Help! Frequency to voltage conversion! - Electronic Circuits Projects Diagrams Free

funkychicken9000 · 12th June 2007, 12:12 PM

I'm building a tacho for a race car, and I was originally going to use an LM2917 f-v converter and a LM3914 LED bar chart chip.

Thing is, the way I was going to supply the LM2917 with a frequency was through the ECU, which would output a logic level square wave of varying frequency.

I read on these boards that the LM2917 needs a AC input, and won't work off a pulsed DC signal. Any way round this?

Cheers all!

**Nigel Goodwin** · 12th June 2007, 12:35 PM

Quote:

Originally Posted by funkychicken9000

I'm building a tacho for a race car, and I was originally going to use an LM2917 f-v converter and a LM3914 LED bar chart chip.

Thing is, the way I was going to supply the LM2917 with a frequency was through the ECU, which would output a logic level square wave of varying frequency.

I read on these boards that the LM2917 needs a AC input, and won't work off a pulsed DC signal. Any way round this?

Pulsed DC is AC - it will work perfectly!.

funkychicken9000 · 12th June 2007, 12:38 PM

Are you sure? In this thread audioguru says it won't...

To clarify, I'm using the 8-pin version - will connecting the pulsed DC straight into pin 1 work?

**Nigel Goodwin** · 12th June 2007, 12:54 PM

Quote:

Originally Posted by funkychicken9000

Are you sure? In this thread audioguru says it won't...

To clarify, I'm using the 8-pin version - will connecting the pulsed DC straight into pin 1 work?

You simply feed it through a capacitor, and a resistor to chassis (0V), it will then be centered around 0V. AC doesn't have to centered around 0V either, it depends entirely where it comes from!.

There is essentially no difference between AC and pulsed DC.

audioguru · 12th June 2007, 02:00 PM

I agree. Capacitor-coupling into a resistor to ground will be OK to feed DC pulses into an 8-pins LM2907/LM2917.

funkychicken9000 · 12th June 2007, 02:53 PM

Cool. Any tips on R and C values?

audioguru · 12th June 2007, 03:12 PM

Quote:

Originally Posted by funkychicken9000

Cool. Any tips on R and C values?

The datasheet has formulas and examples.

funkychicken9000 · 21st June 2007, 05:02 PM

I was wondering about the values I'd need for the resistor and capacitor to connect my pulsed dc input waveform to the LM2917.

Am I right in thinking I connect the signal through a capacitor and resistor in series to ground, and connect the LM2917 input to the joint between the capacitor and resistor? If so, what sort of size resistor and capacitor would I use? I'm assuming I'd want the RC time constant to be large relative to the period of oscillation of the input, but since this is going to vary between around 1kHz and 60kHz, I'm not sure exactly what to choose - would say 10k and 100nF be ok?

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12th June 2007, 12:12 PM	(permalink)
funkychicken9000	Help! Frequency to voltage conversion! I'm building a tacho for a race car, and I was originally going to use an LM2917 f-v converter and a LM3914 LED bar chart chip. Thing is, the way I was going to supply the LM2917 with a frequency was through the ECU, which would output a logic level square wave of varying frequency. I read on these boards that the LM2917 needs a AC input, and won't work off a pulsed DC signal. Any way round this? Cheers all!

12th June 2007, 12:38 PM	(permalink)
funkychicken9000	Are you sure? In this thread audioguru says it won't... To clarify, I'm using the 8-pin version - will connecting the pulsed DC straight into pin 1 work?

12th June 2007, 02:00 PM	(permalink)
audioguru	I agree. Capacitor-coupling into a resistor to ground will be OK to feed DC pulses into an 8-pins LM2907/LM2917. __________________ Uncle $crooge

12th June 2007, 02:53 PM	(permalink)
funkychicken9000	Cool. Any tips on R and C values?

21st June 2007, 05:02 PM	(permalink)
funkychicken9000	I was wondering about the values I'd need for the resistor and capacitor to connect my pulsed dc input waveform to the LM2917. Am I right in thinking I connect the signal through a capacitor and resistor in series to ground, and connect the LM2917 input to the joint between the capacitor and resistor? If so, what sort of size resistor and capacitor would I use? I'm assuming I'd want the RC time constant to be large relative to the period of oscillation of the input, but since this is going to vary between around 1kHz and 60kHz, I'm not sure exactly what to choose - would say 10k and 100nF be ok?