From: Scott McDonnell (netsamurai_at_comcast.net)
Date: 2007-04-07 07:20:53
Doh...right after sending that, I looked up the design of the paddles again. They are NOT being used as voltage dividers. It is actually a rheostat (variable resistor, no center tap.) THAT was why I was talking about current sources. Alright, I will do some math for you tomorrow (unless someone wants to beat me to it) and see if I can give you some values to use. For now, it is late and my brain is unreliable. Scott McDonnell -----Original Message----- From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of Daniel O'Shea Sent: Friday, April 06, 2007 10:26 PM To: firstname.lastname@example.org Subject: Re: Commodore joystick ports ...replying to quite an old message here, but it has given me an answer I've been looking for and I'm hoping that someone will be able to elaborate a little! I have a variable voltage of 0 to 5V, and I want to translate this in to a current to simulate the variable resistance of a potentiometer connected to the SID's ADC - Scott suggests using a transistor, but I want to know what kind of transistor and what value would work best to provide a variable current which duplicates the potentiometer's variable current? and do I need any extra components besides just a transistor? thanks! > ----- Original Message ----- > Re: Commodore joystick ports > From: Scott McDonnell (NetSamurai_at_comcast.net) > Date: 2004-10-22 20:44:06 > > Jim, > > As Hársfalvi mentioned, it is current that needs to be generated, not > voltage. A resistor creates a voltage drop across it, which is a > function of current. (Resistance=Current/Voltage) The voltage is 5V, > the resitance is a range that you know from the potentiometer values, > you simply need to create the corresponding current to emulate the > resistance you need. > > A transistor in this mode (current source) should be thought of as a > valve. The valve is completely shut off when no voltage is applied to > the base. This allows the SID to discharge the capacitor (when it is > shorting for 256 > cycles) without drawing in the 5V. By varying the voltage to the base, you > are opening the valve by a varying degree, varying the amount of current > which is allowed to flow. > > You are correct in assuming that you will need an analog voltage at > the base (but not to bias it, that is for amplication - to carry an AC > signal through the transistor - you are simply using it like a valve.) > If your micro does not have a DAC built in, you can build an R-2R > ladder DAC, like the one showing here: > http://www.hut.fi/Misc/Electronics/circuits/r2r.html or a Summing DAC > like here: > http://www.phys.ualberta.ca/~gingrich/phys395/notes/node162.html > (without the op-amp). Obviously, the values of the resistors in the > circuit may need to be altered a bit to get the range of voltages you > desire. This will be based on some calculations, which are not > extremely difficult. Here's a page you can use for reference: > http://www.williamson-labs.com/480_xtor.htm#voltage-current it > describes and explains how a transistor is used to convert voltage to > current. If you need some more help, drop me a line. I would be > willing to design the circuit for you when I have a spare moment, but > the true satisfaction comes from figuring it out yourself, so I won't > deprive you of that! > > A FET might actually do a better job, but the calculations get a bit > more complex, the cost increases, and is probably not necessary for > the type of resolution you will need. An 8-bit R-2R ladder DAC would > give you 256 voltage values to feed into the base of your transistor, > which would result in 256 different currents at each axis input. > Probably way more resolution than a commodore mouse provides. > > Scott McDonnell Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing list
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