Digital Warning Speedometer

I am trying to build a warning speedometer that will be capable of driving an 8 ohm speaker and the speaker will sound at 60mph and the tone increases and interrupted 10mph thereafter. It must also show a the values in 3 seven segment display. In addition I cannot used programmable chips. Can anyone please help me in my senior design project. I am struggling to build and design it. Thank you.

I think it could be implemented as a sort of frequency counter, taylored to the application. I am assuming you receive a train of pulses whose frequency is directly proportional to the speed. Then you count these pulses over a period of time and display the result on your 7-seg display. You can also use the same info to decide when to sound the warning, etc.

Can you provide more info? Am I correct about the input to your circuit being a pulse train? Can you tell us their frequency in relation to speed?

• The input to the system is taken from one spark plug

• Each pulse from the plug is 0.5V in magnitude, and occurs once for each 2 revolutions of the engine

• Engine speed of 2500 RPM results in a speed of 60 mph in high gear

• Speed is proportional to the RPM of the engine assuming no slippage occurs and stays in high gear

• Two outputs are required from the tachometer

o Visual Display of speed

 Three 7 segment LED readouts are to display the speed to one decimal point accuracy

o Audible tone warning of high speed

 Audible tone increases in frequency as the velocity increases above 60 mph

 Tone is interrupted at a 10Hz rate

 Frequency of the tone is 500Hz between 60 to 69.9 mph, 2KHz between 70 to 79.9 mph, and 5KHz for speeds above 80 mph

 It must drive an 8 ohm speaker with at least 0.5 watts of power

• Unit must be packaged and not to exceed 6”x6”x2” in dimensions

• Chips and other active devices can only be discrete or TTL/CMOS families

• No Microprocessors or other PLA, PAL or other special devices may be used

• PROMS may be used if driven discretely

Added after 1 hours 13 minutes:

the input to the system need not be the actual sparkplug. They are pulses so the function generator can be utilized

OK, here is an idea, although others are possible.

You are dealing with a low-frequency signal: 2500/2/60=20.833 Hz.
In fact, you should account for about 600rpm to 6000rpm, that is 5Hz to 50Hz. Multiplying the frequency by 2.88 you get the speed in mph.
For example, at 2500rpm, you get 1250 pulses per minute, that is 20.8333Hz. Multiplied by 2.88 you get 60, the correct speed in mph.

So you need to measure accurately this frequency, multiply by the constant and display the result. If the result is higher than x mph, do this or that.

You need to refresh the display several times per second. One method to measure low frequencies is to count the pulses over 10 seconds. Obviously, you cannot use this method, since the update rate would be too slow.
Another method to measure low frequencies with high resolution involves measuring the period of the signal and inverting it. f=1/T. This is the method I suggest you use, since you get update rates of 5 to 50 times/ sec. That is fast. You can reduce it to 2.5 to 25 times/ sec. This is what I am going to suggest, for other simplicity reasons.

So, the method I am proposing is this: amplify the signal, shape it into pulses and measure its period: use a high frequency stable oscillator (quartz controlled) and count its pulses between two rising or falling edges of your input signal. The easiest way to do this is to use a D FF and divide the input frequency by two. That way you get a HI between two similar edges. You can use that signal to enable an AND gate that feeds the pulses to a counter. This is how you measure the period.

Now for the calculation, it would be easy to use a micro. Since you cannot, use a trick: a lookup table. The outputs of your counter will address a couple of EPROMs. For each address combination, corresponding to the measured period, enter in the EPROM the actual speed, directly in BCD. Then all you have to do is use the EPROMs outputs to drive BCD/ 7seg decoders and you display the speed in mph. Not math, no nothing, just patience to calculate all those speeds and program the EPROM's.

Since you only need 12 bits for your 3 digits, you can use the remainig 4 bits to implement the alarms: when the displayed speed is between 60 and 69.9mph, have one bit HI, between 70 and 79.9 another and over 80 yet another. Then use these bits to enable some oscillators that will produce the required frequencies (you can use some frequencies derived from the same crystal oscillator, just divide it down to whatever is needed, then enable some gates using those extra bits of the EPROMs and you have the sound, too).

Note that you should use some D- latches (74x174, for ex) between the EPROMs outputs and the BCD/ 7 seg decoders. These will be reloaded every time a new result is available. This will prevent glitches. You will need to develop some logic or simply use one-shots to take care of the timing.

(If you want to get fancy, use a fourth 7-seg display and a switch to change some addresses on the EPROMs. With that, you can build a 4-digit tachometer, so you have a dual function).

I hope these suggestions help. Please post if you need more help.

I have this input of 60Hz into the tachometer which is needed as the first stage of the speedometer. It will go into CMOS 14-STAGE RIPPLE-CARRY BINARY COUNTER/DIVIDER AND OSCLLLATOR then to two flip flops to generate a 60 HZ output. I have attached the file for your review.

Hi fernie,
I am not sure why you say you need the 60Hz as the first stage of the tachometer.
Can you provide more info?

