Faller Paris-Dakar Go Kart Ride 140333 - Construction Log

Here are the 3 pieces that make up the track. Edge decoration has been added.

A close-up of the self-adhesive contact strip that will be applied
to the track and will provide power to the cars.

Contact-strip applied to main section of track.

These trailing pieces will be connected to the Control Box.

Next section of track and contact strip added.

Final section of track and contact strip added.

The ends of the 4 contact strips are threaded through the track.

Underneath the track, the start and end of each contract strip are threaded into a heat shrink tube.

2 reed switches are mounted underneath the track. The Go-Karts will Start/Stop here.

2 assembled (functioning) and 4 unassembled (non-functioning) Go-Karts are supplied.


Homemade Control Box Project

Why build a new Paris-Dakar Go Kart Control Box?
The Control Box supplied with the Paris-Dakar Go Kart model is supposed to drive the Go Karts around the track and then after several laps bring the Karts to a halt back where they started in the Loading Bay. Each Kart has a small magnet mounted on the bottom which is intended to trigger a reed switch mounted underneath the track in the Loading Bay area - this triggering of the reed switch should cause the Control Box to stop the Kart.
The Control Box that came with the original model was faulty and failed to operate the Karts at all. I've had 2 replacement Control Boxes from Faller, and although these drove the Karts round the track, neither brought the Karts to a halt.
I mounted a small LED test circuit and a programmable Picaxe chip on a breadboard. I loaded a short Basic program onto the chip which would light the LEDs if the reed switch circuits were ever activated. I was able to prove that the magnets were indeed activating the reed switches as the Karts passed over them.

Like my homemade Indiago and Octopussy Control Boxes, I used a programmable Picaxe chip to control the motors on the Karts and to detect the activation of the reed switches.

Phase 1 - Prototyping

The first task was to create a prototype circuit on a breadboard that would drive the Karts, detect when the reed switches had been activated and stop the Karts.

And this is the flow diagram of the above circuit (produced using TinyCAD):

I decided to mount some Stop/Go LEDs in the gantry above the track in the Loading Bay area of the model. These have been incorporated into the above prototype circuit and flow diagram.
The final Control Box will have 7 input/output terminals:
1) 9v DC Power Supply
2) Track 1 Power Supply
3) Track 2 Power Supply
4) Track 1 Reed Circuit
5) Track 2 Reed Circuit
6) Track 1 Red/Green LED
7) Track 2 Red/Green LED

Phase 2 - Software

I decided to control the speed of the Karts by using Pulse Width Modulation (basically sending short pulses of power to the motors). Although the Karts will have only one speed, PWM seemed to be the easiest way of finding the right speed for the Karts, rather than messing around building a Voltage Divider, especially as I was already familiar with PWM from building the Control Box for the Octopussy model.
This is the code that I loaded onto the Picaxe chip:

symbol inner = b0
symbol outer = b1
symbol offperiod = b2
symbol onperiod  = b3
symbol blue  = b4
symbol green = b5
	let pins = %00110000
	pause 5000
	let blue  = 0
	let green = 0
	gosub mainlaps
	gosub finallap
	pause 10000
	goto ride
	let pins = %11000000
	let onperiod  = 1
	let offperiod = 3
	for outer = 1 to 30
		for inner = 1 to 200
			let pins = %11000110
			pause onperiod
			let pins = %11000000
			pause offperiod
		next inner
	next outer
	let pins = %00110000
	let onperiod  = 2
	let offperiod = 3
	do while green = 0 or blue = 0
		if blue = 0 and green = 0 then
			let pins = %00110110
		elseif blue = 0 then
			let pins = %00110010
			let pins = %00110100
		pause onperiod
		let pins = %00110000
		pause offperiod
		if pin1 = 1 then
			let blue = 1
		if pin2 = 1 then
			let green = 1
;                    COMMENTS
;	Status of Blue Kart (Track 1)
;	Status of Green Kart (Track 2)
;	set LEDs to Red
;	set LEDs to Green
;       set PWM pulse on/off periods  
;	}
;	}                            
;	}  Repeatedly send pulses to 
;	}  transistors to allow power
;	}  through to both Karts     
;	}                            
;	}                            
;	}
;	set LEDs to Red
;	Adjust PWM time periods to counterbalance    
;	additional lines of code being executed 
;	in following loop.
;	Allow both Karts to continue running:
;		supply power to both Karts
;	Only allow Blue Kart to run:
;		supply power to Blue Kart only
;	Only allow Green Kart to run:
;		supply power to Green Kart only
;	Blue Kart is now above Reed Switch:
;		Stop Blue Kart
;	Green Kart is now above Reed Switch:
;		Stop Green Kart
;	Loop until both Karts have been stopped

Phase 3 - Stripboard Layout

I've arranged the components of the flow diagram (from Phase 1) into the following stripboard layout (using VeeCAD Stripboard Editor):

Key Component Value Purpose (as I understand it)
C10 / C20 / D10 / D20 Capacitor / Diode 100nF / 1N4001 To reduce high-frequency interference and absorb back emf.
T10 / T11 / T20 / T21 Transistor BC337 To amplify the small current from the chip sufficiently to drive the motors.
BR1 Bridge Rectifier W02M Used here to allow DC connection pin to be positive or negative.
R11 / R12 / R21 / R22 Resistor 1k / 10k / 1k / 10k Used to divert a small current to an input pin when the reed switch is closed.
R10 / R20 Resistor 10k Restrict current flow through chip to the transistors.
R5 / R6 / R7 Resistor 4k7 / 10k / 20k Recommended connections / values by Picaxe
R_LED1 / R_LED2 Resistor 330R Limit the current to the LEDs
VR1 Voltage Regulator L7805CV Converts input DC (in this case 9vDC) to 5vDC.
C1 / C2 Capacitor 100nF / 100uF Smoothes the DC output from the Voltage Regulator

Phase 4 - Building

The reverse side of the stripboard with cuts applied and components ready for assembly (original Paris-Dakar Control Box for size comparison):

Clockwise (from lower left):
9vDC, Track 2 Contact Strip, Track 2 Reed Switch, Track 2 LED
Track 1 LED, Track 1 Reed Switch, Track 1 Contact Strip

Bi-colour LEDs installed above Loading Bay, and controlled by the Control Box

And here's a video showing the model working with the my Control Box and the Stop/Go LEDs:


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