Using a Pot to vary resistance to ground?? HELP!!

[kenyan06]

Well satellite's with Infrared emmitters are placed around the room and the satellite is hung from a swivel from the roof. The unit rotates until it detects an infrared signal from the satellites and then stops to fire a laser at the target. The sensors have to be constantly checked to determine if it is still on target because we get more points for a bulls eye. So i assumed the best method was to input the voltages from the sensors into the ADC unit and convert them to a numerical value. If this value is above a certain threshold then we know were are lookin at the target and then can fire the laser.

Is there a bettter way of doing it?

 

[kenyan06]

also what do you meen by "acquisition time change" and how does this affect the values i would recive if i hooked it up to display the acdc values in a hyperterminal?

 

[ericgibbs]

also what do you meen by "acquisition time change" and how does this affect the values i would recive if i hooked it up to display the acdc values in a hyperterminal?

The ADC conversion value will be reduced if the specified source impedance is exceeded, due to the longer time it takes to charge the internal sample/hold capacitor. It will not be a 'big' reduction, bit does effect the conversion accuracy.

Is there a bettter way of doing it?
I would have chosen to feed the outputs from the 3 PT's into opa/compartors
Then used the comp outputs as digital signals to one of the ports and generate an interrupt. Much faster and simpler that 3 ADC conversions,

 

[kenyan06]

Yea that sounds like it would have been a better way of doing it, but how would you account for different distances ( lower voltages) or is there a way to change the compare value of the opamp/comparator from the atmega88?

 

[Ambient]

I know the circuit I am using works just fine with a 10k. I get a 30mV peak-peak signal. I simply decoupled the signal using a 10uF cap from the emitter to an op-amp. After that some simple filtering was done and then more gain, allowing me to get a 0-5V signal using 5V for Vcc.

 

[ericgibbs]

hi ambient,

The problem kenyan has, there is no amplification or level shifting between the PT and the AT88 ADC input.

The IR signal, I suspect will be very small. He could adjust the ADC Vref say down to +2.5Vref, this will double the ADC conversion value by increasing the slope of the ADC.

As he states the ADC input for the AT88 states 10K0 max source impedance.

If kenyan can give us some Vir signal values we may be able to suggest ideas.

 

[Ambient]

At the distances he is talking about it will be a small signal. I believe I am using a gain around 50 for my application, and that is only designed for a 2 foot range to saturate the signal.