# tachometer.

Discussion in 'General Electronics Discussion' started by mechtronics, Aug 7, 2011.

1. ### mechtronics

153
2
Aug 7, 2011

hi does any one know if this tachometer circuit can be calibrated so that it only starts working at 7750 rpm for example: i want it so that at 7000 rpm to be 0 volt output, 7750 rpm to be 1 volt, 8000 rpm to be 2 volts, 8250 rpm to be 3 volts, 8500 rpm to be 4 volts, 8750 rpm to be 5 volts and 9000 rpm to be 6 volts any ideas would be appreciated?

Last edited: Aug 7, 2011
2. ### duke37

5,364
772
Jan 9, 2011
The LM2917 circuit you have shown gives 1V/1000rpm, the list of outputs you require is not linear at the bottom end so there will be a problem. At the top end, you want 1V/250rpm so an amplifier with a gain of 1000/250 = 4. Giving an offset to get 0V at 7000rpm, you get
0V 7000rpm
1V 7250rpm
2V 7500rpm
3V 7750rpm
4V 8000rpm
5V 8250rpm
6V 8500rpm
7V 8750rpm
If you want a non linear response it will be much more complicated, otherwise an op-amp with a gain of 4 and an offset of 3.5V would do the job.

3. ### mechtronics

153
2
Aug 7, 2011
thats actually wat i meant to say sorry!! as in wat u said being:

0v = 7000rpm
1v = 7250rpm
2v = 7500rpm
3v = 7750rpm
4v= 8000rpm
5v= 8250rpm
6v = 8500rpm
7v = 8750rpm

would i need the op amp to do this because it is now linear? and can an output of 7 volts actually be achieved from the lm2917 the reason i am saying this is because of the built in zener diode of 7.56 volts. and i was thinking of getting 0.5 volts to equal every 250 rpm. this would give me a total of 4 volt output up to 9000 rpm which should be achievable.

4. ### duke37

5,364
772
Jan 9, 2011
I was thinking of an extra op-amp to give the required voltage but I have had a look at the National data sheet and see that there is an internal op-amp which can be got at. The gain (rpm/volt) can be set with one resistor and a offset can be connected to the - op-amp input. With the lower voltage requirement you have now specified it should be possible to do this without an extra op-amp.
I have something else to do at the moment but will try to get my other brain cell in gear in the next day or two.

5. ### duke37

5,364
772
Jan 9, 2011
My disturbed brain cell would not let me sleep last night so here we go.
I coudn't get your link working properly but got details of the LM2917 from the National website.
Set the F/V convertor to give 4000rpm/V
7000rpm gives 1.75V
subtract 1.75V
Set op-amp gain to 8 i.e. 500rpm/V
then 7000rpm > 0V
9000rpm>4V
I attach some diagrams with resistor values to do what I think you want. The resistors are E24 standard values except the 50k, for this, use two 100k in parallel.

File size:
10.1 KB
Views:
845
6. ### mechtronics

153
2
Aug 7, 2011
thanks for the diagrams. do i need each of these circuit diagrams to do the job or just one. i understand where you are getting the gain of 8 in the second one but what is the last diagram for? here is the image of the circuit i was wondering could you show me where the op amp circuits that you came up with would be attached

Last edited: Aug 9, 2011
7. ### duke37

5,364
772
Jan 9, 2011
Your link shows a 14 pin chip. the National data sheet gives details on an 8 pin chip. The pins do nor correspond 100%.
The 8 pin chip has the following connections.
1. Input 2. Capacitor 3. F/V output 4. Output 5 +Vcc 6. Zener (7,56V) 7. Op-amp- 8. Gnd

The three diagrams I give are to show the development of the circuit, the third one is the one to use. The opamp is in the LM2917 so all you need are resistors. You will need to set the LM2917 F/V to 4000rpm/V then 0V represents 7000rpm and 4V 9000rpm.
I have had a thought that perhaps you are going to drive a bar graph display. In this case you no not need to modify the LM2917 at all, just set two resistors in the bar graph driver reference to give the sensitivity and offset you require.

8. ### mechtronics

153
2
Aug 7, 2011
so i need to use the 8 pin version so? and im guessing that the + side of the op amp circuit that you showed me connects to the output pin of the lm2917 and then you take your output voltage from the emitter side of the transistor??? ?? could u just briefly explain the last op amp circuit u gave me please?

would something like this be good enough? just found it there on the web!

Last edited: Aug 9, 2011
9. ### duke37

5,364
772
Jan 9, 2011
The 8 pin version is the one which I had information on.
The circuit of the 8 pin version needs to be modified. The op-amp is inside the LM2917. You do not need an extra one. The + pin of the op-amp is internally conected to the F/V stage and to pin 3. See the National data sheet.
To modify the 8 pin diagram you have given:-
1. Replace the link between P4 and P7 with 100k
2. Connect the top end of 50k to P6 (7.56V)
3. Connect the bottom end of 18k to P8
4. Connect a 1k resistor between the 50k/18k junction and P7
5 Replace R3 with 1k to reduce the lift at the output due to the current through the 100k resistor.
6. Take output from P4

You will need to adjust VR1 to get the correct sensitivity of the basic F/V convertor. Set the voltage across VR1 to be 1V at 4000rpm.