Im just following the specs. given of 2500 RPM resulting in 60 Hz output. So, I figured I would use that circuit to initialize a divider at the input. I was going to used MM5369 but it is an obsolete component By the way, what is the best software for simulations. I have workbench, PSpice 9.2 but they are both limited in parts list. Do you know of any other softwares out there. Thanks for the help man.

Added after 1 minutes:

What do you think about this kit!

I am sorry, I do not understand.

At 2500 rpm, you get 1250 pulses per minute (you get one pulse every two complete revolutions of the engine, for a 4-cycle engine). That means the input frequency is 1250/60=20.833 Hz, not 60Hz.
That happens at 60mph and it varies linearly with the speed, such that at 100mph you get 100/60*20.833=34.722Hz.
At 40mph wyou would get 13.888Hz.
So I do not understand where the 60Hz came from. Am I missing something?

About the kit, I think they are doing a regular frequency counter, using as a time base just an RC oscillator. I doubt the precision will be adequate for your application. The refresh rate I think will also be rather low.


I think I know where the 60 Hz came from. The kit manufacturer suggests you calibrate it using 60Hz. The kit uses as input the pulses from the ignition coil, that is it gets 2 pulses per engine revolution, not 1 pulse/ 2 engine revolution, as in your case.
Hence, at 1800rpm, they get 1800*2=3600 pulses/ min. So 3600/60=60Hz. That is why they tell you to apply 60Hz and adjust the meter to read 1800rpm.

In your case you get fewer pulses (since you are getting them from one spark plug only, according to your spec) and so the frequency is lower.

I use PSPICE release 8 demo, which also has limitations on parts and nodes.

I will post my schematic by tomorrow for your review to c if my concept is correct or not. Thank you for all of the help.

Added after 22 seconds:

do you know of other schematic available.

hellooo i hope that you matter has been resolved . i am sendding u the link that can be used ti any speedometer . In case of diesel engines use inputs from the alternater .A slight look at the circuit will solve your problem of taking input and freq to voltage conversion ............



Operation

This circuit has been designed to alert the vehicle driver that he has reached the maximum fixed speed limit (i.e. in a motorway). It eliminates the necessity of looking at the tachometer and to be distracted from driving.
There is a strict relation between engine's RPM and vehicle speed, so this device controls RPM, starting to beep and flashing a LED once per second, when maximum fixed speed is reached.
Its outstanding feature lies in the fact that no connection is required from circuit to engine.

Circuit operation:

IC1 forms a differential amplifier for the electromagnetic pulses generated by the engine sparking-plugs, picked-up by sensor coil L1. IC2A further amplifies the pulses and IC2B to IC2F inverters provide clean pulse squaring. The monostable multivibrator IC3A is used as a frequency discriminator, its pin 6 going firmly high when speed limit (settled by R11) is reached. IC3B, the transistors and associate components provide timings for the signaling part, formed by LED D5 and piezo sounder BZ1. D3 introduces a small amount of hysteresis.
Notes:

D1 is necessary at set-up to monitor the sparking-plugs emission, thus allowing to find easily the best placement for the device on the dashboard or close to it. After the setting is done, D1 & R9 can be omitted or switched-off, with battery savings.
During the preceding operation R8 must be adjusted for better results. The best setting of this trimmer is usually obtained when its value lies between 10 and 20K.
You must do this first setting when the engine is on but the vehicle is stationary.
The final simplest setting can be made with the help of a second person. Drive the vehicle and reach the speed needed. The helper must adjust the trimmer R11 until the device operates the beeper and D5. Reducing car's speed the beep must stop.
L1 can be a 10mH small inductor usually sold in the form of a tiny rectangular plastic box. If you need an higher sensitivity you can build a special coil, winding 130 to 150 turns of 0.2 mm. enameled wire on a 5 cm. diameter former (e.g. a can). Extract the coil from the former and tape it with insulating tape making thus a stand-alone coil.
Circuit's current drawing is approx. 10mA. If you intend to use the car's 12V battery, you can connect the device to the lighter socket. In this case R20 must be 330R.
Depending on the engine's cylinders number, R11 can be unable to set the device properly. In some cases you must use R11=200K and R12=100K or less.
If you need to set-up the device on the bench, a sine or square wave variable generator is required.
To calculate the frequency relation to RPM in a four strokes engine you can use the following formula:
Hz= (Number of cylinders * RPM) / 120.
For a two strokes engine the formula is: Hz= (Number of cylinders * RPM) / 60.
Thus, for a car with a four strokes engine and four cylinders the resulting frequency @ 3000 RPM is 100Hz.
Temporarily disconnect C2 from IC1 pin 6. Connect the generator output across C2 and Ground. Set the generator frequency to i.e. 100Hz and trim R11 until you will hear the beeps and LED D5 will start flashing. Reducing the frequency to 99 or 98 Hz, beeping and flashing must stop.



Regards

What are the values of the components. They are not visible. How can I find out. What type of chips are being utilized. I think this would work though. I just need to come up with a circuit design for the 3 7-segment display. Thank you

I NEED HELP THANK YOU.....IF YOU HAVE THE COMPONENT WITH THE SPECIFICATTIONS LISTED. I WILL BUY THE COMPONENT