#328 555Timer/VoltageControlledOscillator

A voltage-controlled oscillator (VCO) using the 555 timer.

Build

Notes

Conventional astable oscillator configurations of the 555 timer allow frequency (and duty cycle) variation with a variable resistor.

A voltage-controlled oscillator (VCO) on the other hand is one - obviously - where the output frequency is proportional to some voltage input, making it ideal for electronic control.

Is it possible with a 555 timer? Yes, it turns out there are a few ways in fact.

The simplest, which I am testing here, is directly indicated in most 555 datasheets:

The threshold and trigger levels normally are two-thirds and one-third, respectively, of VCC. These levels can be altered by use of the control-voltage terminal.

The simplified schematic of the 555 shows the control terminal directly connected to the upper threshold of the internal three-way voltage divider:

555_schematic

Test Circuit

I’m using a standard 555 astable tuned for 320Hz @ 51% duty cycle. That’s too fast for the eye, but just enough for a good scope trace.

To keep things simple, I’m using a simple voltage divider to allow easy measurement at different levels.

In the following scope traces, the three channels are connected as follows

Channel	Colour	Connection
CH1	Yellow	timer output (pin 3)
CH2	Blue	threshold/trigger (pin 2 & 6)
CH3	Red	control voltage (pin 5)

Basic Astable Operation

Well, it is meant to be running at a theoretical 320Hz. No doubt due to part tolerances and breadboard losses, I have the circuit running hot at 553Hz (50.3% duty cycle) when put on a scope.

This is the trace of the standard circuit with the control voltage disconnected

scope_basic_astable

Attaching the Control Voltage

After attaching the voltage-divider to the control terminal, and before even starting to make measurements, I noticed some severe oscillation on the rising edge of the output:

scope_vco_unfiltered

I tried some ferrite beads to no great benefit. But adding a 10nF capacitor (C3) on the power rail solved the problem nicely:

scope_vco_filtered

Voltage-controlled Oscillator Samples

Here is a tabulation of a sampling of results for different control voltages. Note that I’m using a nominal 9V supply (batteries).

Control Voltage	Output Frequency	Duty Cycle
0.70 V	-	0%
0.80 V	411 Hz	7%
2.40 V	670 Hz	17%
3.52 V	620 Hz	29%
8.32 V	364 Hz	74%
8.60 V	-	100%

Conclusions:

oscillation breaks down at the upper and lower voltage ranges
at the upper limits, it swings to full “on”
at the lower limits, it swings to full “off”. It begins to tailoff below about 2.4V
there’s good control of frequencies between 361 Hz (8.52V) and 670 Hz (2.4V)
but the effect on duty cycle is even more pronounced