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

Audio Controlled Mains Switch

It is often useful for audio or video equipment to be switched off automatically after there has been no input signal for a while. The function of the on-off switch in such equipment is then taken over by switch S2 in the accompanying diagram. It remains, however, possible to  switch off manually by means of Si. Automatic  switch-off occurs after there has been no input  signal for about 2 minutes: this delay makes it possible for a new record or cassette to be placed in the  relevant machine.

 

The audio input to the proposed circuit may be  taken from the output of the relevant TV set, amplifier, or whatever. The input earth is held at + 6 V  with respect to the circuit earth by potential divider  Ri-R2-R3-R4. The two 741s function as comparators: the output of ICi goes high when the in- put signal is greater than + 50 mV, whereas the out- put of IC2 goes high when the input signal  becomes more negative than -50 mV. Resistors  R6, R7, and R8 form an OR gate that drives transistor Ti. If the output of either ICi or IC2 is logic  1, Ti conducts.

 

Circuit diagram :

Audio Controlled Mains Switch Circuit Diagram

The 555  operates as a retrigger able monostable,  whose period is determined by Rio and Ci. The  device is triggered when its pin 2 is earthed by the  closing of S2. Its output, pin 3, then remains high  for 1 to 2 minutes, depending on the leakage cur- rent of the 555. 

The monostable resets itself as soon  as the potential across Ci exceeds a certain value.  As long as there is an input signal to the circuit, Ti conducts and Ci remains uncharged. As soon as  the audio signal ceases, Ti switches off, and Ci  charges until the potential across it is sufficient to  reset the 555. The monostable may also be reset by  closing Si, which connects pin 6 of the 555 to + 12 V.

 

When IC3 is reset, Ci is discharged via its pin 7. Resistor Rrn serves as protection, because without it Ti could short-circuit the supply lines. When the output of IC3 goes high, T2 conducts,  the relay is energized, and the relay contacts switch on the mains voltage as appropriate. To counter the induced potential when the relay contacts close, which could damage T2, diode Di has been connected in parallel with the relay coil. 

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