Analog voltage to duty cycle control using 555?

[SittingBear]

Hello EEVBlog Forums,
I'm trying to use the voltage output from a peak detector amplifier to convert to a fixed frequency, variable duty cycle signal. I thought about using a micro controller to do this, but that seems like a bit overkill to me and I would rather learn about a possible new use for the 555 timer.

The positive voltage rail will either be +5Vdc or +12Vdc with the negative or low rail being 0V / GND. I feel that there must be a simple way of doing this.


Any help is greatly appreciated!
-Bear

[David Hess]

How accurate do you want it to be?  An operational amplifier can be used to integrate the difference between the desired and output duty cycle.

[oldway]

You can do this of a very easy way with a TL494 or with a SG1525, but it will not work with +5V, it is too low voltage.
They need more than +8V.

[T3sl4co1l]

One chip, a handful of resistors.  Will work as shown at 5V, though you may want to tweak values to adjust the exact voltage ranges or thresholds or whatever.

The ramp waveform is sloppy (exponential curves), so the transfer function won't be linear.  You can fix this by using gated current sources instead of R5.  R5 * C1 sets the frequency, R1-R2-R3 sets thresholds.

Tim

[SittingBear]

Sorry. I found the solution, untested, on page 13 of this datasheet. I attempted to delete this thread, but I guess it might have been better to hear other ideas instead of going with an untested method! I'll be able to construct and take measurements of the circuit tonight.

Cheers!