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My first project is encountering some problems

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long0900

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Hi
I have made a voltmeter using PIC and some 7 segment display. Basically, it's worked but having some problems:
1. When there is nothing connected to the measuring pin, 7 seg leds keep showing random numbers.
2. While displaying a digit, other segments aren't completely off, they are still glowing dimly.

Please help me solve these problem. Thanks
 

hi long,
For #1, try connecting a 1meg Ohm resistor from the measuring pin to 0v, this will mean your measuring into a 1meg impedance, if you want a higher input impedance consider using a non inverting CMOS OPA, I would suggest a 10megOhm to 0v when using a OPA.
So do not let the measuring input 'float' as this will give random numbers on the display.

You should also consider adding a series resistance with the input pin, just in case you probe a voltage higher than the rating of the PIC or OPA input.

#2, This can happen when the program for timing the LEDs On/Off periods is incorrect.

E
 
Hi,

long0900 said:
1. When there is nothing connected to the measuring pin, 7 seg leds keep showing random numbers.
Click to expand...

It seems like the random number is due to the noise in the adc channel.

2. While displaying a digit, other segments aren't completely off, they are still glowing dimly.
Click to expand...

The other segments are still glowing because the delay between switching the segments may be greater than 30ms.
 

Hi,

usually these problems are generated by hardware and/or software.
None of both we can see.

Therefore all we can do is guessing.

Klaus
 

Hi E,
Thanks for your help
You are partially right about my 1st problem. I have found another weird problem

When I disconnect all digit selecting pins and GND pin from the 7 seg display, the fourth digit still glowing ?!

- - - Updated - - -

Hi,
I dont think it is caused by the noise, because the range of fluctuation is very big

Praveen Kumar P S said:
It seems like the random number is due to the noise in the adc channel
Click to expand...
 
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