# 555 voltage controlled oscillator question

Discussion in 'LEDs and Optoelectronics' started by Thedrive2112, Jun 19, 2014.

1. ### Thedrive2112

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Nov 17, 2013
I have a ne555 timer setup as a voltage controlled oscillator. My understanding is I can control the output frequency by injecting a voltage on pin 5 (control pin). This actually works well if I put a pot on pin 5 to vary the voltage that is already present on pin 5 (10.6volts) . I'm looking to control this with a varying 0-5 volt signal from a vehicle when I input this signal onto pin 5 I get nothing. Does my input signal (0-5v) on pin 5 need to be greater than the voltage that is already present on pin 5 (10.6v) to work? Or am I way off base here? Is there someone who understands the VCO operation on the 555 better than me.

Thanks

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2. ### Rleo6965

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Jan 22, 2012
Please read the datasheet of NE555. NE555 have have 2 comparators and 3 voltage resistor. Voltage control pin5 was connected to comparator with 2/3 reference voltage. Your 0-5V was not enough amplitude for 2/3 VCC of NE555. That's the reason that it does not affect frequency of NE555.

Last edited: Jun 19, 2014
3. ### Thedrive2112

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Nov 17, 2013
Thanks for the response. I read the data sheet but I'm still rather new at electronic circuits so I guess I didn't catch it. I figured this was the reason. Does anyone have a fix or is there a way to drop the reference voltage so a low input signal (0-5volts) will work? Or is the best solution to feed the input signal (0-5 volts) through a op-amp to boost the voltage higher than 2/3 VCC?

4. ### Rleo6965

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Jan 22, 2012
NE555 can still function on +5V on its VCC and therefore matched with yun 0-5V input voltage.

But if your installing the NE555circuit to +12V supply. You can place a series resistor and 5.1V zener diode for the supply of 5.1V of NE555 VCC. This will solve matching of 5V vco input. Swap connection of pin7 and pin 3. Then pin 7 will be your open collector output. But you need a pullup resistor of around 1k and connect to +12V and pin 7. Pin 7 oscillator output will swing from 0.7V to 10V.

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6. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Rleo is wrong; you can operate a 555 with pin 5 over quite a wide range. But it's not a very ideal VCO - the frequency vs. voltage graph is not linear, and an increasing voltage causes a decreasing frequency (over most of the range, at least). You can improve it by adding external components - see http://www.electroschematics.com/7170/wide-frequency-range-555-vco/ (I have not tried and don't necessarily endorse that design.)

The 566 is a very old device and appears to be obsolete.

There are many devices that can be used as VCOs. A popular and easy-to-use one is the CD4046B (see http://www.digikey.com/product-detail/en/CD4046BE/296-2052-5-ND/67295). It is an old device, but is still used, and manufactured by three companies: NXP (formerly Philips), ON Semiconductor (formerly Motorola) and Texas Instruments (who acquired it from National Semiconductor).

Avoid versions of the 4046 that include 74 in the name; most of these require a 5V supply voltage. The 4046 operates from supply voltages up to 15V (some versions can tolerate 18V).

The 4046 WILL be damaged by voltage surges and spikes on the automotive +12V power supply rail; you NEED to include clamping (using a zener diode and/or voltage suppressor with series resistors) and/or an automotive voltage regulator if you're operating ICs in an automotive environment, which is harsh (Google load dump). Regulation is a good idea anyway, since supply voltage variations will affect the VCO behaviour somewhat. Clamping may also be needed on the control voltage, to avoid damaging the high-impedance control voltage input of the 4046.

The 4046 is easy to use as a VCO because it has two auxiliary resistors that can be used to determine the frequency range covered by the full range of control voltages. For example, if your control voltage range is 0~10V, you can set the 4046 to produce a corresponding frequency range of 10~1000 Hz, or 100~200 Hz, or 10,000~10,010 Hz, or 10,000~15,000 Hz, or whatever you need. There is a single timing capacitor (as in most VCOs).

It also has a very high-impedance control voltage input, which is helpful if you want to add filtering on the control signal because it will not cause a voltage offset.

I suggest you get all of the following data sheets, because different manufacturers provide different types of detail:
NXP: http://www.nxp.com/documents/data_sheet/HEF4046B_CNV.pdf

And buy a few devices and play around with different values for C, R1 and R2 to get the idea of how to use it.

As I said, there are other VCO devices available. Have a look at Digikey's selection table at http://www.digikey.com/product-search/en?FV=fff40027,fff80205&stock=1&quantity=1&pageSize=250 but beware, most of the options are SMT (surface mount technology). If you filter those out, there aren't many options left; most of those are variants of the 4046.

Thedrive2112 and Ehsan like this.
7. ### Rleo6965

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Jan 22, 2012
My mistake.KrizBluenz was correct. Pin6 that tied with pin2 will be a sawtooth voltage from charging and discharging of capacitor and within the range of threshold voltage 0-5V pin5.. NE555 will still oscillate even with +5V or lower voltage with corresponding change in frequency.

8. ### Thedrive2112

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Nov 17, 2013
I'm grateful to all who replied.

I ordered and when it arrives I'm going to try the 4046ic as recommended by chrisblue sounds like a perfect solution for what I'm trying to accomplish. I actually got the below circuit to work decent by changing the R and C values. The problem that I'm struggling with now is pin 5 has 2/3vcc voltage on it as you all know. When I connect the throttle position sensor of the vehicle (0-5v depending on the posistion of the throttle) to pin five it will accelerate the vehicle as the vehicles ECM is now seeing a voltage and thinking that the throttle pedal is being pressed.

