controlling ADC0808 using parallel port

[Harry1234]

Hey guys,

i have this problem and i was wondering if you can help out.

My project is the following: i use the parallel port of the PC, and using C++ code i read values from In0 and In1 of the ADC0808. on the in0 i have connected a thermistor and on the In1 a humidity sensor. The range goes from 0v to 5v and with 8-bit resolution from 0 - 255.
the adc0808 is clocked by a 555 timer.

I use a parallel to serial shift register to read the values and display them on the screen.


testing:

for start, i connected 0v to the input and the output of the conversion of the ADC was 0.
when i connected 5v, the output was 255.

for testing purposes, i connected 2 x 10KΩ resistors acting as voltage divider and the result from the conversion is as expected. 128 is the output of the ADC0808 @ 2.5V.

when i connected 3 resistors of 10KΩ in series, the input voltage of the ADC0808 was 1.6667V and the expected result of the ADC would have been 85 ( 255 equals 5v, for 1.6667V what is the equivalent adc value? = 85). but instead i get 120. I had it running for 3 hours and at every
check, the result was 120

the way i operate the ADC0808 is as follows:

1. Select analog input 3 on MUX of ADC0808 (i.e channel 0)
2. Turn ALE ON
3. Turn START=1. i.e Start conversion
4. Turn ALE OFF
5. Make START=0
6. I monitor the pin until End of Conversion(EOC) goes HIGH.


can you please advice on what is going wrong?




thanx in advance

Harry

 

[Harry1234]

in case you find this relevant, for power supply i dont use the 5v and 12v of the computer but instead a homemade power supply consisting of a trnasformer, 2 electrolytic capacitos and a voltage regulator.

thinking fo using Pc's voltage though.


thus eliminating any noise or voltage variations