Controlling a Drag Strip

 

v  How could/should a slot drag strip be built in terms of controls? There have been many discussions on several boards over the years; based on what I understood from these discussions, I have tried to summarize here the main things I've come across and also propose some electronic solutions.

 

v  Basic Functionality:

o    First the power supply goes to the controller as usual, but instead of going from the controller directly to the track, the output of the controller is connected to a 4-position switch.

o    The first position switch could be a 4 position rotary switch; the driver would have his throttle controller in one hand and control this switch with his other hand. Alternatively, the switch could be a 4-position lever (as seen on some of the car simulator video games) that performs the same function.

o    At each output of the four outputs of the switch, rectifier diodes (a single rectifier diode drops the voltage by 0.7 V when current flows through it) are mounted in series to drop the voltage from the power supply. Thus, the voltage will be increasing in steps from zone 1 to zone 4. For example:

§      Zone 1 has 6 diodes mounted in series, dropping the voltage from the supply by 4.2 V

§      Zone 2 has 4 diodes for a 2.8 V drop

§      Zone 3 has 2 diodes for a 1.4 V drop

§      Zone 4 has no diodes and gets all the voltage from the power supply

§      This can be changed to a different number of diodes (3,2,1 or any combination) if desired.

o    There is also an argument to be made to have the sections powered up in the reverse order, i.e. the latter sections have less voltage than the earlier ones. This way, there is more acceleration early on and less later on. This assumes that the car will not reach it's terminal velocity (maximum speed) before the end of the strip or it might actually slow down on that last zone.

o    In any case, it could prove useful to have a programmable/adjustable number of diodes per section. Here is something that does just that for a few dollars.

o    Some other people (namely thatguy01) in these discussion mentioned that a single double-pole double-throw switch could be used. One position of the switch would control zones 1 and 3 while the other would control zones 2 and 4 providing the same end result. In, this case the switch would be flipped back and forth 3 times as the car goes through the successive zones.

o    The zones can be isolated easily by simply cutting the rail on the track (only on the positive side of the voltage) between each zone. The negative connection to the power supply is common to all the zones

o    This way, a driver has to time perfectly his switching (similar to changing gear???) as the car goes from zone to zone so he experiences no power loss. Switch too early and the car will coast for a little while until hitting the next zone, switch too late and the car will coast when entering the new zone before picking up power.

o    To help with this, one can add visual signals, as the car is about to change zone. This can be done by Reed switches (magnetically actuated switches) mounted under the track that will switch when a car is driving over them. When this happens the corresponding LED turns on. If the turn on time of the LED is too short, one can add a small relaxation circuit, which can extend that turn-on time. This could also easily be replaced with optical sensors if so desired. Also, after the cars go through each zone, simple electronics could be added so that the LEDs stay on rather than turn off once a car has passed. These LEDs could be mounted on the side of the track, at the driver station, or both.

 

 

v  Electronic Control:

o    It is very easy and cheap to replace the mechanical switch above with an all-electronic control system to perform the same function.

o    First a push button is needed and could be mounted on the back of the throttle controller so that it is thumb accessible, or it could be mounted at the driver station.

o    A second push button needs to be mounted at the driver station or maybe in front of the "race director" (whoever that is and wherever he is J ).

o    Before the race starts, the "race director" pushes his button to reset the circuit. This way, power is applied only to zone 1 and not the others.

o    As the race starts each driver has to activate his push button for a brief time and power will be switched from one zone to the next.

o    The way it works is through the use of a serial-to-parallel shift register where the driver's push button is connected to the clock input while the race director push-button is connected to the "reset/clear" inputs for zones 2,3, and 4 and the "set" input for zone 1 (this assumes the use of four D latches to build the serial-to-parallel shift register). Such a circuit is also called a ring counter.

o    The debouncer circuit is present to avoid multiple triggering of the shift register when pushing the driver's mechanical push-button. Note that a debouncer circuit is also probably needed on the race director push button. A debouncer circuit is simply a resistor (say 10 Kohms connected to the supply voltage) and a capacitor (say 4.7 mF connected to ground) in parallel with the push button (connected to ground) and followed by an inverter to reshape the signals. Maxim semiconductor also makes a debouncer chip (the Max811 and Max812) that could be used here.

o    The outputs of the shift register control power relays that then connect the main power to each track zone where it is dropped by the rectifier diodes before the track.

 

 

v  What else can be added:

o    You'll need a timing Software and Hardware. Many of the existing timing software will work by using one sensor at the start and one sensor at the finish mounted in parallel to the same computer input.

o    Visual indicators of which zone is currently powered could also be useful: one LED and one resistor per zone will do it.

 

v  How much would all this cost?

o    About $5 for a 5V DC mini adapter wall wart

o    About $4 for the three reed switches and associated LEDs and resistors.

o    About $5 for a 4 position rotary switch, knob, and a dozen high current (6 Amps) rectifier diodes

o    About $12 to $20 for the electronic solution (relays, push buttons, electronic chips, rectifier diodes), depending on how much current the relays really need to pass to the track.

o    In Summary:

§      About $15 per lane for the mechanical solution

§      About $15 to $25 per lane for the electronic solution

 

Last Update: 8/20/06                                                                                           Drop me a note