A voltage-controlled oscillator (VCO) using the 555 timer.
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:
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
|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
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:
I tried some ferrite beads to no great benefit. But adding a 10nF capacitor (C3) on the power rail solved the problem nicely:
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|
|0.80 V||411 Hz||7%|
|2.40 V||670 Hz||17%|
|3.52 V||620 Hz||29%|
|8.32 V||364 Hz||74%|
- 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
3.52 V, 620 Hz, 29% Duty
8.32 V, 364 Hz, 74% Duty
Credits and References
- LM555 Datasheet
- Voltage-controlled oscillator - wikipedia
- How to Build a Voltage Controlled Oscillator (VCO) with a 555 Timer Chip - learningaboutelectronics
- LEAP#016 AstableOscillator
- ..as mentioned on my blog