Fig1 is the basic op-amp follower with gain. The gain is 1+R3/R4
Fig2 is a modification with an output transistor to give more output current and the ratio of the resistors is 7:1 to give a gain of 8. By lifting the 14.3k resistor to 2V the resistor tap will be at 1.75V which is where you want to see the output start to lift.
Fig3 provides a 2V source by dividing down the Zener voltage and, ideally the source resistance should be 14.3k (50k and 18K in parallel). I didn't get this quite right so I added the 1k resistor.

As I said previously, if you are driving a bar graph display this modification can be done in the driver with only two resistors and using the 8 pin LM2917 diagram without modification.

10. ### mechtronics

153
2
Aug 7, 2011
cool thanks. ya i will be using the led driver the lm3914 IC to drive the leds in a bar display.
if i am using this do i need to modify the circuit i have given above to the components you specified to change?

i am looking for a circuit that will work with this using the lm3914.

can the lm3914 be actually calibrated so that to turn on led's every 250 rpm for example every rise in 250 rpm led comes on?

i think you can do this using variable resistors on pins 6, 7 , 8 of the lm3914.

but i think the reference output voltage on the lm3914 is for led brightness.

Last edited: Aug 10, 2011
11. ### duke37

5,364
772
Jan 9, 2011
LM3914
The output from P7 has two functions, a stabilised voltage and a current sense to set the led current. The led current is about 10 times the current supplied by P7. So with 1.25V, a 1k resistor to ground will give a led current of about 12mA. P8 should be connected to ground.

The 3914 has a chain of ten 1k resistors and if a voltage is put across this chain, an led is lit when the input voltage exceeds the voltage at that point.

So, set the V/F convertor to 2000rpm/V, then 7000rpm = 3.5V and 9000rpm = 4.5V
To get a display running from 7000rpm to 9000rpm with ten steps of 200rpm (01V/step), the current through the chain will be 0.1mA.
To raise the voltage at the bottom of the chain by 3.5V, use a 35k resistor to ground (P4 to P2).
To get 4.5V at the top end of the chain you can use the zener (7.56V) in the F/V convertor. A resistor is needed to drop 3.06V at 0.1mA = 30.6k. Connect this between P6 of the LM2917N and P6 of the LM3914.
It depends on what accuracy you require but to get the odd value resistors it is normally best to select a resistor slighly high and parallel it with a much higher resistor, you can then get within a percent or two. 1/R = 1/R1 + 1/R2

Connect the output (P4) of the LM3917N to the input (P5) of the LM3914. Make sure you have a common ground.

12. ### mechtronics

153
2
Aug 7, 2011
so your basically saying by raising the botton end voltage to 3.5 volts, any rpm under 7000 rpm will not light the led's right and as soon as you hit 7200 rpm the first led will light and so on up to 9000rpm.??

i.e as soon as the lm2917 gives a 3.5 volt output to watever frequency there is at 7000 rpm the first led will light and so on as the revs approach 9000rpm.??

so:

7000rpm = 3.5v
7200rpm = 3.6v
7400rpm = 3.7v
7600rpm = 3.8v
7800rpm = 3.9v
8000rpm = 4v
8200rpm = 4.1v
8400rpm = 4.2v
8600 rpm = 4.3v
8800rpm = 4.4v
9000rpm = 4.5v

Last edited: Aug 10, 2011
13. ### duke37

5,364
772
Jan 9, 2011
That is it exactly.

14. ### mechtronics

153
2
Aug 7, 2011
do i have to still modify that 8 pin lm2917 circuit i showed you before ya?

the 200rpm increments on the lm3914? will that automatically work out when i set the lower end to be 3.5volts because of the ten 1k resistors on the chip??

i will draw up a circuit and attach it of wat i think so far

Last edited: Aug 10, 2011
15. ### mechtronics

153
2
Aug 7, 2011
have you got an email address i can send the schematic to because the file is too big to upload on the forum.

16. ### mechtronics

153
2
Aug 7, 2011
just sent a rough drawing of it there to ur email hope it came out ok! let me know ehat u think.

17. ### duke37

5,364
772
Jan 9, 2011
No, this is not what you want, you have modified the F/V convertor and the display, you want one or the other not both. I will draw what you want in the next day or so.

18. ### mechtronics

153
2
Aug 7, 2011
oh ok so if i just use the normal 8 pin schematic version i showed you before and leave the lm3914 the way it is right??

il draw it up again so .

Last edited: Aug 11, 2011
19. ### mechtronics

153
2
Aug 7, 2011
just sent u another schematic to ur email there might be better!

20. ### duke37

5,364
772
Jan 9, 2011
You seem to have got it.
Note 35k use 39k//330k giving 34.8k
30.6k use 33k//430k giving 30.6k
Do not forget input protection circuitry, ignition systems can be very vicious.