1 Voltage-controlled oscillator

Introduction

The 555 timer is possibly the single most useful integrated circuit in history.Since its introduction in 1972, it is estimated that over one billion 555sand closely-related variants (such as 556 dual-555 chips and 7555 low-powerchips) have been produced every year. Youll have seen some demonstrationsof 555 uses in lecture. These demonstrations ignored the control input onthe 555; today well use that input pin to change the internal trigger andthreshold levels in the 555 and make a Voltage Controlled Oscillator (VCO).

Finally, well use some of our newly-developed skills with op-amps tomake this VCO into an optical theremin.

Procedure

1. Build the VCO circuit shown, using the 1k potentiometer input.Note that either potentiometer can change the frequency of the output:the potentiometer in series with C1 changes the frequency by changingthe rate at which the capacitor charges, and the potentiometer at pin5 (control) changes the frequency by changing the voltage at whichon/off transitions occur.

Once were sure the VCO is working, well replace the control poten-tiometer with a different variable voltage source, and use the capacitor-charging potentiometer for tuning.

2. We would like to control the circuit with a photodiode, so that theoutput frequency depends on the light level instead of on a knob po-sition. The most obvious way to do this is to send the voltage acrossa resistor in series with a reverse-biased photodiode directly to thecontrol input, but there are two problems with this:

The photodiode signal level is fairly low ( 200 mV). To changethe frequency range of the VCO over a large range of values, youneed a signal that ranges from about 0 V to about Vcc.

The output impedance of the photodiode circuit shown is veryhigh ( 1 M). If you attach this directly to the relatively low-Zcontrol input of the 555 timer, the photodiode output voltage willbe pulled even lower.

Youre going to need an amplifier between the photodiode and thecontrol input. Here are some desirable characteristics of this amplifier:

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Figure 1: VCO circuit, with two input options.

It should have a high input impedance. It should have a DC gain that can change a 200 mV input to a

1012 V output.

The output should be positive; thats what the 555 control inputneeds.

It does not need to be a great amplifier: your signal input willbe somewhat hand-wavy, so building a 3-op-amp instrumenta-tion amplifier is probably overkill.

It does NOT need high bandwidth. Much of the photodiodeoutput is just noise: Johnson noise on the 1 M resistor, shotnoise from the quantized current of the photodiode, power-supplynoise at the 555 output frequency (every time the 555 outputswitches it causes a tiny but noticeable glitch in Vcc) and 120 Hzflicker from the fluorescent lamps; none of which we want to sendinto the control input. Accordingly, set the corner frequency ofthe amplifier you build to be above the highest frequency at whichyou could manually change the light level on the photodiode, andbelow the expected 120 Hz fluorescent-lamp noise.1

1This is an important concept: you never want to amplify noise. Always set youramplifier high-frequency cutoff to just above the highest frequency of interest.

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Design your photodiode signal amplifier in your lab book includingcalculation of component values and have your instructor check itbefore continuing.

3. Build your complete circuit, and demonstrate it by playing a recog-nizable tune. Singing along with your theremin is optional. http://www.youtube.com/watch?v=_YYABE0R3uA

Questions

1. The 220 resistor R2 at the discharge pin is very important. Why?

2. Its most likely that your theremin produces higher frequencies forlower light levels. How would you change your circuit so that increasingthe light level increased the frequency?

3. The sound quality of your theremin is low. In large part, this is becausethe output to the speaker is a square wave (with high-frequency Fouriercomponents) rather than a smooth sinusoid. How would you changethis? Hint: You could add a volume control at the same time.

Clean up

Put all your components away and clean up all trash and scrap in yourarea. Also check room 123 for any important papers. Throw out or recycleanything unimportant!

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