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9. ### KrisBlueNZSadly passed away in 2015

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Haha! Yep. You could fix this, but since the control voltage works backwards (lower voltage = higher frequency), there's not much point. The usefulness of pin 5 on a 555 is limited to making siren circuits. You'll like the 4046. Just be careful not to damage it - keep the inputs within the supply rails, and buffer the output if you need more than a few milliamps from it.

10. ### Thedrive2112

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Nov 17, 2013
Really appreciate the advice as I'm really good at burning things up! Wishing now I payed better attention in electronics class in high school. Good thing there are forums like this with good people like you to help.

I'm feeding the output to a lm339 so I can control duty cycle. From there it's off to N channel mosfets and then the magnetic coil.

11. ### KrisBlueNZSadly passed away in 2015

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You can't vary the duty cycle by feeding the output through a comparator - the output is a square wave with a duty cycle of 49~51%. You would have a similar problem with the 555 circuit - the main output is a square wave, and the waveform at pins 2 and 6 would vary in amplitude and average voltage as you varied the control voltage.

You might be able to do what you want by triggering a monostable with a time constant that's controlled by the VCO voltage in conjunction with the duty cycle control voltage.

Perhaps if you describe your project in detail we will be able to offer some suggestions.

12. ### Thedrive2112

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Nov 17, 2013
If I can have have as much control over the linear frequency as it seems with the 4046 I won't need duty cycle control.

I thought I could run the square wave output through a pass filter to get a sawtooth-ish wave to send to the lm393 compair

I experiment with alternative fuels in vehicles. I'm currently looking to deploy DME as a fuel suppliment. I'm trying to build an injector pulser where the frequency can be linearly increased based on a voltage input from a vehicle (such as 0-5V from a throttle posistion sensor or 0-5v from a map sensor). The frequency will need to increase as the voltage from the input increases. I also need to be able adjust the frequency to be slower or faster based on the same 0-5v input signal so it will work for smaller and larger engines alike. The injector rail I'm using has 4 magnetic coil injectors that have 3ohm impedance each . I have run these injectors in the past with other fuels using a 555 and lm393 but the frequency of the 555 was set at one frequency and the 393 handled duty cycle. I then used another lm393 with relays to turn on or off each injector based on the 0-5v from the vehicle. So at low RPM one injector would be on at the set frequency and as rpm would climb the second injector would turn on at the set frequency and so on...

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13. ### KrisBlueNZSadly passed away in 2015

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The amplitude of the triangle-ish wave will vary depending on frequency. So you would have to either vary the resistor in the integrator, or vary the comparator threshold, to compensate.

If you need variable duty cycle independent of frequency, a good option would be the two-op-amp "integrator + Schmitt trigger" oscillator configuration, using a transconductance amplifier to drive the integrator. Actually there was a circuit posted here recently that did that - see https://www.electronicspoint.com/threads/which-capacitor-is-best.269126/#post-1612612

You wouldn't need most of that circuit. The important bits are IC2A, C4, IC3C and IC1 (I give the designer extra points for gratuitous use of a 555 in a proper circuit). The signal on IC3's output is a triangle wave with fixed amplitude (swings between about 5V and 10V, determined by the 555's thresholds) with a frequency determined by the current going into pin 1 of IC2A. Let me know if you want to go down that road.
I thought the injectors had to be synchronised to the rotation of the engine...?

14. ### Thedrive2112

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Nov 17, 2013
Normally that's true. Im working mostly with Diesel engines and are supplementing the diesel fuel with alternative fuels to cut back the use of diesel not completely replacing it.

15. ### Thedrive2112

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Nov 17, 2013
Sorry, bad use of the reply function. I'm still figuring this place out.

16. ### Thedrive2112

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Nov 17, 2013
Found some 4046's at a local electronic supply and started playing around with it. Is there a problem taking the output off the pins 6 or 7 (capacitor pins) vs the square wave VCO output pin??
I'm able to control duty cycle fairly well when I take the output off pins 6 or 7 and run it into a lm393.
Or Would it be better to take the square wave on VCO output pin and run it through a pass filter?

17. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Connecting anything to pin 6 or pin 7 would not be recommended by the manufacturer and it might not work properly with a different brand of 4046. But if that doesn't worry you, and your load has a very high input resistance, it may be OK. It's a lot simpler than using an integrator or low-pass filter.

18. ### Thedrive2112

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Nov 17, 2013
Thanks for advise, I need to do it the right way I guess I'm probably going to making this circuit many times in the future and I'm going to get some PCB's made.

What is the best option: low pass filter or integrator. I'm aware of and have used a low pass filter in the past. I have not used an Integrator as of yet.

19. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
If you want a VCO that can generate arbitrary duty cycles, it's probably best to start with one that generates a triangle or sawtooth wave with constant amplitude (and therefore, variable steepness). If pin 6 or 7 of the CD4046 has a suitable signal, you can use that, but I'd be concerned that different brands might have different signals on those pins. That's just because no manufacturer says anything about what those pins do, other than the fact that you connect a capacitor between them.

If you have an oscilloscope, you should check the waveforms on those pins to confirm that at least one of them is a triangle or sawtooth. I've never looked into it myself. But you might want to check CD4066es from several manufacturers.

Passing the square wave through an R-C filter will not give you a triangle or sawtooth wave, so your duty cycle control voltage will not map linearly to duty cycle. You could use an integrator or a resetting ramp generator, but in either case your current will need to be made proportional to the frequency, otherwise the amplitude of the triangle or sawtooth wave will vary with frequency. That's why I suggest it's best to source the triangle or sawtooth waveform directly from the VCO.

20. ### Thedrive2112

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Nov 17, 2013
Got it! I will put it on the scope and report back if anyone has need to reference it in the future.

Thanks for your continued help Kris.