Discussion in 'General Electronics Chat' started by evo.motorsport, Jun 20, 2007.

1. evo.motorsport Thread Starter Member

Jun 20, 2007
21
0
Im building digital tachometer to display spindle speed on my hobby CNC machine.

Im using optical censor to pick up the RPM signal. Signal from the optical sensor connected to LM2917. Im reading output from the LM2917 using digital voltmeter built on ICL7107 to display the output voltage. When Im increasing the spindle RPM the voltage increases and when Im decreasing spindle RPM voltage decreases.

But here is the problem:
The output voltage increases only to 3.6V. Then it stays on 3.6V, does not get any higher.

The supply to LM2917 is 6V

2. kender Senior Member

Jan 17, 2007
263
0
May be your frequency is too high for your selection of C1. The last formula on page 7 of the LM2917 datasheet shows how to calculate the maximum frequency that LM2917 can convert.

3. Papabravo Expert

Feb 24, 2006
10,340
1,850
Can you post a schematic diagram of your circuit?

4. evo.motorsport Thread Starter Member

Jun 20, 2007
21
0
My frequency is 0Hz to 670Hz.
Can you help me to calculate? How can I output 5V or even better 10V on max frequency?

5. kender Senior Member

Jan 17, 2007
263
0
I'd be happy to help you with calculations, although I need to know your circuit. Please post a schematic or tell us the document (datasheet, app note, web site etc.) and a figure where the circuit diagram can be found.

6. evo.motorsport Thread Starter Member

Jun 20, 2007
21
0
Here is the schematic.

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7. kender Senior Member

Jan 17, 2007
263
0
I think, the capacitor connected to pin 2 is too big. You should be able to go to 670 rpm, if you put a 0.033uF capacitor or smaller.

By the way, at what RPM does your output reach 3.6V an stop changing in your original design?

8. evo.motorsport Thread Starter Member

Jun 20, 2007
21
0
Actually my full RPM is 16000 but I'm dividing frequency by 24 usig decade counter. So my maximum frequency is 667Hz. The output reaches 3.6V at 480Hz

9. evo.motorsport Thread Starter Member

Jun 20, 2007
21
0
Tried 0.033uF.... now voltage stops at 2.8 on 177Hz

10. beenthere Retired Moderator

Apr 20, 2004
15,815
283
Can you post a more complete schematic? We need to see the rest of the circuitry to be able to help out. Generally, 2917's are pretty straightforeward to set up.

11. evo.motorsport Thread Starter Member

Jun 20, 2007
21
0
That's the complete diagram or the the module. Frequency go In to 2917 and digital voltmeter connected to the Vout.

is it any way to calcullate the pars values base on input parameters?

The frequency in from 0Hz to 667Hz I need the output voltage to be 0V to 10V