reading atari pot, not getting linear

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#1
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I'm using a 555 timer to read an atari paddle.

I get a vary radical exponential reading. Like 12:0 to 9:00 is a difference of 300us, 9:00 to 6 is 100us, and 6:00 to 3 is 50us. I'm guessing some math could correct this but was wondering if anyone knew why its so outrageously off like that. Using an atari paddles is not something new, but I dont see much on the matter. in regards to this.

BTW: the atari is not a true pot, its used as a variable resistor. Only two leads or connected by design.

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With two lead variable resistor, I would NOT expect linear.

Jim

 

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As I figured, then I must do the math, thx.

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Capacitor charging with fixed voltage through resistor is exponential, therefore your time is not linear either.

Note that charging capacitor with fixed current is linear in time, so time is linear if pot is linear.

But first you should figure out if the variable resistor in the paddle is linear or logarithmic.

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No it does not say "paddle resistance is logarithmic".

It just says that when doing the voltage measurement by putting a resistor from paddle output to GND, the output voltage behaves logarithmically. This would mean that the actual paddle resistance is linear.

But this is how C64 does it too - it counts how long it takes for the cap to charge. And the charging threshold is about VCC/2, which gives more linear results than charging up to VCC*2/3 like with NE55x devices.

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Ok there was a bug in the circuit that sort of vanished on its own. I hate that when that happens. and its now quite linear. All 4 paddles I have where extremely jump and I was able to fix them by opening and cleaning with alcohol.

The only issues left is only 40% of the paddle is usable. These pots are 30 some years old, but they all act the same way. The Pot ( Variable resistor ) goes up to about 1 Meg, but the timer stops working around 500k. I tried to put a 300k resistor across it but it didnt help much.

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So junk all of the original electronics.

Tie the pot to V+ and Gnd, and read the wiper voltage with an ADC. You can use a look up table to lineraize it, or calculations.

In the days of the Atari ADCs were expensive, tended to drift a lot, and not designed for inexpensive consumer electronics. Those issues are no longer a factor, and lots of AVRs have ADCs which can read the pot just fine.

If you put a small cap from the wiper to Gnd it will help with the noise of the wiper.

JC

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In this case the device is designed to work with original old paddles. Since there is not wire attached to the 3rd pin , I'm out that option.

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One way would be to construct a constant current sink, then constant current would flow through variable paddle independent of resistance and voltage over the resistance would be linearly dependent on the resistance.

Problem just is the 1Mohm paddle, to get readings from 0-5V you would need 5uA current..