Digital Mains Voltage Indicator

Digital Mains Voltage Indicator Circuit diagram. Continuous monitoring of the mains voltage is required in many ap-plications such as manual volt-age stabilisers and motor pumps. An ana-logue voltmeter, though cheap, has many disadvantages as it has moving parts and is sensitive to vibrations. The solidstate voltmeter schema described here indicates the mains voltage with a resolution that is comparable to that of a general-pur-pose analogue voltmeter. The status of the mains voltage is available in the form of an LED bar graph. Presets VR1 through VR16 are used to set the DC voltages corresponding to the 16 voltage levels over the 50-250V range as marked on LED1 through LED16, respectively, in the figure. The LED bar graph is multiplexed from the bottom to the top with the help of ICs CD4067B (16-channel multiplexer) and CD4029B (counter). The counter clocked by NE555 timer-based astable multivibrator generates 4-bit binary ad-dress for multiplexer-demultiplexer pair of CD4067B and CD4514B. 

Digital Mains Voltage Indicator Circuit diagram

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Digital Mains Voltage Indicator Circuit Diagram

The voltage from the wipers of pre-sets are multiplexed by CD4067B and the output from pin 1 of CD4067B is fed to the non-inverting input of comparator A2 (half of op-amp LM358) after being buff-ered by A1 (the other half of IC2). The unregulated voltage sensed from rectifier output is fed to the inverting input of com-parator A2. The output of comparator A2 is low until the sensed voltage is greater than the reference input applied at the non-inverting pins of comparator A2 via buffer A1. When the sensed voltage goes below the reference voltage, the output of com-parator A2 goes high. The high output from comparator A2 inhibits the decoder (CD4514) that is used to decode the out-put of IC4029 and drive the LEDs. This ensures that the LEDs of the bar graph are ‘on’ up to the sensed voltage-level pro-portional to the mains voltage.

The initial adjustment of each of the presets can be done by feeding a known AC voltage through an auto-transform and then adjusting the corresponding pre-set to ensure that only those LEDs that are up to the applied voltage glow. 

EFY note.  It is advisable to use ad-ditional transformer, rectifier, filter, and regulator arrangements for obtaining a regulated supply for the functioning of the schema so that performance of the cir-cuit is not affected even when the mains voltage falls as low as 50V or goes as high as 280V. During Lab testing regu-lated 12-volt supply for schema operation was used.)

Author : Pratap Chandra Sahu - Copyright : EFY

Bridge Rectifier LED Indicator

Bridge-Rectifier  LED Indicator Circuit Diagram. Using a few diodes and a LED, you can make a nice indicator as shown in associated schematic diagram that can be used for a lot of applications (with a bit of luck). It’s quite suitable for use in series with a doorbell or thermostat (but don’t try to use it with an electronically con-trolled central-heating boiler!). This approach allows you to make an attractive indicator for just a few pennies.

The AC or DC current through the schema causes a voltage drop across the diodes that is just enough to light the LED. As the voltage is a bit on the low side, old-fashioned red LEDs are the most suitable for this purpose. Yellow and green LEDs require a somewhat higher forward voltage, so you’ll have to first check whether it works with them. Blue and white LEDs are not suitable. You also don’t have to use modern high-efficiency types (sometimes called ‘2-mA LEDs’ or ‘3-mA LEDs’). If a DC current flows through the schema and the LED doesn’t light up, reverse the plus and minus leads.

Circuit diagram :

Bridge-Rectifier  LED Indicator Circuit Diagram

When building the schema, you’ll notice that despite its simplicity it involves fitting quite a few components to a

small printed schema board or a bit of prototyping board. That’s why we’d like to give you the tip of using a bridge rectifier, since that allows everything to be made much more compact, smaller and more tidy, and it eliminates the need for a schema board to hold the components. Besides that, you can surprise friend and foe alike, because even an old hand in the trade won’t understand the trick at first glance and will likely mumble something like “Huh?

That’s impossible.”

A bridge rectifier contains four diodes, which is exactly what you need. If you short the + and – terminals of the bridge, you create a schema with two pairs of diodes connected in parallel with oppo-site polarity. Select a bridge rectifier that can handle the current that will flow through it. In the case of a doorbell, for example, that can easily be 1 A. Select a voltage of 40 or 80 V.

Never use this schema in combination with mains voltage, due to the risk of contact with a live lead.