Urgent! Simple circuit or IC to convert input voltage into output resistance.?

Question

I am making as part of an urgent school project something that maps acceleration and deceleration on a scalextric track onto a computer chart, and need to know what and if there is an IC tor some kind of circuit that can turn the variable DC (12-18) volts in to resistance 0-10K
can anyone respond quickly as i need to order parts and assemble quickly before school starts!

Additional Details:
I am using a usb experiment board and plan to use the 2 analog inputs that produce a digital value from 1 to 255 on the software, this is the value i am programming with. It works with resistance, not voltage, so i need some sort of ic or circuit to convert the variable voltage of a slot car track (12 to 18 volts) to a form of resistance.
The voltage that is being applyed to the resistance can be altered from 0 volts to 5 volts with 0.1volt increments

Answer 1

A transistor will vary its resistance in response to a voltage. The existing circuits input mostlikely has a pullup or pulldown resistor in which combined with the unknown resistance form a voltage divider to an ADC converter. Merely place a transistors collector and emitter leads on the input. Then apply your voltage to the base using the emitter as a common. You will need to bias the base to place it in the linear region and you will need a potentiometer to act as a voltage divider to allow you to adjust the input voltage aplied to the base.





http://www.eskimo.com/~ddf/Theory/Transistors.html





EDIT:

http://www.vellemanusa.com/downloads/0/illustrated/illustrated_assembly_manual_k8055_uk_rev3.pdf



I just looked at the schematic and instruction manual. The analog input is designed for a 0 to 5 volt signal and not a rsistance. The instructions on Pg18 state that the internal 5V supply can be used to simulate an analog input via RV1 or Rv2 potentiometers. Basically these potentiometers form a voltage divider that is feed by the 5V supply.



Disconnect the jumper2 SK2 and SK3 to remove the 5V supply. You can then input a voltage directly to the analog inputs and use the RV1 & RV2 pots to adjust the input voltage for a max readout at max input voltage (18V).



Problem here is that you will loose resolution if you just adjust RV1/2 as 18V would be dropped to 5V.



RV would form a voltage divider set to 27777 ohms & 72223 ohms. Applying 18V to the total 100K and extracting a scalled down voltage across 27777 ohms gives you 5 Volts (max input) to the op amp.

However drop that voltage to 12V with the same RV settings and you get 3.3V so your readings would go from a low of 168 @ 12V to 1 high of 255 @ 18V. Your 8bit ADC (analog to digital converter) effectively becomes a less than 7 bit ADC



What you really need to do to get the full scale of your ADC is add an offset voltage to counteract the voltage at the lowest input value (12V) and adjust the GAIN of the opamp to scale the 6V swing (18-12 = 6) so that the voltage swings from 0V to 5 V to give you a full 255 count from min to max inputs



The schematic shows that the gain of your op amps can be changed by the R8 & R9 values. See the instructions on Page 10







I would suggest that rather than messing to much with your board that you use an input conditiong circuit to elimeinat the DC voltage offset and hand the gain adjustment. This leave your USB board untouched at 0to5V inputs.



You can buy an instrument amp at digikey for less than 50 cents



http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=NJM4558D%23-ND



Wire it up as a differential amplifer where you input a 12V reference and your 12V to 18V input.

http://en.wikipedia.org/wiki/Differential_amplifier



Op Amp Data Sheet:

http://focus.ti.com/lit/ds/symlink/tlv272.pdf

part is available at www.digikey.com while supplies last.

Answer 2

Voltage To Resistance Converter

Answer 3

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I think it would be profitable to look more closely at the specification of the analogue input to the USB board, and to check whether it is indeed the case that it responds only to the resistance connected to the input. What is in my mind is that if the board uses a conventional analogue to digital converter (ADC) then it might be that the external resistor is part of a potential divider chain (the other part being on the board) which provides a variable voltage to the ADC input. If this is the case the system would respond to an externally applied voltage in the way that you want it to. This seems to me to be very likely, since an interface board that responds to a voltage input is a very useful thing to have, whereas one which responds to resistance only is rather specialised, and restricted in its usefulness. In the worst case, the variable resistance input could be connected to something like a 555 timer, providing pulses to the PC, the frequency of the pulses depending on the resistance attached to the analogue input terminals. If this is the case, a common junction FET or even a bipolar might serve as an interface, the gate or base voltage controlling the current drawn from the terminals, and thereby producing the required effect. Good luck....

Answer 4

if you check the schematic diagram for the USB experiment board you might see that it has an analog to digital converter to measure voltage that produces the 1 to 255 in software, and you might be able to bypass the resistance measurement input circuits if the USB experiment and connect a scaled voltage source as an input to the board from your variable DC.



you need to be careful to not exceed the voltage input of the components on the USB experiment board if you do hack into the circuitry. However, it will be a lot easier to make a small mod to the USB board and convert the DC voltage to an appropriate range with a few resistors rather than build a voltage to resistance converter which will take a lot of components.



the resistance measurement input circuitry on the USB board probably has a voltage source in series with a resistance connected to one analog input on the USB board and the other analog input might be connected to ground. If that was the input circuitry then you could disconnect the resistor and connect the DC source to some scaling resistors and apply that scaled voltage to the analog inputs.

Answer 5

You can try using a digital potentiometer IC such as the Microchip MCP41xxx and 42xxx. Use an MCU with onboard ADC to convert the voltage (which you may need to scale down with a voltage divider to within the MCU limits) to a digital value. Then control the digital pot (via SPI in the case of the MCP part) to output the corresponding resistance value. You may want to use the digital pot in rheostat mode