Converting RC car to computer controlled car

Let me start by saying I am not very good with electronics (yet). I am currently trying to learn. I have an old RC car that I want to control
via my parallel port. After several nites of research I know how the port works and have seen examples of converting models that are simple all or nothings (100% speed forward/ back). But my remote uses a pot on each of the remote's sticks. My idea is to use the 8 data pins on the parallel port to control both the speed and directional controls at the same time. The first 4 would be for speed, and the 2nd four for direction. to replace the pot (analog) with the digital parallel port I decide give up on a true DAC, instead I think a step system would be better. the first byte (pin) would set forward/back (left/right) and the remaining three would set speed (angle). For instance: Pin 2 = 20% of power into Pot. pin 3 = 30% of power into Pot. pin 4 = 50% of power into Pot.
In this method i would be able to combine the pins to reach 100% and still have some of the controle that the original analog system had. (ranges of 0,20,30,50,70,80,100)
So here are my questions/problems at last: Is this a sound plan? Is there a better way?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
1. Use parallel port to generate the signal which modulates your RF carrier. Need to probe around in your transmitter (Tx) with a scope if your Tx does not have a buddy port (look for signals like in links below).
2. do '1' except use a simple microcontroller (e.g. 8-pin PIC) in between parallel port (or COM port..) and Tx to un-burden PC. somewhat like http://www.mh.ttu.ee/risto/rc/electronics/pctorc.htm http://www.rc-electronics.co.uk/PCBuddyProgManV1.pdf
3. If the goal is to learn more electronics, then one other option which should also work with non-standard Txs too is to use a digital potentiometer - wide range here;
http://www.digikey.com/scripts/dksearch/dksus.dll?Criteria?Ref320&Site=US&Cat2375472 one example; http://rocky.digikey.com/WebLib/Analog%20Devices/Web%20Data/AD5162.pdf
Have fun,
Kevin
So here are my questions/problems at last:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
It would give you more possible speeds if you set it up as a pair of 4-bit adcs. That way with n pins you get 2^n options (16) instead of n options as you suggest. First, though, you should make sure that the transmitter uses the stick pots as variable voltage sources. If it uses them in some other way, such as part of a variable frequency oscillator as many computer gameports do, this won't work.
The digital pot option, as previously mentioned, would also be pretty easy to implement.
chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I was trying to avoid the digital pot, I thought that they require a microcontroller? Don't think I'm ready for that yet.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Most digital pots use a serial interface (eg. SPI) which can be readily emulated on a couple of lines of the parallel port (one for data, one for clock etc).

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

...
You need to emulate the receiver, not the transmitter as suggested elsewhere. Skip past all that, and research what the servo needs to see for control. The servo expects a timed pulse as its control signal, roughly 30 times a second. A high pulse of a certain duration means to hold center. A shorter pulse means to move in one direction; a longer pulse means the other direction. You'll find the specifics in a web search.
This means you can control up to eight channels without multiplexing 8 digital output lines. (How?) There's no messy digital-to-analog conversion.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mike, I think you missed the point. I'm only trying to connect the car's remote to my computer, not rebuild the car. I only need to replace the physical act of moving a joystick with a signal from my PC.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 20 Oct 2005 18:49:35 -0700, "Golgathus"

So what have you tried so far or experimented with in your setup? I'd think you would make a resistor bank that you would put in parallel with the joystick pot. These resistors would be put in/out of service via NPN transistors controlled via the parallel port pins. You probably can get the components from the local Radio Shack. If they have the 74HCT259 chips (276-2868, $1.49) in stock you can use it to have more resistors available. The below links are for controlling a homemade webcam video switcher, but should apply for controlling resistors via the parallel port.
http://www.geocities.com/zoomkat/ppswitcher.htm http://www.geocities.com/zoomkat/ppswitcher3.htm
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Currently have drawn up a circuit that would act as a binary decoder, using npn and pnp transistors. Since the remote uses both outs from the pot, I set up a chain of risitors that act as the resistance before the sweeper and after the sweeper to sum to 4.8K Ohm (original value of pot). I haven't finished building the circuit yet so don't know how well it will work yet.
Thanks for the link.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Have you tried to use an electronic simulator? Are you using 2n3904 and 3906 ? thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
was using 2n2222 and 2n3906.. but i recently chnged my mind and ecided to use two 4 to 16 decoders.. Decided on the 74154 So now the transistor relay (60 total transistors) wil be replaced with two 24 pin chips. Much cleaner design and less soildering. Logic remains the same. Just waiting for parts to arrive.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I'm not sure why you've decided to drop the DAC route...
The pot in your remote will provide a voltage somewhere between 0v and the supply voltage depending upon the position of the stick. The electronics use this voltage to set the transmitter output such that the center position (1/2 supply voltage) gives the neutral (stopped or center steering) signal. As you move the stick forward the voltage increases (or decreases depending on the make of your transmitter) and the transmitter sends a corresponding signal. Commonly 0 volts will correspond to full speed reverse and supply = Full forward.
I'm supposing you realise how the POT works...
From what you describe you would probably manage with 2 x 4 bit DACs, giving you 16 voltage levels for each channel from your 8 bit port. Which is pretty much a 'Step system' anyway.
The only issue would be getting your center position to provide the correct voltage for 'neutral'.
The voltages might not be 0V and supply - you would need to check this with a meter or scope and then arrange for your DAC to provide the same voltage range. Perhaps using an OPAmp to provide buffering and a little voltage offset.
Hope this helps.     Jon.
Golgathus wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Good info, thank you.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.