide range of power, from 8v to 18v DC, D3 protects reverse power supply connections, and U1 regulates the supply to +5v , -5v is derived from U2 555 oscillator which serves dual purpose.

Circuit Operation

Part of – InfraRed LED Flasher for Optical Switch

The infra red diode D2 detector gets the reflected light from LED and some ambient light, The forward voltage drop of D2 will vary with the amount of light falling on it. Ambient light causes a DC component and the pulsing light from D1 causes an AC component.

Proximity Switch - Driver Supply

The capacitor C6 blocks DC and only transfers AC pulses if any to opamp amplifier U3A whose gain is set by R18, D9 rectifies the pulses to DC and this DC voltage is used by opamp comparator U3B which drives Q1 through Q2 for an open collector output for relays. LED D7 turns on when relay Output is high.

R14 and R13 can be replaced with potentiometer for threshold adjustment if required.

Connect 12v DC supply to +V and GND Ports, Connect a relay coil Between OUT and GND Ports, you can use the relay contacts as you require to turn on a lamp, heater, fan or motor.

If all connections are ok and ICs are working you should see a +5V at U3 pin8 VCC and around -4 to -5 at U3 pin4 VDD.

See also – Mixed and Interface Circuits

Optical Proximity Switch - Detector


The Optic switch can be used for both reflecting detection (retro reflective) or obstacle detection. The mechanical construction will decide this, for obstacle detection the diodes D1 and D2 could be put in two different tubes and can be kept far apart 2mts+ and both should be exactly opposite each other, any obstacle like a passing person will be detected.

To make a retro reflective proximity switch this circuit is ideal, it can be housed in a cylindrical 30mm by 70mm metal unit with m30 threads and nuts for mounting, both D1 and D2 have to be fitted in the front of this tube on a plastic plug optically insulated from each other yet beside each other.

This circuit shows the voltage doubler working with a 555. LM555 has good drive 200mA, both Vcc and Gnd.

555 has the advantage of having a high drive as well as being a Mixed Design, Analog Programmable chip. That may be a High Title for such humble a chip.

It has the capability of a Mini ADC due to its VCO function. It could form even a simple switching supply. Power Line Modems have been designed using this chip.

Timers, Modulators, Trip Relays and even a Timer for The Humble Bread Toaster. Musical circuits, Piano and Metronome Galore, it drives Speakers directly.

The Star of what we used to Know as Chip as IC. Too small today in the days of ASIC and FPGA. But ideal for Education of Electronics and Simple Real times Solutions.

This is a constant current source using a FET. This is the most simple replacement to series resistor to limit current. The N-Channel FET BF256C can give 15mA current.

Simple Methods

Before you get to use chips, experiment with some methods, which will help you learn about the LEDs better. The first is just One Resistor in series. This is to Limit the max current in a Series LED Chain. If you have a Regulated Supply with a Fixed Voltage, then you can use this method.

Let us take a 12V SMPS, Each HB White LED has a drop of around 3.2 (please see datasheet). If you put 3 LEDs in series it is a drop of 3 X 3.2 = 9.6 V.

12V – 9.6V = 2.4 V. This is the drop across the Resistor, let us keep the current at 20mA for a Long life for LED. Some LEDs will get damaged at 30mA some take more that that. We now have LED Modules which can take even 1 or 2 A.

V/I = R as per OHM. 2.4V/20mA = 120 E or Ohms.

How Hot? W = VI Power in Watts. 20mA x 2.4V = 48mW. This is where you lose the Money. Keep it low, else the Green Goblin will frown. Unless you want LED Lighting to double up as a Room Heater, Nice idea if you are in the Artic.

Now you have a chain of 3 LED with one R, make many such chains and put it in parallel to around 70% of SMPS capacity. If you have 20 Strips of 3 LEDs each, 20 X 20mA = 400mA. You will need a 12V 600mA SMPS .

MOSFET Drive for LED Constant Current

Let us assume, you have a supply that is varying and not stable. Then use a SMPS and Resistor as shown above. Closely matching the LED Chain to the SMPS voltage to keep the heat loss minimum. At Low voltages the above idea may not work. So you can try a MOSFET circuit shown.

You can use Transistors too but The Heat is more, as the Drop is more. When you use batteries, you cannot afford to lose even 0.5V. So the MOSFET is the answer.

This circuit is a nice design idea about LED drive with low voltage and watts burden. In combination with a Joule Thief – and PWM you can make many White LED utilities like Lanterns and Flashlights. PWM is to modulate brightness and also Extend LED Life.

The Essence is The LED has to have a long life, constant current is the answer. The Efficiency has to be High, Switching MOSFET is the answer. For just a LED or Two, you do not need to bother about Efficiency, but Constant Current, No Compromise. Why? Because it is in the Absolute Maximum Rating.

“You do not cross the road, when the light is RED. You do not Drive Faster, than the Speed Limit. You do not Eat, more than you can Digest.You do not Stress the Absolute Max in the Ratings.”

The Elektrik Jedi

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