Its a personal lap timer meaning a single car at time, but you can have a lot of fun setting hot laps and then challenging your mates to beat them, thats the model we are currently using at our club track in Dubai.
My soon to be beaten m-Chassis hot lap at the Pro RC Track in Dubai-
The single best feature of the lap timer is the big buzzer in the bottom right of the pciture, this gives instant feedback while you are driving, with 10 corners in 19 seconds theres not a lot of time to check the display, but a single beep for a new lap and a double beep for a new best is suprisingly effective and motivating !
Part One of the build shows how to build a manual version of the system on breadboard, in order to build the full automated version of the system we need to build the infrared transponder.
Part One - http://rcarduino.blogspot.com/2012/07/lap-timer-build-along-part-one.html
If you want to time an automobile or kart ask your track whether they already have an infra red beacon, if so you can skip this step.
The Parts List -
I have used a 7805 regulator, this takes the 8.4-7.2 volts supplied by the car battery as input and produces a steady 5volts as output. We use the 5 volt output to drive the timing components which actually produce the transponder signal.
You can replace the 7805 regulator with more modern alternatives, just ensure to use the supporting components recommended in your chosen regulators datasheet.
1 - 7805 Voltage Regulator
Regulator Supporting Components
2 - 100uf Electrolytic Capacitors
1 - 0.1 uf Ceramic Capacitors
1 - Diode
The diode is include to protect the rest of the circuit from accidental reverse power connection.
The Signal Timing Circuit
The timing circuit is based on two 555 timers, these are the basis of many introductory electronics projects so are widely available and inexpensive.
Here is a nice introduction to the 555 Timer - http://www.kpsec.freeuk.com/555timer.htm
There are also many online calculators giving required component values for the frequencies you enter -
For our transponder we want two on/off circuits running at different frequencies, the first is a 38Khz circuit which switches the Infra Red LED on and off 38,000 times a second. The second circuit is much slower and simply switches the first circuit on and off at 100Hz (100 times a second).
The transponder signal is the 100Hz signal, this means that in a car travelling at 60Km/h we should receive six pulses in the time it takes the car to travel one meter, this is enough for use to reliably detect an RC Car. For full size cars the detection zone is much bigger as the tracks are wider, its actually easier to detect a faster full size can than it is a fast RC Car on a narrow track.
For a great introduction to Infra Red detectors and why the 38Khz signal from the first timer is important, see the following link and tutorials - http://learn.adafruit.com/ir-sensor
Timing Circuit Parts List -
2 - 555 Timer ICs
1 - 1uf Electrolytic Capacitor
2 - 0.1 uf Capacitors
1 - 0.01uf Capacitor
2 - 1K Resistor
1 - 10K Resistor
1 - 220 Resistor
1 - 680 Resistor
To build the full lap timer with audio feedback you will need the following additional components -
1 - 38Khz IR Detector
1 - Peizo Buzzer of your choice
1 - NPN transistor to drive the buzzer
1 - LED
Next Step - building the transponder, stay tuned