Frequency to Voltage

[jesusfreak]

Hi,
I need to get the Frequency of a guitar. The Amplitude must be 0V and 5V, i need to get the changes low to high with a Microcontroller to calculate the frequency.
My Problem is now, how to get the Guitar Frequency to Volotage, I used to do it with a bc547 Transistor, but the highest Voltage is about 2-3V, that is not enough.

 

[Nigel Goodwin]

Hi,
I need to get the Frequency of a guitar. The Amplitude must be 0V and 5V, i need to get the changes low to high with a Microcontroller to calculate the frequency.
My Problem is now, how to get the Guitar Frequency to Volotage, I used to do it with a bc547 Transistor, but the highest Voltage is about 2-3V, that is not enough.

Just provide more amplification - an extra BC547 would do, or you could use an opamp. With a twin opamp you could use one half as an amplifier, and the second half as a comparator - this would give a really nice squarewave output.

 

[jesusfreak]

how do i connect the extra bc547? Parallel to the first bc547? or do i connect them behind each other.

 

[Nigel Goodwin]

how do i connect the extra bc547? Parallel to the first bc547? or do i connect them behind each other.

As an extra amplifier following the first one, post your existing circuit so we can see how that's wired.

 

[jesusfreak]

The Amplifier Part is copied from:
**broken link removed**

But in my case, it doesn't amplify enough.

 

[Nigel Goodwin]

The Amplifier Part is copied from:
**broken link removed**

But in my case, it doesn't amplify enough.

You could try duplicating C3, R1, R2, R3 and T1 - placing it before the existing C3 - if that gives too much gain, you can lower it by inserting a resistor from the emitter to ground on the new transistor.

You could also try replacing the BC547 with a BC549, which has higher gain and lower noise - but you might need to alter resistor values.

Presumably you have a scope?, and that's how you know the signal is too low.

 

[ljcox]

Your subject is a bit misleading. It reads as if you want to convert the frequency to voltage. But it appears that you want to amplify the signal so the micro can count the pulses and thus calculate the frequency

Len

 

[jesusfreak]

Your subject is a bit misleading. It reads as if you want to convert the frequency to voltage. But it appears that you want to amplify the signal so the micro can count the pulses and thus calculate the frequency

Len

yes, you are right, that is what I want. Sorry for the bad topic.

 

[ljcox]

The amp shown at the web site is a bit crude.

What is the input voltage in Volts RMS or peak to peak?

Len

 

[jesusfreak]

The amp shown at the web site is a bit crude.

What is the input voltage in Volts RMS or peak to peak?

Len

I don't know exactly the input Voltages, it's from a guitar Input, it's some mV, not much.

 

[ljcox]

I suggest you take Nigel's advice. Put a transistor amp in front of the existing one. Decrease R1 (in the new stage) to 220 k and insert a 470 Ohm resistor between the emitter and gnd.

This will give a gain of about 10 and this should be more than enough to give you a 5 Volt signal at the collector of the second transistor.

You will need a capacitor between the collector of the new transistor and the base of the second one (ie. the existing one) to block DC.

 

[jesusfreak]

here is the schematic of my new amplification design. is this right, or did I understand you wrong?

PS: i forgot the power and GND, Power is connected to the top part, GND to the bottom...

 

[Nigel Goodwin]

Yes, that's looking OK - as already suggested, you may have too much gain now - you can reduce it (if required) by fitting a resistor in the emitter of the first transistor.

 

[ljcox]

here is the schematic of my new amplification design. is this right, or did I understand you wrong?

No you did not misunderstand me - except that you did not include the emitter resistor (between the emitter of Q1 and gnd) in the first stage (call it R7)

R1 = 220k, R2 = 56k, R7 = 470R will make the gain of the first stage about 10.

R7 will both reduce the gain and increase the input resistance - both are desirable.

Len

 

[jesusfreak]

do i need a capacitor at the output of this circuit, too? somebody said i should use one.

 

[ljcox]

do i need a capacitor at the output of this circuit, too? somebody said i should use one.

No, provided that the PIC and the amp share the same voltage supply.

Len

 

[jesusfreak]

okay, they have both 5V power supply

 

[ljcox]

That's good. No need for a capacitor.

Len