# Need help with seven segment display + LED bar graph tachometer!

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#### Wiskey

##### New Member
Hey guys. Let me start off by saying I’m a 17 year old young man and I’m incredibly interested in electronics. I have a fair understanding in the field in terms of comprehending and then constructing a circuit diagram. I learned what I know by taking my dad’s old university books and actually reading! But unfortunately most of the books are badly damaged and some completely destroyed. But none the less, I can also piece together several circuits or modify an existing one but I have absolutely no idea where to start or how to go about designing my own circuit. And that’s how I turned to the internet for help.

I want to build a digital tachometer for a Honda CBR400RR. It should consist of 5 seven segment numerical displays on which the RPM should be indicated and more or less 50 LED’S as a bar graph with another LED indicating “It’s time to shift”.

This is how the story went... I found a bar graph tachometer circuit using LM3914’s that i would have initially used. All I needed now was to add a circuit that will indicate the RPM on 5 seven segment displays. This is where I got confused... If you look at the original tachometer of the Honda CBR400RR (which is analog), it goes from 0 to 16 and on the right it indicates “x1000”, which, if we take 16 for example, will be 16 000 (16x1000). But when I searched for a circuit to display the RPM on seven segment displays, all I could find was circuits consisting of only 4 seven segment displays, which would not have worked. Since the bike is capable of producing a RPM of 5 digits, 4 digits implies that the value displayed be multiplied by ten. Since I want the indications simple but still very accurate, only 5 digits will be completely sufficient to my liking, but unfortunatley I lack the knowledge to adjust the curcuit to consist of 5 numerical displays. But then I had an Eureka moment! Since there is very little space (if any) on a bike for extra stuff, my circuit would have to be really small (And it would be placed in potting compound to prevent the vibrations of the bike from loosening the soldered equipment, which makes it even bigger). But these circuits are generally large in size due to the amount of components used. The first answer that came to mind was “Code”. Microcontrollers is the solution to really restrict the size of the circuit! But unfortunately my knowledge is even more restricted in terms of microcontrollers... And this is where you fine gentlemen and ladies would be of great help...

I need someone to be so kind to please design me a circuit diagram. You can use whatever components you wish (preferably microcontrollers) as long as the final schematic would be small in size if built (bare in mind I can’t work with SMT components since I don’t have a 10ft magnifying glass and neither do I have a solder station with a super fine tip). I will be using 0.28” seven segment displays (common anode or cathode, basically designers choice), 50x5mm LED’s and the “shift” LED will be 5mm as well. The signal will be retrieved inductively by wrapping the signal wire around the spark plug cable.

I hope someone can help me. Thanks guys! I think that’s all info you need!

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#### MikeMl

##### Well-Known Member
You have a significant problem getting five significant figures on a digital RPM display.

It goes like this. To get the last digit on the right to be meaningful (significant), you have to count Revolutions for a full minute. You will not be very happy if your RPM display updates only once per minute..., You likely want the display to update several times per second...

There is a way to do this, but forget doing it with a cascade of decade counters. It takes a microprocessor capable of doing some math, like decimal division.

This is a job for an Arduino. Are you willing to do some more study???

btw-I have built digital tachometers for aircraft to replace a mechanical tach. About three significant figures is all you are gonna get, so the last two digits to the right might as well as be hardwired to read 00, or you might as well mentally multiply a three digit readout by 100 in your head...

Also, digital tachs are almost worthless on an engine that can accelerate as fast as your motorcycle. You might as well focus on building a bar-graph. Actually, there is something to be said for using an analog meter movement for the readout...

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#### dr pepper

##### Well-Known Member
An analogue meter gives a better representation of a live trend I must agree.
You can measure 1/10's of a rev by timing the interval between the pulses, as mentioned though its tricky to do that with discrete logic.
Theres probably a couple of projects on the web for an arduino.

#### Wiskey

##### New Member
To be honest guys. I did some more thinking and came to the conclusion that trying to fit seven segment displays and a LED bar graph on a fairly small circuit trying to fit it in the original socket of where the bike analog tacho is located, just won't be worth all the trouble. Instead I'm gonna use the circuit I have and construct only an LED bar graph. With the right choice of colour of the LED's and set in an unique shape, I'm pretty sure I can make a fantastic tacho! Again guys... I'm incredibly grateful for your comments and input ! I'll be sure to keep you updated on my project and post some pictures when it's done! Thanks guys!