Can a free working LMC555 VCO discharge its timing cap to zero?

Frequent design thought (DI) contributor Nick Cornford not too long ago revealed a synergistic pair of DIs “A pitch-linear VCO, half 1: Getting it going” and “A pitch-linear VCO, half 2: taking it additional.”

Wow the engineering world together with your distinctive design: Design Concepts Submission Information

The principle theme of those articles is design methods for audio VCOs which have an exponential (a.ok.a. linear in pitch) relationship between management voltage and frequency. Nice work Nick! I turned significantly within the subject throughout a energetic dialogue (typical of editor Aalyia’s DI kitchen) within the feedback part. The controversy was about whether or not such a VCO might be constructed across the venerable 555 analog timer. Some stated nay, others yea. I leaned towards the latter opinion and determined to attempt to put a schematic the place my mouth was. Determine 1 is the end result.

Determine 1 555 VCO discharges timing cap C1 fully to the damaging rail by way of a Reset pulse.

The nay-sayers’ case hinged on a perceived lack of ability of the 555 structure to fully discharge the timing capacitor, C1 in Determine 1. They appeared to have a superb argument as a result of, in its common mode of operation, the discharge of C1 ends when the set off enter stage is crossed. This usually occurs at one third of the provision rail differential and one third is a good distance from zero! However it seems the 555, regardless of being such an previous canine, is aware of a distinct trick, it entails a really seldom used function of this historic chip: the reset pin 4.

The 555 datasheet says a pulse on reset will override set off and in addition pressure discharge of C1. In Determine 1, R3 and C2 present such a pulse when the OUT pin goes low on the finish of the timing cycle. The R3C2 product ensures the heart beat is lengthy sufficient for the 15 Ω Ron of the Dch pin to precisely evacuate C1. 

And that’s it. Downside solved as sketched in Determine 2.

Determine 2 The VCO waveforms; reset pulses on the finish of every timing cycle, and is triggered when Vc1 = Vcon, to pressure an adequately full discharge of C1.

Determine 3 illustrates the ensuing passable log conformity (due largely to my shameless theft of Nick’s intelligent resistor ratios) of the ensuing 555. VCO, displaying good exponential (linear in pitch) habits over the specified two octaves of 250 to 1000 Hz.

Determine 3 Log plot of the frequency versus management voltage for the two-octave linear-in-pitch VCO. [X axis = Vcon volts (inverted), Y axis = Hz / 16 = 250 Hz to 1 kHz]

In truth, on the worth of an additional resistor, it is perhaps doable to enhance linearity sufficient to select up one other half a volt and half an octave on each ends of the pitch vary to span 177 Hz to 1410 Hz. See Determine 4 and Determine 5.

Determine 4 R4 sums ~6% of Vcon with the C1 timing ramp to get the development in linearity proven in Determine 5.

Determine 5 The impact of the R4 modification displaying a linearity enchancment. [X axis = Vcon volts (inverted), Y axis = Hz / 16]

Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.

Associated Content material

• A pitch-linear VCO, half 1: Getting it going
• A pitch-linear VCO, half 2: taking it additional
• VCO utilizing the TL431 reference
• Extremely-low distortion oscillator, half 1: how to not do it.
• The right way to management your impulses—half 1
• A two transistor sine wave oscillator

The publish Can a free working LMC555 VCO discharge its timing cap to zero? appeared first on EDN.

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