'Coke Can' RC car conversion

Rick100

Senior Member
#5
Hello Buzby,

I bought one of these for the very same reason you did. I intended to replace the electronics completely but I like your way better. Could you give more details on the wiring? Specifically, which pins of the 16 pin chip were for left/right/fwd/rev and the power supply? You also mentioned a boost convertor. How much voltage does it supply? Do you have to be careful about driving the left/right transistors, so they don't both turn on and short out? Same with fwd/rev?

What photo sensors to you plan to use? I ordered some QRE1113 from Ebay. They were the smallest I could find at approximately 3MM square but they are probably to big. I measured the current draw on the steering actuator at around 250 milliamps and the resistance at about 5 Ohms. That makes it hard to stick to my original plan of replacing all the electronics and using a lipo. The current through the actuator would be around 750 milliamps. It's hard to appreciate the scale of Edmunds project until you try to work on something this small. I believe these cars are 1/58 scale and Edmund's is 1/87. Sorry for all the questions .

Thanks,
Rick
 

Buzby

Senior Member
#6
Hi Rick,

No problem about questions, that's what a forum is for !.

The pinout details I got by searching for the datasheet of the RX-2 chip, which is used as the basis of many cheap RC toys from China. ( I'll post a copy and my notes when I am back on my own PC. )

My plan also was to replace the board, but when I got the car opened I decided I couldn't build circuitry as small as was needed, and why would I need to, as the only part I needed to replace is the processor ?.

So I got the pinouts from the datasheet, with the intention of seeing if I could replace the RX-2 with a 'drop in' tiny PICAXE. That's when I realised removing the original chip would be a Bad Move !. The RX-2 is not just an RC receiver, it's the boost convertor as well. It boosts the 1.4v from the tiny lithium cell to about 3.4v. I definitely couldn't build one that small !.

I've not measured the current draw of the steering coil. The transistors on the PCB must be rated big enough to drive it, so I'm not really bothered. Same with the motor. Also, I don't know what happens if I energise both directions at once, so I make sure not to !.

My biggest concern is the behaviour of the steering. The steering is done by energising the coil in opposite polarities, which then attracts one of two magnets attached to the steering bar, pulling the bar left or right accordingly. The RC chip just drives the steering bang-bang left or right, but I don't know how this will respond to PWM if I need to get 'smooth' steering.

Regarding photosensors, I've not looked yet. Whatever I choose I'm sure that some of the car chassis will need cutting out to make space. Your QRE1113 look like they will do the job, but I was thinking more along the line of 3mm 'LED style pkg' photodiodes and a couple of SMD LEDs.

Regarding using a 20X2, I'm not sure that is the best choice I could have made. I think I should maybe have used a 20M2, as it's got more PWM options. Now I might need to roll-my-own PWM, cos' I'm not changing the 20X2 after all that effort of getting in !.

I've re-installed the red tail light, and am going to add another, not because I like tail lights, but to use for diagnostics. ( My original intention was to use the ERF/URF for remote program and diagnostics, but there is no way I could fit it in the car. )

There is no proper code yet, just some routines to drive and turn. From these simple tests it looks like the car is going to be a bit of a challenge to control, it's steering is more than a bit erratic.

It's going to be a few more days before I can get back on this, how far have you got ?

Cheers,

Buzby
 

Rick100

Senior Member
#7
Hello Buzby,
Thanks for the info and part number. I just found and downloaded the RX2 datasheet. As for the steering; actuators in micro RC aircraft are driven with PWM for proportional control. I'm not sure what frequency they use but it's probably not high, so bit banged PWM should work. I built an IR receiver from an 8 pin pic and wound my own coils many years ago. The program, written by someone else in C, bit banged the PWM. The RC aircraft often use the 2 actuators(rudder and elevator) lined up so there magnets attract each other and center themselves. Although others just use the spring of the hinge. This car seems to have a weak spring for centering the steering so that may be a problem. Playing with that spring may help with proportional steering.

I disassembled mine and filed on the steering parts to get a tighter turning radius. I'm not sure how much difference it made since the steering linkage is so loose. I would like to redo the steering linkage but it's just to small for my aging eyes. I think I'll try a dip package 14M2 since it's the smallest chip I have with enough I/O to work with. I' probably work on it again over the weekend.

Again, thanks for the info.

Rick
 

Rick100

Senior Member
#9
Hello Buzby,
The 14M2 has 4 pwm pins. PE6 shows at 4MHz, the frequency can be as low as 62 Hertz. Looks like 20M2 also has 4 pwm outputs.

Good luck,
Rick
 

Rick100

Senior Member
#11
Hello again Buzby,

I've followed your lead and have my 14M2 controlling the car. The Picaxe is mounted on a small breadboard and the car barely carries it, but it shows I didn't destroy the original circuit board during the modifications. The solder on that board is terrible. I have pwm outputs to all four control pins and have tested the pwm on the motor. I included pull downs on the outputs to the control pins just in case. My plan is to make a pcb for the 14M2 and mount it on top. It won''t be as small as your dead bug style. My sensors came in from China and I'm thinking of putting an IR receiver for start/stop control. The steering looks like it will be a problem but the weight of the breadboard may be preventing it from working very well. I will probably wait until next weekend to work on it again.

Good luck,
Rick
 
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