The following article consists of a circuit diagram and set up notes
Parts List:
- C1 = 1nF
- C2 = 22nF
- C3 = 1uF
- VR1 = 50K (25 turns)
- VR2 = 5K (25 turns)
- D1 = 1N4148
- other parts are listed on the circuit diagram
Adjustment Instructions.
Connect circuit to a 12 volt supply and adjust VR2 so that the output voltage from LM317T is 8 volts. VR1 is adjustable to set the full-scale rev limit. Fitting a resistor in series with VR1 will increase range. Changing the values of these resistors (VR1 & it's series resistor) alters the “rev range” of the circuit.
The calculation for this resistance is:
Vout = Vin x VR1 x C2 x Frequency
The LED circuit is designed to be at full-scale at 4 volts, and the input voltage is 8 volts (from regulator circuit) so:
4V = 8V x VR1 x C2 x Frequency
Since C2 is fixed at 22 nF (0.000000022) the calculation works as follows
6 cylinder with max revs set to 5200: Frequency = 5200 / 120 x 6 = 260Hz
4V = 8V x VR1 x 0.000000022 x 260
To get VR1 rearrange formula:
4 / (8 x 0.000000022 x 260) = 87412.5 (87.4K)
NOTE: Put a large variable resistor in to get a good amount of adjustment as I have found that this resistance calculation isn’t always as accurate as is needed. This is due to manufacturing tolerances on the components values etc. (suggest 50K for VR1 and a series resistor to get the total desired resistance).
It is best to set the cicuit up on a frequency generator, but I set the circuit up on the car to give half scale (5 led's) at half max revs. You could work out how many led's would be on at whatever revs and set it up that way if you wish...
e.g. if max revs is 5000, then 10 led's = 5000, 5 led's = 2500, 2 led's = 1000 etc...
This circuit is designed to flash all the led's when it hits full scale (#10 led lights). This can be used like a shift light and as the rev range is adjustable, it can be set to be at full scale at your own personal choice of rev limit
Rev counter built up into project box
The layout of the led's can be altered to suit if they are on "flying leads" from the cicuit board. The colours and sizes of the led's can also be changed to suit personal preference. I led's that I have used are 3mm, (5 green, 2 yellow, 1 orange and 2 red).
I am currently developing this project further. Click the 'next generation' link below to see how this project will evolve.
Stripboard Layout and Parts List Next Generation Digital Rev Counter 