#### evo.motorsport

Joined Jun 20, 2007
21
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

#### kender

Joined Jan 17, 2007
264
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.

#### Papabravo

Joined Feb 24, 2006
20,384
Can you post a schematic diagram of your circuit?

#### evo.motorsport

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

#### kender

Joined Jan 17, 2007
264
Can you help me to calculate? How can I output 5V or even better 10V on max frequency?
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.

#### evo.motorsport

Joined Jun 20, 2007
21
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.
Here is the schematic.

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#### kender

Joined Jan 17, 2007
264
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?

#### evo.motorsport

Joined Jun 20, 2007
21
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?
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

#### evo.motorsport

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

#### beenthere

Joined Apr 20, 2004
15,819
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.