Digital Mains Voltage Indicator

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 circuit 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. 

Circuit diagram:

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 circuit 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 circuit operation was used.)

Author : Pratap Chandra Sahu - Copyright : EFY