In order to ensure that the car performs to its best in terms of speed and reliability we need to be able to accurately set up the engine.The most critical statistic for us to know is the power output of our engine, whilst tuning using different engine settings or adding tuning parts we will want to know the effect it is having. Measurement of power can be derived by loading a dynamo coupled to the engine, this is fairly straightforward electro-mechanical engineering. Measuring how fast the engine is running presents quite a different challenge. The engine’s peak power is given at around 21,000 RPM, to measure this accurately requires the use of an electronic counter and display system.

The most difficult part of the system to produce for reasonable expenditure in terms of time and money. We will use a miniature 6 digit display counter capable of counting up to 10kHz with a 5μA current consumption. Relevant electronic specifications :

Supply voltage 2.6 -3.4 V d.c. Current consumption 5μA Count range 0-999999 rolling over to 0 Display 6mm characters Inputs negative edge triggered, threshold 0.7V Count 10kHz max. 50μs min pulse length Reset 50 Hz max. 15ms min. pulse length Connection identification

Many optical sensor systems are available. Visible light systems must be rejected because of artificial light interference problems, mains lighting flashing at 100Hz. Sensors responding to Infra Red radiation, which can be provided by an infra-red LED provide a solution.
Digital output Tachometer encoders are made specifically for our purpose, the Honeywell HLC 2705 is available from RS Electronics, it incorporates two photodiodes behind a daylight cut-off filter and provides a tachometer pulse and a direction indication pulse. The tachometer has an open collector output, which sinks current during the output pulse.

Alternatively use can be made of the BP103B phototransistor, available from Rapid. This also functions as an open collector device. The downside to this solution is that it will be affected by the infra-red component of both daylight and artificial light.

Relevant electronic specifications: Supply voltage 35V max. Peak wavelength 850nm Light Current 50mA max allowed Dark Current 100nA max A suitable Infra-red emitter is the SFH485 diode, also available from Rapid. Relevant electronic specifications: Forward voltage 1.5V max. Reverse voltage 5V max. Current 100mA max Peak wavelength 880 nm

The connections to these devices are confusing! Notice that the shorter leg is always connected to positive

The sensor would detect suitable white and black bars on the engine flywheel, producing a series of pulses.

The counter requires input pulses with a negative leading edge and duration of at least 50μs, these can be provided by arranging the sensor to trigger a 555 timer i.c. configured as a monostable.

A convenient arrangement seemed to be resetting to zero, counting pulses for 1 second and then displaying the total count for one second before repeating the process.

A suitable clock pulse for this can be provided by a 555 timer i.c. configured as an astable.

Pulses from the sensor are gated to the counter by the clock.

Clock subsystem (astable)	Typical values for components
	i.c. 555 R1 100k + 100k variable R2 1M + 2M2 variable to allow calibration C1 470nF C2 10nF

Pulse shaper subsystem (monostable)	Typical values for components
	i.c. 555 R1 100k C1 1nF C2 10nF

To aid reliable operation the unit is powered by a 9V battery, the 5V logic supply and 3V supply for the counter/display are stabilised. For reliable results the capacitors should be connected close to the i.c.

Power supply sun-system for the Counter/display

The series resistor can be of any suitable value provided that the maximum current of the LED is not exceeded. Operating from 5V try 270R.

Note that the infra-red output is completely invisible. A suitable source of test pulses to count is any infra-red controller for a TV or Video.

The probe sub-system This sub-system utilises the IR LED/IR photo transistor combination, an original was constructed to fit in a probe reaching into the model engine.
	Values used were: D1 SFH485 T1 BP103B I.C. CA3140E R1 270R R2 10k R3 10k R4 1M C1 100nF

Tacho Subsystem
	This shows how simple the tachometer i.c. is to use. The output is compatible with logic and 555 inputs. The direction output is not used in this application.

Reset Subsystem
This was achieved by a bit of a cheat, simply a 0.022μF capacitor from the clock to the reset input. This uses the internal circuitry resistance of the counter/display to produce an input pulse (as a differentiator!).

Full Circuit Diagram and Components List